Bulletin ILLIN'OIS iturail History Survey BULLETIN' Dynamics of One-Species Populations of Fishes in Ponds Subjected to Cropping and Additional Stocl(ing NATURAL HISTORY SURVEY SEP 24 1970 LIBRARY IT OF REGISTRATION AND EDUCATION HISTORY SURVEY DIVISION LLINOIS IH£ UBRARl QE IHE JUL 2 01970 UNIVERSITY OF ILLINOIS AT. URBANA-CHAMPAIGN VOLUME 30, ARTICLE 2 MARCH, 1970 ILLINOIS Jsktwi^rsLl History Suir-vey BULLETIN* Dynamics of One-Species Populations of Fishes in Ponds Subjected to Cropping and Additional Stocicing Iter Buck !S F. Thoits III ILLINOIS FMENT OF REGISTRATION AND EDUCATION lAL HISTORY SURVEY DIVISION lA, ILLINOIS VOLUME 30, ARTICLE 2 MARCH, 1970 STATE OF IL,LIN01S DEPARTMENT OF REGISTRATION AND EDUCATION BOARD OF NATURAL RESOURCES AND CONSERVATION William H Robinson, Chairman; Thomas Park, Ph.D., Bioloijy ; U U Sloss, Ph.D., Geology: ROGER Adams. Ph.D., D.Sc, Chemistry: Robert H. Anderson, B.S.C.E., Engineering: Charles E OLMSTED, Ph.D., Forestry; W. U Everitt, E.E., Ph.D., Bepresentimg the President of the University of Illinois; Roger E. Bevler, Ph.D., Representing the President of Southern Illinois Umvrrsit I/. NATURAL HISTORY SURVEY DIVISION, Urbana, Illinois SCIENTIFIC AND TECHNICAL STAFF George Sprugel, Jr., Ph.D., Chief Alice P. Campbell, B.A., Secretary to the Chief Assistant Wildlife Assistant Wildlife Assistant Wildlife Section of Economic Entomology William H. Luckmann, Ph.D., Entomologist and Head Willis N. Bruce, Ph.D., Entomologist Wayne L. Howe, Ph.D., Entomologist Stevenson Moore, III, Ph.D., Entomologist, Extension Howard B. Petty, Ph.D., Entomologist, Ex- tensimi James E. Appleby, Ph.D., Associate Entomolo- gist Edward J. ARMBRUST, Ph.D., Associate En- tomologist Marcos Kogan, Ph.D., Associate Entomologist Joseph V. Maddox, Ph.D., Associate Entmnolo- yist Ronald H. Meyer, Ph.D., Associate Entomolo- gist Robert D. Pausch, Ph.D., Associate Entomolo- gist Ralph E. Sechriest, Ph.D., Associate Ento- mologist CiAitL-NCE E. White, B.S., Assistant Entomolo- gist Keun S. Park, M.S., Assistant Chemist Sue E. Watkins, Supervism-y Assistant Donald E. Kuhlman, M.S., Instructor, Ex- RoscoE Randell, M.S., Instructor, Extension Stephen Sturgeon, B.S., Assistant Specialist, Extension Jean G. Wilson, B.A., Research Associate Keturah Reinbold, M.S., Research Assistant Stephen Roberts, B.S., Tech/nical Assistayit Douglas K. Sell, B.S., TecJvtiical Assistant Section of Faunistic Surveys and Insect Identification Philip W. Smith, Ph.D., Taxonomist and Head Wallace E. LaBerge, Ph.D., Taxonomist Milton W. Sanderson, Ph.D., Taxonomist Lewis J. Stannard, Jr., Ph.D., Taxonomist Robert W. Poole, Ph.D., Assistant Tajcoiiomist John D. Unzicker, Ph.D., Assistant Taxono- mist Donald W. Webb, M.S., Assistant Taxonomist Bernioe p. Sweeney, Technical Assistant Section of Aquatic Biology George W. Be.vnett, Ph.D., Aquatic Biologist a/nd Head D. Homer Buck, Ph.D., Aquatic Biologist R. Weldon LARIMOP.E, Ph.D., Aquatic Biologist William C. Starbett, Ph.D., Aquatic Biologist Robert C. Hiltibran, Ph.D., Biochemist William P. Childers, Ph.D., Associate Aquatic Biologist Donald F. Hansen, Ph.D., Associate Aquatic Biologist Richard J. BAUR, M.S., Research Assistant warren U. Brigham, M.S.. Research Assistant Dennis L. Dooley, Technical Assistant Mary Frances Martin. Teclvnical Assistant C. Russell Rose, Field Assistant Section of Botany and Plant Pathology J. Cedric Carter, Ph.D., Plant Pathologist and Head Robert A. Evers, Ph.D., Botanist Junius L. Forsberg, Ph.D., Plant Pathologist CONSULTANTS : Parasitology, Norman D. Levine, Ph.D., Professor of Veterinary Parasitology a/nd Veterinary Research, University of Illinois; Wildlife Research, Willard D. Klimstra, Ph.D., Professor of Zoology and Director of Cooperative Wildlife Research, Southern Illinois University ; Statistics, Horace W. Norton, Ph.D., Professor of Statistical Design amd Analysis, University of lllmoirS. ii Eugene B. Himelick, Ph.D., Pla/)it Pathologist R. Dan Neely, Ph.D., Plant Pathologist D. F. ScHOENEWEiss, Ph.D., Associate Plant Pathologist J. Leland Crane. Ph.D., Assistant Mycologist Walter Hartstien, Ph.D., Assistant Plant Pathologist Betty S. Nelson, Technical Assistant Gene E. Reid, Technical Assistant Section of Wildlife Research Glen C. Sanderson. Ph.D., Wildlife Specialist and Head Frank C. Bellrose, B.S., Wildlife Specialist Richard R. Graber, Ph.D., Wildlife Specialist Harold C. Hanson, Ph.D., Wildlife Specialist William L, Anderson, M.A., Associate Wild- life Specialist W. W. Cochran, Jr., B.S., Associate Wildlife Specialist William R. Edwards, M.S., Associate Wildlife Specialist Jack A. Ellis, M.S., Associate Wildlife Spe- cialist Ronald F. Labisky, Ph.D., Associate Wildlife Specialist Stanley L. Etter, M.S., Assistant Wildlife Specialist Robert E. Greenberg, M.S., Specialist G. Blair Joselyn, M.S., Specialist Paul J. Matthews, B.S., Specialist George B. Rose, Ph.D., Assistant Wildlife Spe- cia list Keith P. Thomas, M.S., Assistant Wildlife Specialist Ronald L. W^estemeier, B.S., Assistant Wild- life Specialist Ronald E. Duzan. Tecli/nical Assistam.t Caeoly'n S. Evers. B.A., Technical Assistant Mary Ann Johnson, Tech/nical Assistant Helen C. Schultz. M.S.. Technical Assistant Eleanore Wilson, Technical Assistant Robert D. Crompton, Field Assistant Section of Administrative Services Robert O. Watson, B.S., Administrator and Head Supporting Services WiLMA G. DiLLMAN, Property Control and Tru^t AccoitAits Robert O. Ellis, Supervisory Assista/nt Lloyd E. Huffman. Stockroom Ma/nager J. William Lusk, Mailing and Distribution Services Melvin-E. Schwartz, Fitiancial Records James E. Sergent, Greenhouse Superintendent Publications and Public Relations Owen F. Glissendorf, M.S., Technical Editor Robert M. Zewadski, M.S., Associate Technical Editor Shirley McClellan, Assistant Technical Editor Richard M. Sheets, Technical Illustrator "WiLMER D. Zehr, Technical Photographer Technical Library Doris F. Dodds, B.A., M.S.L.S., Technical Li- brarian CONTENTS Acknowledgments 70 Description of the Area and Ponds 71 Experimental Design and Method of Operation 73 Presentation of Data 75 Smallmouth Bass 75 Standing Crop Data 76 Effects of Cropping and Adding Stock 83 Growth and Condition 84 Mortalities 87 Spawning Success .90 Foods of Smallmouth Bass 92 Review and Discussion of Smallmouth Bass Data 96 Largemouth Bass 98 Standing Crop Data 98 Effects of Cropping on Fish Production 105 Growth and Condition 105 Mortalities 110 Spawning Success ^ 1 13 Review and Discussion of Largemouth Bass Data, and Comparison with Smallmouth Data 115 Yellow Perch 1 18 Standing Crop Data 118 Effects of Cropping on Fish Production 121 Growth and Condition 122 Mortalities 126 Spawning Success 1 28 Related Environmental Factors 1 29 Review of Perch Data, and Comparison with Largemouth and Small- mouth Data 130 Bluegills 131 Standing Crop Data 131 Effects of Cropping on Fish Production 134 Growth and Condition 134 Mortalities 137 Spawning Success 1 39 Review and Discussion of Bluegill Data 139 Brown Bullheads 140 Standing Crop Data 141 Growth and Condition 142 Mortalities 145 Spawning Success 145 Review and Discussion of Brown Bullhead Data... 145 White Crappies 146 Standing Crop Data 146 Growth and Condition..... 150 Mortalities 151 Spawning Success 151 Review and Discussion of White Crappie Data 151 General Discussion 152 Literature Cited 160 Index 163 This report is printed by authority of the State of Illinois, IRS Ch. 127, Par. 58.12. It is a contribution from the Section of Aquatic Biology of the Illinois Natural History Survey. I 1350S—4.000—3-70 ) Dynamics of One-Species Populations of Fislies in Ponds Subjected to Cropping and Additional Stocking D. Homer Buck Charles F. Thoits III THIS REPORT is based on several years of intensive studies of the produc- tion and related population dynamics of six kinds of warmwater fishes main- tained as single species in 1-acre ponds. Species involved included the large- mouth bass, Micropteriis salmoides (Lacepede); smallmouth bass, M. dolo- mieiii Lacepede; bluegill, Lepomis ma- crochiriis Rafinesque; yellow perch, Perca flavescens (Mitchill); brown buD- head, Ictahiriis uebulosus (LeSueur); and the white crappie, Pomo.xis annu- laris Rafinesque. Most production data published for these species have origi- nated primarily from studies involving complex, multispecies populations. Only Bennett & Childers (1957), Cooper et al. (1963), and Hipper (1964) are known to have made comprehensive studies of the dynamics of populations having only a single species. The principal aims of this investiga- tion were to 1 ) increase our knowledge of the carrying capacities of ponds for warmwater fishes, 2) consider the rela- tionship of carrying capacity to standing crop and to rate of production, and 3) measure the influence of controlled pop- ulation increases and decreases on fish production. It was assumed that physically simi- lar, contiguous ponds would have reas- onably similar characteristics and/or production potentials, and that differ- ences in production could be related to treatments administered. Thus, empha- sis was placed on the collection of fish population data, as opposed to intensive environmental analyses. An attempt was made in these inves- tigations to determine how nearly stand- ing crops approximated carrying capaci- ties. Although the data provided clear indications in some instances, we were dealing with a complex relationship that merits some preliminary examination. The evolution of the carrying capacity concept has been quite thoroughly treated by Edwards & Fowle (1955). Somewhat later, Krumholz et al. (1957) reviewed the status of wildlife terminol- ogy and proposed the following defini- tion of carrying capacity as one widely applicable to both fish and game biology: "the maximum number (or weight) of organisms of a given species and quality which can survive in a given ecosystem through the least favorable environmental conditions that occur within a stated interval of time." These writers further observed that in general the most appropriate "stated interval of time" is one year. By the terms of this definition, carrying capacity involves a minimum quantity which places limita- tions upon its practical application. If the interval of time is a year, the carry- ing capacity becomes the minimum population present during the annual cycle. Frequently, however, it is of more value to determine maximum rather than minimum rates of abun- dance. Only maximum rates, for exam- ple, permit the proper evaluation of the harvestable crop, or of the comparative production potentials of two species or combinations of species, or permit the proper evaluation of a remedial treat- ment in fisheries management. It would therefore seem desirable to measure carrying capacities at seasons other than when conditions for survival are poorest. A more useful concept might there- fore be that of Bennett (1962:59), who proposed the following definition of carrying capacity as one applicable for fishes: "the maximum poundage of a 69 70 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 given species or complex of species of fishes that a limited and specific aquatic habitat may support during a stated interval of time." In eliminating the restrictive clause "through the least fav- orable environmental conditions," Ben- nett effectively removed the requirement for a minimum quantity, and in this respect his definition resembles the "cur- rent carrying capacity" of Edwards & Fowle (1955). One then becomes free to evaluate carrying capacity at any season for which suitable data may be obtained. The question logically arises as to why standing crop may not in itself pro- vide a satisfactory measure of carrying capacity, or of potential rates of abun- dance. It may, of course, at times. It is widely recognized, however, that the two are not necessarily the same, and that standing crop might be lower than, equal to, or in excess of the carrying capacity (Krumholz 1948, Bennett 1962, and others) due to changes in habitat, food supply, or population structure. Carrying capacity is often conceived to be a relatively stable quantity, chang- ing little from year to year. Theoretic- ally, under comparatively stable condi- tions such as an extremely sterile lake or an extremely fertile, continuously fertilized pond, the carrying capacity could conceivably be quite stable. In the first instance one or more factors would be definitely and continuously deficient, or limiting, so that production would be at a continuously low level, as in a northern oligotrophic lake. In the second instance the essential nutrients would be supplied in a continuous abundance so that production might tend to remain near its potential maxi- mum. Such a condition might be found in a southeastern pond so fertilized that available nutrients were channelled di- rectly into the production of a continu- ous abundance of phytoplankton, zoo- plankton, and tertiary feeders with none diverted into the growing of higher plants. In both instances the flow of energy would tend to follow a reason- ably regular and direct path. Between these two extremes exist those inter- mediately productive, unfertilized waters where the factors influencing production may exist in an almost infinite variety of combinations and the flow of energy may be channelled through a great vari- ety of paths. The ponds of the present study typify such an intermediate category. References made to carrying capacity in this paper are intended to represent the current or temporary carrying capa- city which existed at the time of obser- vation and may not necessarily repre- sent the carrying capacity at the time when conditions for survival were poorest. The investigations were conducted at the former McGraw Hydrobiological Laboratory near Dundee, 111. Limited investigations were begun in 1956, but the majority of the data were collected in the years 1958-1963. The project was designed to continue for a minimum of 15 years as a cooperative program of the North American Wildlife Founda- tion, The Illinois Department of Con- servation, the Illinois Natural History Survey, and the McGraw Foundation. As a result of organizational changes within the McGraw Foundation, the program was prematurely terminated in November 1963. IACKNOWLEDGMENTS The investigations were centered in 16 1-acre ponds constructed especially for this study. Pond construction was financed in part and an office and labor- atory work space were provided by the North American Wildlife Foundation. Additional financial support for pond construction and other items of use in the program was provided by the Illinois Department of Conservation, and for the period 1956 through Octo- ber 1961, that agency assigned to the project a full-time biologist. From Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 71 October, 1961, partial financial support was received through the Dingell-John- son program, including salary and ex- penses for the assistant leader. Planning and administration of the program, financial support for the program leader, summer field assistance, and major equipment support was provided by the Illinois Natural History Survey through- out the course of the investigations. We gratefully acknowledge the con- tributions of M. A. Whitacre of the Illinois Department of Conservation during the early years that he was assigned to the project as a fisheries biologist. We also owe a special debt of gratitude to the late Sam A. Parr for his many valued contributions to the program in his former capacity as Ad- ministrative Assistant to the Director of the Illinois Department of Conservation. For assistance with summer field work, we are grateful to Robert M. Eichler and Warren U. Brigham. The program was conceived and de- signed by Dr. George W. Bennett, and we gratefully acknowledge his sugges- tions and encouragement throughout the investigations as well as his critical review of the manuscript. We are addi- tionally grateful to O. F. Glissendorf for his editing of the final manuscript, and to Richard M. Sheets for drawing the figures. DESCRIPTION OF THE AREA AND PONDS The study area, in Kane County, Illinois, is approximately 35 miles south of the Wisconsin state line, and 30 miles west of Lake Michigan. It lies within the Wisconsin glaciation, with a rolling topography formed by the irregular dep- osition of glacial material. Major re- lief occurs only along waterways. The land is fertile and intensively devoted to the culture of corn and soybeans. Physi- ography and characteristics of the soils have been presented by Hopkins et al. (1917), and the climate of the area is described by the Atlas of Illinois Re- sources (1958). The study ponds were located within a block of approximately 1,200 acres operated as a hunting and fishing club. This was an area of irregular topog- raphy, lying partly on high ground over- looking the Fox River to the west and dropping rather abruptly to the flood- plain from points as high as 170 feet above the river. Steeper slopes were heavily wooded with native hardwoods and cut by frequent natural drainage ways to the river. Within this area were two series of club fishing lakes at two distinct levels. These contained mixed populations of warmwater sport fishes and were the source of water for most of our experiments. The only excep- tions were three ponds used in our first series of smallmouth bass experiments which were filled by a partial diversion of an artificial spring-fed trout stream. The major system of 15 ponds was constructed in five units, each contain- ing three contiguous ponds. Because of differences in location, in types of bottom materials, and in age. or com- pletion dates, the ponds presented a rather wide range of characteristics. Table 1 presents the names, dates of completion, and various chemical and physical characteristics of the ponds in each of the five units, plus the single pond Alpha. All ponds were designed to have a surface area of 1 acre and a maximum depth of 8 feet, with average depths of between 3 and 5 feet. Pond banks were steep in slope (approximately 1:1) to minimize growth of shoreline plants. Grade of pond bottoms was established to provide 1 foot of rise in the first 50 feet from the drain, and 1 foot addi- tional rise for each 100 feet thereafter. Pond shapes varied from approximately round to an irregular L-shape, depend- ing upon topography, but the most com- mon shape was triangular— wide at the shallow end. and forming a narrow apex at the drain (Fig. 1). 72 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 »0 t^OOCD Trj4^CC ^hC; Or^CD CDCq^ TfOlO 00 ooQooo 000500 060606 060606 060606 00 5 CD [i 00 5 bS i o.^ s — -k^ n^ ^ (-? hn -3 n. SSciiJoJos ago t:3M.„ ecies Populations of Fishes 85 tional growth data are presented in Fig. 2 and 3 where growth curves are based on samples collected during the months indicated. Linear growth in our single-species populations was somewhat intermediate to growth made in other waters through the first 2 years of life, but in some cases was quite poor thereafter (Table 7). Extremely poor growth by the third, fourth, and fifth year fish was due to overcrowding (most notably in Unit 4), or to the scarcity of large food items such as small fish or crayfish. Fig. 2 presents growth and condition curves for the dominant 1958 brood through the 1960 season in ponds Beta, Gamma, and Delta. Near cessation of growth in ponds Gamma and Delta from mid-May to mid-June was paralleled by a deflection in condition curves. Con- dition of the cropped population made a sharp improvement in June and re- mained high for the remainder of the season, and this was paralleled by rapid and continuous growth through Septem- ber. Data from the control pond illus- trates an inconsistency that was rather common throughout this study. Here a high rate of growth was paralleled by a declining rate of condition. During this same period both growth and condition in the add-stock population remained low. The growth and condition curves showed only moderate conformity to the differing densities of stock. When stocked in October, 1959, each pond had received approximately 455 bass of this dominant age group (Age II in 1960). Cropping and transfers of stock during 1960 included 321 bass of Age II weighing 75.5 pounds, cropped from pond Delta, of which 175 weighing 41.4 pounds were marked and released in pond Gamma. Comparative densities at the time of the October, 1960 censuses were 406 in the control pond, 126 in the cropped pond, and 506 in the add- stock population (Table 6). With a APR MAY JUN JULY AUG SEP OCT I960 Fig. 2.—Growth and condition (C) of small- mouth bass of the dominant 1958 year-class in the control pond Beta (dots), add-stock pond Gamma (rectangles), and cropped pond Delta (circles) during the I960 growing season. final ratio of densities as great as 4:1 (506 to 126) between the add-stock and cropped populations, one would have expected a greater influence on growth and condition than seems to have oc- curred. Again, we suspect that the influ- ence of the treatment was masked by differences in the ponds" productivities, with pond Delta again indicated to have a low productivity. As mentioned earlier, growth by larg- er fish was extremely poor in the infer- tile, generally overcrowded ponds in Unit 4. This is emphasized in Table 8 which shows total length and weight in- crements of the dominant 1958 year- class over the period 1961-1963. Growth and condition curves of the 1958 year-class through 1960 were plotted in Fig. 2, and following the transfer of the population to the new unit of ponds we may again trace this dominant age group through its 3-year 86 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 Table 8. — Length and weight increments over three seasons, 1961-1963, and final average coefficients of condition for the dominant 1958 year-class of smallmouth bass in ponds Sigma, Tau, and Upsilon. Pond Average Length Average Weight Increment, Inches Increment, Pounds Final Average Condition (C) bigma Tau Upsilon Control Cropped Add-stoek 2.68 4.17 1.51 0.80 1.61 0.61 4.9 6.8 4.8 history in ponds Sigma, Tau, and Up- silon (Fig. 3). Again growth and condi- tion were computed from the same samples. While mid-season samples were in some cases quite small, and a source of some error, those collected at the beginning and end of each season were large and believed to be highly representative. Because of initial overstocking of all three ponds with approximately 130 pounds per acre, growth was negligible, conditions remained poor, and the influence of the experimental treatment was only barely discernible in 1961. All three popula- tions were heavily and equally thinned in early 1962, and the effects of subse- quent cropping and transfer of stocks became increasingly apparent through- out 1962 and 1963. The degree of conformity between the growth and condition curves indi- cates that for the 1958 year-class in Unit 4 ponds there was a rather close relationship between rate of growth and body condition, with fastest growth as- sociated with best condition, and vice versa. The most dramatic response to population manipulation occurred among the dominant 1958 year-class in pond Upsilon. Average condition for these fish declined from 4.5 in April, 1961, to 3.6 in the following Septem- ber, due to increased density of stock. 7.0 n 6.0 5.0 4.0 3.0 U) Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 87 A combination of overwinter mortality, and additional thinning by us in May, 1962, caused condition of this popula- tion to improve from 3.7 in May, 1962, to 5 by the following June. Mortalities Mortality data for the populations of smallmouth in ponds Beta, Gamma, and Delta were for three categories of fishes: 1) those of the original stock, 2) those transferred from the cropped to the add- stock pond, and 3) those fish captured, measured, weighed, marked, and re- leased when and where caught. Mortali- ties over the 1958-1959 period for bass of the three year-classes transferred from Gamma to Beta in 1959 ranged from 50 percent for the 1956 year class, to 100 percent for the 1958 year class, averaging about 70 percent; whereas mortalities over a single season for those marked and transferred from Gamma to Beta in 1959 ranged from 38.1 to 75 percent, averaging about 51 percent. Among 294 fish of the 1958 brood trans- ferred from Gamma to Beta during 1959, the mortality by the end of that season was 38.1 percent, while for 144 fish of the same age caught in Beta, marked, and returned to Beta, the com- parable mortality was 34 percent. For older fish of the 1957 brood, com- parable figures were 40 and 36.7 per- cent, in almost the same ratio. In this instance the transferred fish survived quite well in the new population, and the act of transfer was in itself no cause for a notable increase in mortality. In 1960, however, we found the mortality among 175 fish transferred to pond Gamma was 25.1 percent, as compared to 17 percent among the 168 fish which were caught in Gamma, marked, and returned to the same population in that pond. As in the 1959 data, mortalities of the younger and older fish in 1960 generally were higher than those of in- termediate ages. Finally, average rates of mortality were greater in the add- stock pond, intermediate in the control pond, and lowest in the cropped pond. Mortalities as related to densities of stock are best known for the 1958 year- class of bass during their third year of life in the 1960 season. When restocked in October, 1959. each pond received from 449 to 455 fish of this age class. Recoveries, numbers unaccounted for in the census, and percents of mortali- ties by October, 1960, for various cate- gories of these bass are shown in Table 9. Following the removal of 321 indi- viduals from the cropped population, only 8 fish were unaccounted for in the October census, indicating a mortality of 6 percent among that portion of the original population that was not cropped. Comparable figures were 7.9 percent in the control population, and 13.2 percent among the original stock in the add- stock population. Among those fish Table 9. — Numbers of the dominant 1958 year-class of smallmouth bass originally stocked in ponds Beta, Gamma, and Delta in 1959, numbers cropped and added in I960, and numbers of the different categories recovered in the draining censuses, with numbers of fish unaccounted for, and percents of mortality. Number Number of of Number Number Number Number Original Transferred Unaccounted for Percent Mortality Originally Cropped Transferred Stock Fish Stocked, from to Recovered Recovered Oct., 1959 Pond Pond in Census, j'n Census, Original Transfer- Original Transfer- in / 960 in 1960 Oct., 1960 Oct., 1960 Stock red Fish Stock red Fish Beta Illinois Natural History Survey Bulletin Vol. 30, Art. 2 transferred to the add-stock population, mortalities were 25.1 percent, approxi- mately twice as great as among surviv- ors of original stock in this same pond. It should be pointed out here that neither the marks used, nor the handling of fish involved in the transfer of stock, were in themselves causes for increased mortalities. The transfers were accom- plished in conjunction with our regular sampling operations, and frequently more fish were caught, marked, and returned to the control pond than were marked and transferred from the cropped to the add-stock population. The transferred fish were simply carried a few feet farther, in water, before their release. Thus, where mortalities were greater among fish that were marked and transferred than among resident in- dividuals that were marked and returned to the same pond, those mortalities must be attributed to something other than the purely physical effects of the mark- ing and transfer. In this instance it was probably an inability to compete on even terms with those fish already resi- dent in the overcrowded population. The inability of transferred fish to compete effectively with those resident in an overcrowded population also was evident in a study made of rates of seasonal mortalities of smallmouth bass in 1960. Rates of mortalities were based on recoveries in the draining censuses of fish given distinctive fin clips for each month of the 1960 growing season (Table 10). Marked fish were princi- pally of the 1958 brood. Errors due to incomplete recoveries are believed to have been small, and such errors would operate in favor of lower mortalities. Mortalities due to marking were erratic, and high for some marking periods, as is clear from a study of Table 10. For example, the returns of fish marked in April implied that percentages of from 26.7 (in Beta) to 30 (in Gamma) of all fish of the group in question that were living at the time of marking had died by October. Actually, total rates — fp 3 -i E-, g. ^^ s a. « I So ^ a So 2rt d.^ So ^ a ^Pi CO(M Mi-i (N .-IN OOOOS o -H 00 r^ c^ ,-( Tt^ CD COt^ ^ OiO O -H Ol o (N CO CI t-H O -t^ J CO CO (M oo o ^ oo CO CO CO CO CO CO Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 89 of mortality for the year-class in ques- tion (1958) over the entire 12-month period were only 9.6 percent in Beta, and 17 percent in Gamma. Marks used were clearly distinguishable at the time of the draining censuses, and there seemed to be small margin for error in their interpretation. These data indi- cated that such studies must be con- ducted with great care, and should in- volve large samples. In the present instance the "unnatural" death of as few as four or five fish projected serious errors. Mortalities were not proportional to length of time marked, due possibly to greater deaths among marked fish in certain months than others. However, if such errors (deaths) may be assumed to have been reasonably proportional for each population, a significant and logical pattern appears when rates of mortality for fish marked each month are averaged for each category over the season (see averages in Table 10). Average rate of mortality was lowest in Beta, the control pond, intermediate for those marked and returned in the "add- stock" pond, and highest for those transferred from Delta to the add-stock pond. Our principal data on mortalities in the ponds in Unit 4 are from fish of the dominant 1958 brood in the control and add-stock populations. As observed for our earlier smallmouth populations (Table 10), mortalities were lowest in the control pond and highest in the add- stock pond, and seemingly related to density of stock. For example, in the control pond Sigma, with an original stock of 343 Age III fish on October, 1960, the year-class had an unaccounted for loss of only 5 individuals by Octo- ber, 1961, and an additional loss of only 29 individuals by October, 1963. In this instance, 105 fish had been removed in 1961 and five were unaccounted for, leaving 233 survivors in the 1961 fall census. Assuming removals and deaths had been nonselective as to size, over the 12-month period the 233 survivors had made an average length increment of only 0.24 inches, and had shown an average loss in weight of 0.012 pounds and a loss in condition of from 4.5 to 4. Living conditions were therefore quite poor, but not so poor as to have induced mortality. Such low rates of mortality in pond Sigma in 1961 were verified by the fol- lowing experiment. For each month from April through September, from 28 to 30 individuals of the 1958 brood were caught and released with a mark distinctive for that month. When cen- sused following pond draining in Octo- ber, 1961, recoveries were made of 28 of the 30 fish marked in April, 28 of the 30 marked in May, and of 100 per- cent of fish marked in each of the four remaining months. Of the two fish missing from each of the April and May samples, one for each month, or two of the total of four, were observed, accidental deaths. There were therefore only two unaccounted for individuals among the 1 76 fish marked, and mortali- ties for this group were less than 2 percent. In the add-stock pond Upsilon, with its steadily increasing density of stock, mortalities became quite high. Assum- ing no mortalities, we would have en- tered the winter of 1961-1962 with 325 individuals of the 1958 year-class. Our 1962 spring estimate indicated a popu- lation of only 180, and a mortality to that date of 184 individuals. Average condition had declined through 1961 from 4.5 in April to 3.6 in September, so large overwinter mortality was not unexpected. Our next inventory by Pet- ersen estimate in April, 1963, indicated that an additional 78 fish of this group were missing and unaccounted for. While both estimates could have been in error, the sum of estimated mortali- ties from both was verified by the 1963 final census. The two estimates had in- dicated 81 survivors by April-May, 1963, and a transfer from Tau raised 90 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 the total to 82. Thus, a recovery of 83 individuals in the final census indicated that the estimates had been exceptionally good. While mortalities were high in the add-stock pond, this population was subjected to considerable stress by vir- tue of the greater population density and a limited food supply. There may have been an additional stress owing purely to the transfer and adjustment to a new environment. Quite possibly the impact of the transferred fish may have created stress for the resident fish as well. On the other hand, mortalities in the con- trol pond were extremely light. Results from the control pond indicated the adult smallmouth bass to be unusually hardy, even under the relatively poor conditions afforded by pond Sigma, and even when subjected to frequent samp- ling by seine, shocker, or hook-and-line, and to multiple marking by fin clip. In this respect, we experienced the small- mouth bass to be a much tougher and superior experimental fish than the largemouth. Spawning Success In 1960 an effort was made to study success of spawning and survival of young in relation to the differing densi- ties of populations in the three ponds in Unit 1 . Table 1 1 presents 1 ) esti- mates made of the number of potential spawners in each pond, 2) counts made of total numbers of nests and numbers of successful nests, 3) percentages of successful nests, 4) ratios of numbers of total nests and of successful nests to numbers of spawning adults, and 5) estimated numbers of fry produced and their rate of mortality by the time of the October draining censuses. The potential number of spawning adults was taken as the number of fish of Age II (1958 brood) or older stocked in each pond the previous November, plus or minus those added to or removed from each pond prior to the advent of E.o .1 s: fe; '*'(« '^I'^fSc i^ii "fe; o ia; •11" CO oS SJ moo Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 91 spawning in late May, 1960. The as- sumption of no overwinter mortality is undoubtedly erroneous; however, deaths through the winter were known not to have been large. Counts of nests, and of successful hatches of fry were made by dividing each pond into small sections and making daily counts of all sections throughout the spawning period. Esti- mated production of fry was computed from an average number of fry per nest based on actual counts of fry taken from the nests. The black but still largely immobile sac-fry were siphoned out of the nests and spread over the bottom of a large white enameled tray having a grid marked in the bottom. Pictures were taken and later enlarged so that counts could be made in the laboratory. The fry were sucked off the nest by means of a %-inch tube at- tached to the bottom of a 5-gallon bucket. Slow submersion of the bucket from above created a gentle suction by which the fry could be moved from the nest into the bucket. When fry were re- turned to the nests the guarding males resumed their patrols, and all such spawns were observed to develop in a normal fashion. Successful counts were made of broods removed from eight nests, six from pond Beta, two from pond Delta. The overall range in num- bers of fry per brood was from 190 to 787. The six broods from pond Beta averaged 542 fry per brood; the two from pond Delta averaged 568. The average of the eight nests was 548.25. and this figure was used as the basis for computing the total productions of fry listed in Table 1 1 . It may be significant that pond Gam- ma with both the largest number of potential spawners and the largest num- ber of nests, had by far the lowest rate of spawning success (Table 11). This would appear to be a density-related factor whereby overcrowding brought about a reduced reproduction. Ricker (1954) presented reproduction curves exhibiting this phenomenon in a variety of animal populations, and Rose (1959) presented evidence of reduced repro- duction as a result of overcrowding guppies. However, the relation of spawning success to density of the spawning population was not consistent in data from ponds Beta and Delta. Pond Beta had an intermediate number of spawners and the smallest number of nests, but had the greatest number of successful nests, whereas Delta with the smallest number of potential spawners had an intermediate number of nests and an intermediate number of success- ful nests. Though density related, rate of spawning success in the present data appeared related to additional factors as well. One such factor may have been the relative locations of the ponds, and the manner in which they received water. As mentioned earlier, the supply stream first entered Beta, from which it overflowed to fill Gamma, from which it again overflowed to fill Delta. It may be significant that pond Beta, the first pond in the series, is believed to have had the most successful rate of reproduction and/or survival in aU 3 years, 1958- 1960. Reproduction in Delta, the last in the series, was extremely limited in 1958, and believed to be nonexistent in 1959, and in both years was less than that achieved in pond Gamma, the middle pond in the series. Thus, in both 1958 and 1959 the rate of spawning success decreased in the exact order in which the ponds were located in the flow series, from Beta to Gamma to Delta. The order was partially reversed in 1960, with a greater production of fry in Delta than in Gamma. In this instance, how- ever, the much greater density of breed- ing stock in Gamma than in Delta (Table 1 1 ) may have had an overriding influence. The significance of the position of the ponds in the flow series could be related to an accumulation of an inhibitory product. Rose (op.cit.) demonstrated in aquaria that some fishes release waterborne products that limit growth 92 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 and survival of their own or closely re- lated species, though not limiting that of certain other distantly related forms. Rose's studies also suggested that self- inhibiting products produced by one species may be metabolized away by other species in the community. Thus, a species maintained alone might be more subject to such an inhibitory influence than when in a mixed population. Al- though the ponds contained no fish when filled, they received additions of "make-up" water throughout the course of the experiments. Thus, an accumula- tion of waterborne products would have been progressively greater in Gamma and Delta, and if such an inhibitory in- fluence was operative it would have de- creased production and survival of young in the order in which it occurred. Total counts of nests and of successful nests were not possible in the Unit 4 ponds because of higher water turbidi- ties. However, in the 1961 spawning season a reliable count was believed made of broods of fry just risen from the nests. Each such brood was marked with a float at the point first seen, which in most cases was very near to a nest. The totals of floats in each pond is be- lieved to have closely approximated the totals of broods. Table 12 presents the number of broods seen, the time inter- vals over which they were seen, the number of potential breeders believed to have been present, and computed ratios of numbers of broods to number of po- tential spawners. As earlier observed in Unit 1 (Table 11), the fewest num- ber of hatches was associated with the greatest density of breeding stock. Pond Upsilon with an estimated 348 breeders (1958 year-class and older), produced only 7 hatches, for a ratio of about 1 :50. Comparable ratios were 1:14 in Sigma, with an estimated 292 breeders, and about 1:17 in pond Tau with approxi- mately 237 potential spawners. Brood counts were also made in 1962 and 1963, but poorer visibilities, due principally to frequent strong winds, probably made these counts less com- plete than those of 1961, and they were not included in the table. In 1962 our notes listed only five hatches observed in the add-stock pond, as compared to nine in the control, Sigma, and either "eight or nine" in the cropped pond Tau. No hatches were observed in pond Tau in 1963, due, possibly, to the ab- sence of a breeding pair because of overcropping, or to predation of eggs or larvae by an abnormally large popula- tion of crayfish. Foods of Smallmoufh Bass Information on foods eaten in ponds Beta, Gamma, and Delta is based on examination of 20 stomachs each from ponds Beta and Gamma and 18 from pond Delta, all of Age II, collected in June and August, 1960 (Table 13). Based on frequency of occurrence in stomachs, scuds and mayfly nymphs ranked first and second, respectively, in both June and August. Cladocerans and crawling water beetle larvae were also prominent in June, but much less so in August. Nymphs of damselflies were Table 12. — Estimated numbers oi potential smallmouth bass spawners, numbers of broods seen, time intervals over which they were seen, and ratios of nunnbers of broods to numbers of potential spawners in ponds Sigma, Tau, and Upsilon, 1961. Number of Potential Spawners Number of Broods When Broods Were Seen Ratio of Broods to Potential Spawners Sigma Tau Upsilon 292 237 348 June 1-16 June 6-9 June 6-16 1:14 1:17 1:50 Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 93 Table 13. — Food orqanisms found in stomachs of the dominant 1958 year-class of smallmouth bass collected from ponds Beta (control). Gamma (add-stock). and Delta (cropped) in June and August, i960. Length ranges of the fish, in inches, were 7.8-9.8 In Beta, 7.4-9.3 in Gamma, and 7.2-9.7 in Delta. 94 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 they were forced to prey on fish larger than their gullets could accommodate. In 1960 an attempt was made to measure the standing crops of inverte- brates in ponds Beta, Gamma, and Delta by collecting them in a large filter box of saran screen as described by Buck & Whitacre (1960). As the ponds were drained the water was screened through the box and the or- ganisms collected and weighed. The only separation possible was that be- tween "plankton" and scuds. The "plankton" consisted in the main of cladocerans, a moderate number of phantom larvae, and occasional cope- pods, water mites, small beetles, various nonburrowing mayfly nymphs, and scuds. This group was collected at a fairly continuous rate throughout the draining, except that the scuds drained out at the end of the operation, or con- centrated in and around the sump at the end of the drain tube inside the pond. The technique worked reason- ably well except for the fact that an unknown percentage of such forms as scuds, and the immature stages of cad- dis flies and mayflies, remained in the abundant dewatered vegetation in Beta and Gamma, while a greater percentage flushed out of the much less densely vegetated Delta. Recoveries of scuds and other larger forms could not be considered complete or comparable for all ponds, but that of "plankton" may have been reasonably so. Weights obtained were as follows: Beta "Plankton" 37 lbs. Scuds 20 lbs. Gamma "Plankton" 142 lbs. Scuds 20 lbs. Delta "Plankton" 670 lbs. Scuds 13 libs. Differences in standing crops suggest a greater productivity for pond Delta, a fact not substantiated by the fish data. The vegetation factor, as mentioned above, would have accounted for a part of the differential, and may have ac- counted for it all. It should also be pointed out that standing crops of smallmouth were at least three times as great in both ponds Beta and Gamma as in Delta. Heavier rates of cropping of the "plankton" and scuds by the larger populations of fish could have caused the difference in standing crops of the invertebrates. With heavier rates of cropping, rates of production of in- vertebrates actually may have been greatest in the ponds having the smallest standing crops of invertebrates. Information on foods eaten by small- mouth in Unit 4 ponds is based on a collection of 119 stomachs, 40 each from ponds Tau and Upsilon and 39 from pond Sigma. About 10 specimens were collected from each pond in each of the four months June through Sep- tember. All samples were collected by hook-and-line, and the monthly samples from each of the three ponds were col- lected on approximately the same dates— June 16-20, July 24-25, August 22, and September 19. All specimens were adults of the dominant 1958 brood and ranged in length from 7.7 to 10.2 inches total length in June and from 8 to 10.8 inches in September. A total of 105, or approximately 88 percent, of the 119 stomachs contained food; 14 were empty. Determinations were made of the number of items of each identifiable taxonomic group, their total volume in each stomach, their fre- quency of occurrence, and the average volume of food in stomachs containing food. A total of 23 items were identi- fied in the stomachs (Table 14), but many were occasional or rare, and only six items were encountered with suffi- cient frequency, or in sufficient volume, as to be considered prominent. On the basis of frequency of occurrence the six most common items were 1 ) midge lar- vae and pupae in 46 stomachs, 2) bur- rowing mayfly nymphs in 42, 3) nymphs of smaller, nonburrowing mayflies in 39, 4) caddis fly cases in 24, 5) damselfly nymphs in 18, and 6) crayfish in 14. By Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 95 Table 14. — Food organisms found in stomachs of the dominant 1958 year-class of smallmouth bass collected from ponds Sigma (control), Tau (cropped), and Upsilon (add-stock) in June, July, August, and September, 1961. Length ranges of the fish, in inches, were 8.5-10.9 in Sigma 8 3-10 8 in Tau, and 7.7-10.4 in Upsilon. 96 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 July, and in one each in August and September. Wiiile young smallmouth bass were preyed upon when available, and their numbers frequently decimated, their remains were found in only two stomachs (one each in July and August), and on this basis they must be consid- ered an item of minor importance. Scuds were not present in these ponds in 1961, but were undoubtedly a major food item following their establishment in 1963. While a large proportion (88 per- cent) of the stomachs contained food, average volumes were small. In the control pond Sigma, 37 of 39 stomachs contained food, with an average volume for the 37 of only 0.49 milliliters. In the cropped pond Tau, 39 of 40 stomachs contained food, with an average volume of 0.39 milliliters. In the add-stock pond Upsilon, only 29 of 40 contained food, with an average volume of 0.92 milliliters. However, the picture of com- parative volumes was distorted by two occurrences of comparatively large vol- umes of crayfish. If we remove crayfish volumes from the calculations, we then have averages of 0.25 in the control pond, 0.34 in the cropped pond, and only 0.12 in the overstocked pond. Though possibly fortuitous, these aver- ages conform with the expected, where volume of food in stomachs was greatest in the cropped pond having the fewest fish, and smallest in the add-stock pond having a more dense population. Review and Discussion of Smalimoul-h Bass Data A survey of the literature revealed records of only five standing crops of smallmouth bass when maintained as a single species (Table 15). Maximum previous poundages were recorded by Regier (1962) from mark and recapture estimates made in a 0.09-acre pond in New York. These were at rates of 143, 146, and 139 pounds per acre over 3 consecutive years. The author observed that successful reproduction occurred in each of the 3 years, in spite of what he thought were high standing crops. Our three largest standing crops ranged from 169 to 180 pounds per acre (Unit 1 ponds), and our populations also re- produced successfully in all 3 years of census. While numbers of young-of-the- year surviving until fall in our three populations were in some cases small (188, 889, and 23), it should also be observed that none of our three popula- tions was cropped to any substantial degree (maximum in any year: 8.4 pounds). Cropping of adult fish would be expected to enhance both reproduc- tive success and survival of young, par- ticularly when the populations were dominated by older and larger fish, as was true in both Regier's populations and ours. Bennett and Childers (1957) record- ed the next highest standing crop of 100.3 pounds (not including Age O) in a 1.4-acre gravel pit pond of "below medium" fertility in central Ilhnois. In the four years preceding the final census of this pond in June, 1955, it had pro- duced successive hook-and-line yields ranging from 78 to 123 pounds per acre. The catches of 119 and 123 pounds per acre, made in 1952 and 1953 respectively, both exceeded the standing crop measured in 1955, and the authors believed this indicated that there was an annual replacement of fish flesh in this pond which exceeded the standing crop that the pond could sup- port. Our smallmouth populations produced no tangible evidence of a replacement potential of this species comparable to that provided by Bennett & Childers. We believe that this failure was due in part to the relative immaturity of our populations, to severe overcropping in at least one instance, and partly to chance. Our first cropped population (in Gamma, 1958-1959) had been initi- ated in May, 1958, with only 10 adults and 40 fingerlings, and in 1959 was so heavily cropped that too few fish re- mained to replace the flesh of those Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 97 Table 15. — Known published records of standing crops per acre of single species fish popuh tions in small ponds. Location 98 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 LARGEMOUTH BASS Bass used in these studies were ob- tained mostly from private ponds, partly from a state hatchery, and partly from a public reservoir, all located within the northern half of the state. Data presented here were obtained over two periods of approximately one and one-half growing seasons each ( 1958-1960) in Unit 2 ponds, and from a 3-year period (1961-1963) in Unit 1 ponds. Ponds Zeta, Theta, and Iota (Unit 2) were completed and first filled with water in the spring of 1958. Each pond was stocked with 10 adults 10-13 inches long in May, and with a total of 61 subadults over the period June through August, for a total in each pond of about 16 pounds. In this series, pond Zeta was the control, pond Theta was cropped, and pond Iota received addi- tions of stock. In July, 1959, largemouth bass in all Unit 2 ponds were found to be severely infested by a gill parasite, Dactylogyrus sp. Since it would be extremely difficult to measure or evaluate the extent of deaths due to the parasites, it was de- cided to census the ponds before mortal- ities became a serious factor. On July 27, all three ponds were treated with rotenone. Based on returns of marked fish, the recoveries were judged to be comparable and reasonably complete in all ponds. Following drainage and spraying of residual pools with a solution of HTH (available chlorine 15 percent), the ponds were refilled and restocked by August 25. Each pond was stocked with 514 young-of-the-year largemouths, plus an assortment of yearlings and adults to a total of approximately 30 pounds. The ponds were next drained in October, 1960. The 1960 drainage censuses revealed the only notable contamination by un- wanted species that occurred throughout these investigations. Pond Zeta con- tained three adult bluegills and an esti- mated 15-20 pounds of young-of-the- year bluegills. Pond Theta contained a single white crappie. Pond Iota con- j tained two adult bluegills and between j one and two pounds of young bluegills. ; The impact of these contaminants is be- ' lieved to have been small. Other things being equal, the most likely influence would probably have been an increase in condition of bass in proportion to the degree of contamination. There was, in fact, an inverse correlation. Pond Zeta, with the greatest contamination (15-20 pounds) contained bass with the poorest average condition (3.8) in the three ponds, and pond Theta, with only the single adult white crappie con- taminant had the best-conditioned bass (4.5). There are, however, additional considerations. Cropping from Theta would be expected to cause the remain- ing bass to have a higher rate of condi- tion than bass in the uncropped ponds, which it seems to have done. At the least, then, we may say that the influ- ence of the contaminants was not such as to have masked the effects to be expected from cropping. In the remain- ing analyses we have assumed the influ- ence of the contaminating fishes to have been negligible though this may not have been strictly true. Following the draining censuses of the Unit 2 ponds in October, 1960, the bass were regrouped by size and age and stocked in Unit 1, with supplemen- tal stock added from other local sources. The ponds in Unit 1 had just been re- filled following the draining censuses of smallmouth populations previously de- scribed. Standing Crop Data Table 16 presents standing crops in pounds per acre as measured by drain- ing censuses of 1 1 populations of large- mouth bass. The data are arranged according to experimental treatments given the populations, with an average standing crop presented for those ponds receiving identical treatments. Table 17 presents the same standing crop data Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 99 Table 16. — Standing crops in pounds per acre of largemouth bass in ponds receiving different experimental treatments. All censuses were by pond drainage on the dates or seasons indicated. Unit 100 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 -D-a- -So* :_o .- o .E ( 3 °4? .•D-D O s o § i : l+o I + Oi Oi 05 o o o lO W3 iC CO CO CD N H hH tS3 H HH oo I + 1—I CO CC CO CD CO CO CO Oi o o^ c^ moiOQ Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 103 tion was significantly high in the cropped ponds, intermediate in the control, and lowest in the add-stock ponds. It also seems significant that in both instances surplus production in the cropped pond exceeded final standing crop in the con- trol pond. The consistent similarities suggest that the eflFects of the experi- mental procedures were valid and reas- onably free from additional unrecog- nized influences. If true, we then had a valid expression of the relationship of total production to rate of cropping or exploitation wherein cropping stimu- lated growth, reproduction, and/or sur- vival in the residual population with the consequence of an increased total pro- duction. In 1958-1959, surplus produc- tion (pounds gained) in the cropped pond was approximately 3.8 times that of the control pond; and in 1959-1960, it was about 2.1 times that of the con- trol pond. It is interesting to speculate as to whether the standing crops might have been the same in pond Theta with or without cropping. It may be that we were achieving an optimum rate of turn- over for this population, and that the additional production was a "bonus" stimulated by an optimum cropping rate. If we assume final standing crops to have approximated carrying capacities, as interpreted here, carrying capacity appears to have approximately doubled in the period from July 1959 to October 1960. While parasitism probably de- pressed growth to some degree in 1959, the greater production of fish in 1960 was believed due primarily to an increase in available food, principally crayfish. In our second series, ponds Beta, Gamma, and Delta (Unit 1) were inven- toried by both drainage and Petersen estimation procedures. As shown in Table 18, the control pond Beta was inventoried by a draining census at the end of the first growing season, by Pet- ersen estimate at the end of the second, and by draining census at the end of the third. Our intimate knowledge of this population, based on frequent sampling, indicated that the 1962 estimate was within acceptable limits. If we accept this estimate we find that the standing crops were almost identical at near 125 pounds per acre in October of both 1961 and 1962, but dropped rather un- accountably to 87.4 pounds in October, 1963. The surplus productions in 1961 and 1962 were 40.8 and 3.7 pounds, respectively, being either moderately high or low as necessary to attain the presumed maximum. That these gains were not larger probably indicates that the standing crops in 1961 and 1962 approximated the carrying capacities of the pond at the times of census. These standing crops greatly exceeded those for largemouth bass measured in Unit 2 (Table 17), but were considerably be- low those of 170-180 pounds of small- mouth bass recovered from this pond (Beta) in earlier years. Following the 1961 draining census, pond Beta was restocked with 105.7 pounds of bass, as shown in Table 18. The loss (pounds lost) of 36.7 pounds by October 1963 indicates that 1) the carrying capacity of pond Beta de- clined significantly during its third seas- on, or 2) that the 1963 population was less efficient than those of 1961 or 1962. Decrease in standing crop im- plies a loss of efficiency by the popula- tion, or a decrease in available food. While no quantitative comparisons can be made, there was no evidence of a decline in available food. On the other hand, differences in population struc- ture in the 3 years suggested possible differences in efficiencies of conversion of available food. From 1961 through 1963, progressively higher percentages of total weights were to be found in the larger sizes of bass. For example, in October 1961, 56 percent of the popu- lation (by weight) was made up of fish more than 8.5 inches long (Age II and older), while by October 1963, nearly 90 percent of the total weight of the 104 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 population was represented by fish long- er than 8.5 inches, most of which were now Age III or older. While an abun- dance of food may have been available for small bass in 1963, it possibly went largely unutilized because of the relative scarcity of small bass. With a more optimum size distribution of fish, the standing crop of pond Beta probably would have been higher. The obvious trend was a progressively greater deple- tion of small bass through predation by a progressively increasing proportion of larger bass. Had the larger bass been cropped by fishing the population would have assumed a more optimum compo- sition and total production undoubtedly would have been higher. As previously observed, largemouth bass of larger sizes (in this case 8.5 inches and above) seem to lack the ability of smallmouth of similar sizes to forage for inverte- brate foods, and are more dependent upon fish or other large food items. For the cropped pond Gamma we have only the single final inventory of this population plus records of original stock and pounds cropped over the 3- year period. The total of about 148.8 pounds in the "pounds removed" col- umn (Table 17) includes 31.04 pounds cropped in 1961, 81.34 pounds in 1962, and 36.40 pounds in 1963. In retro- spect, it now appears that the removal of 81.34 pounds in 1962, followed by a cropping of 36.40 pounds in 1963, may have been excessive. While it caused accelerated growth and improved condi- tion among the survivors, it left too few individuals of larger sizes to permit re- placement of the pounds removed. This seems evident because the final stand- ing crop was low as compared with those of companion ponds, and was comprised of 61 percent Age O fish by weight, and only 50 individuals older than Age O. The final standing crop included only one fish in the Age I group, and only six fish in the 6.5-9.5- inch length range. We thus created quite the opposite condition of that found in the uncropped pond Beta. In that first instance we had an excessive number of slow-growing, poorly conditioned older fish (243 Age I or older) comprising about 97 percent of the population by weight, as compared with too few, faster-growing older fish (50 Age I or older) comprising about 39 percent of the population weight. In each instance, production was limited, and final stand- ing crop was less than the carrying capacity of the pond due to an imbalance in population structure. The add-stock pond Delta was inven- toried by a draining census in the spring of 1962, by Petersen estimates in the fall of 1962 and the spring of 1963, and again by draining census in the fall of 1963 (Table 18). There is a reasonable probability that the estimates are usable because 1 ) both mark-and-recapture samples were large (probably equalling or exceeding 50 percent of the total population), 2) the estimates were cred- ible' when compared with known num- bers stocked and numbers later recov- ered by draining census, and 3) esti- mates of standing crops made in the spring and fall agreed closely with those obtained by actual measurements (drain- ing censuses) at the same seasons in other years. On the basis of experiments conducted to evaluate the estimates (Buck & Thoits 1965), such errors as existed probably would have been nega- tive. If we are to accept these estimates, , certain inferences may be drawn: (1) Standing crops in all inventories of pond Delta at least equalled and ' probably exceeded carrying capacities. This seems evident from the fact that all quantities in the pounds gained or lost columns are negative (Table 18), indicating that the additions of stock had overtaxed available resources. (2) Overwinter mortalities were large and fall carrying capacities ex- ceeded those for spring. This seems apparent because standing crops inven- toried in the fall exceeded those obtained Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 105 in the spring by approximately 44 pounds in the first instance and approxi- mately 32 pounds in the second. Since pounds were lost in all instances, we believe that the carrying capacities at times of censuses were probably close to, but possibly somewhat below, the figures for standing crops. As in Unit 2, surplus production was highest in the cropped pond, intermediate in the con- trol pond, and lowest in the add-stock pond. Effects of Cropping on Fish Production The influence of cropping on fish production is best illustrated by refer- ring again to Table 17 which features the figures on "pounds lost or gained." The data from ponds in Unit 2 (Zeta, Theta, and Iota) are most rewarding. The significant points in the data are that 1 ) in both periods of observation, standing crops of largemouths in the control and cropped ponds were similar, 2) in both periods surplus production in each cropped population equalled or exceeded the final standing crop in both cropped and control ponds, and 3) sur- plus production in the cropped pond was 3.8 times that of the control pond in 1958-1959 and 2.1 times that of the control pond in 1959-1960. The com- bination of these data indicate that we here approached the optimum rate of cropping for these populations and that the replacement of fish flesh during each period was approximately the same as the ponds could support at any one time ( 100 percent). Data from the Unit 1 ponds were less precise. The fact that the final "pounds gained" figure of about 138 pounds made in the cropped pond was so much larger than the amounts of the com- panion ponds indicated that cropping did indeed stimulate production. This, however, was an accumulated gain over three seasons, which translates to an average gain of approximately 46 pounds for each of the 3 years. We may note that the final standing crop of 84.2 pounds in this cropped pond was lower than that for either companion pond, and was comprised of a larger number (10,601) of young-of-the-year with only one fish of Age I and 49 that were of Age II or older (Table 19). The total of about 148.8 pounds cropped from this pond (31.04 pounds in 1961, 81.34 pounds in 1962, and 36.40 pounds in 1963) may have represented overcrop- ping, particularly among fish of larger sizes. A more optimum distribution of sizes in this population— specifically, fewer fish of Age O and more of Age I —would undoubtedly have increased its potential for replacement of the fish flesh that was removed. Growth and Condition Table 20 presents growth data for fish of known age in the final censuses of 10 of our populations of largemouth bass along with growths made in other waters. Precise growth data for large- mouth in ponds Zeta, Theta, and Iota were limited because of the variety of origins, sizes, and ages in the original stock, and because small confidence was placed in the interpretation of scale samples collected. The data in Table 20 are, therefore, limited to first-year growths made in these ponds. For other purposes, however, we used length fre- quency distributions to make the less precise separations presented in Table 21, and these provide limited additional information on growth in these waters. Growth rates listed for 1959 were those which were terminated at the time of the premature census on July 29. First-year growths over the full 1960 season ranged from 4.3 inches in Theta to 4.8 inches in Zeta, and these were quite intermediate to first-year growths made in other waters with which they are compared (Table 20). The 1959 section of Table 21 shows numbers of fish within four different size ranges in the populations censused in July, 1959, the average condition for each size grouping, and a designation 106 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 Table 20. — Growth of largemou+h bass in ponds used for carrying capacity experiments on the McGraw Foundation grounds, along with sinnilar data on largemouths in several other waters. Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 107 3.8) for 166 fish in the control pond, Zeta, and the high averaging about 5.2 for 9 fish of the oldest class in the cropped pond Theta. The tendency for increasing condition with increase in size of bass was present in these popula- tions due in part to a change in shape as the bass become larger and in part to a greater abundance of food (notably crayfish) for larger fish in 1960 over that of 1959. The correlation between strength of year-class and average condition in the respective ponds was also less apparent in the 1960 census. As an example, 166 fish of the "mostly Age I" category in pond Zeta had an average condition of 3.8, as compared to 4.2 for 233 fish of the same class in the add-stock pond. We may note, however, that average condition of all three of the oldest year- classes in 1960 ranked higher in the cropped than in either companion pond, due probably to their lesser abundance as a result of cropping. Transfer of better-conditioned fish from the cropped to the add-stock population could have contributed to the higher condition of certain age classes in pond Iota than in the control pond, but we believe the differences were due primarily to a slightly greater fertility of the add-stock pond. Growth data of two types are also presented for largemouth from ponds in Unit 1 (Beta, Gamma, and Delta). Data presented for comparison with growths made in other waters (Table 20) are restricted to fish of known age, and represent an average of all fish in the final draining censuses for which ages were known. Additional growth data are presented in Fig. 4 where growth curves are based on samples collected during the months indicated. As is evident in Table 20, growths in these single-species populations were slower than in most mixed populations with which they are compared, even in some instances when growth was stim- ulated by relatively heavy cropping. ijei^S £ « Q S -9 1 ^O" M 05 -X> 108 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 6.0-1 APR MAY JUN JULY AUG SEP OCT MAY JUN JULY AUG 1962 1963 Fig. 4.—Growth and condition (C) of largemouth bass of the dominant 1961 year-class over the 1962 and 1963 growing seasons In the control pond Beta (dots), cropped pond Gamnna (circles), and add-stock pond Delta (rectangles). Cooper et al. { 1963) demonstrated that heavy cropping of a population of year- ling bass in a Pennsylvania pond caused substantial improvement in growth and condition, and in surplus production. It should be pointed out that the Pennsyl- vania study was a comparison of data from a single pond in different years while our comparisons were of data from different ponds in the same year. As emphasized earlier, differences in environments frequently masked the in- fluences of our experimental treatments. While our rates of surplus production were consistently greatest in cropped ponds, and lowest in the add-stock ponds, rates of growth and condition showed less conformity. Our most complete data comparing growth with condition are for the 1961 brood over the 1962 and 1963 growing seasons since this was the age group that was most abundant and the most heavily cropped over that period. Fig. 4 presents growth and condition curves for the 1961 year-class based on approx- imately bimonthly samples over most of the 1962 and 1963 growing seasons, with the terminal points in 1963 repre- senting averages of all fish in the final censuses. Growth and condition curves are based on identical samples. After mid-June, 1962, the growth curve repre- senting the cropped population ranked higher than that of either companion population and the margin of difference increased steadily throughout 1963. Coefficients of condition computed from the same samples were extremely variable from one sampling period to Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 109 the next throughout 1962. but all three populations exhibited similar patterns of seasonal variation. Condition in all ponds improved steadily from spring to a mid-summer peak, dropped sharply over the period from mid-July to mid- August, rose to a second but lower peak by early September, and then declined again. Differences in patterns between individual populations were not great in 1962, and showed little influence from the experimental treatment. In 1963, however, condition of the cropped pop- ulation had achieved a distinct superior- ity by early June, and this was main- tained throughout the 1963 season. While the cropped population ranked high in both growth and condition in 1963 (Fig. 4). growth in the add-stock population was higher than, and condi- tion was similar to, that in the control pond throughout much of both 1962 and 1963. Such an advantage in the add-stock population could have been influenced to the degree to which faster- growing, better-conditioned fish trans- ferred from the cropped pond contrib- uted to the samples obtained from the add-stock population. However, in- spection of our samples showed that the transferred fish composed such a small part of the samples as to render their influence negligible. Faster growth and better condition of fish in the add-stock pond than in the control pond, in spite of the greater density of stock in the add-stock pond, were believed due to a higher productivity for the add-stock pond than for the control pond Beta. As mentioned before, this indicated a re- versal in the comparative productivities of the two ponds since the earlier period with the stock of smallmouth bass. The condition curves for our Age I fish in 1962 (Fig. 4) illustrate the de- gree to which the condition of such a population may fluctuate over short per- iods of time. The similarity of curves in all ponds suggests that the fluctuations were due to some natural phenomenon common to all ponds. Since our Age I bass were largely dependent upon zoo- plankton and other small invertebrate foods, the fluctuations were probably re- lated to normal variations in the availa- bilities of these foods. The relative sta- bility of the condition curves for these same fish when larger and one year older in 1963 suggests that they were then subsisting on larger food items of a more stable supply. This may well have been crayfish, or the 1963 spawn of bass, or both. Fig. 5 shows rates of condition plotted against lengths of bass recovered from ponds Beta, Gamma, and Delta in the draining censuses of October 1963 and allows a comparison of the three curves u 6.0 70 8.0 9.0 10.0 U.O IZO 130 140 150 160 17.0 150 19.0 TOTAL LENGTH IN INCHES Fig. 5.—Coefficients of condition (C) in relation to lengths of largemouth bass for populations in the control pond Beta (snnall dots), cropped pond Gamnna (circles}, and add-stock pond Delta (rectangles) In 1963 compared with similar data (large dots] for bass from a mixed species popu- lation in an Illinois reservoir (Ramsey Lake; Stinauer 1966). Points plotted are the averages of condition for bass of convenient length groupings against the mid-length of each length grouping. 110 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 with a similar one for bass from a normal, mixed population from an Illi- nois reservoir (Stinauer 1966: Ramsey Lake). Conditions of our bass were commonly highest in the cropped pond, lowest in the add-stock pond, and inter- mediate in the control pond. While our bass showed a common though slight tendency for increased plumpness with increase in length, the tendency was less marked than in the normal population with which they are compared. This tendency may be typical of single-species populations of bass having no compan- ion fish as a source of forage. Mortalities Rates of mortality for largemouths in ponds, Zeta, Theta, and Iota were poor- ly known because of incomplete knowl- edge of year-class strengths. Informa- tion is limited to those fish which were marked and transferred from Theta to Iota or were marked and released where caught. Since ages were poorly known, they usually were designated only as "Age I and Older" or "Age II and Older." The limited data permit three general observations: 1) mortalities of those fish marked and transferred were consistently a little higher than for those resident fish marked and released in the same (add-stock) population, 2) mor- talities in the add-stock populations were slightly higher than for fish in the less densely populated companion ponds, and 3 ) known rates of mortality for largemouth were not notably differ- ent from those for similar categories of smallmouth in ponds Beta, Gamma, and Delta in 1959-1960, insofar as they could be compared. In 1 960 we attempted a study of rates of seasonal mortality based on recoveries in fall draining censuses of fish given distinctive fin clips at earlier periods in the growing season. Numbers marked in each period, numbers recovered in the final censuses, and percents of mortality are presented in Table 22. Because the fish marked were of a variety of sizes, and not of a single age class as in the earlier, similar experiment with small- mouth bass, the range in length and average length of each lot of marked fish are included in the table. Mortali- ties by the end of the 1960 growing season ranged from a low of 27.3 for 22 fish marked in pond Zeta in August to a high of 69.2 for 13 fish marked in pond Iota in April. Averages of mortalities (all ponds combined) for bass marked in each month showed a decrease from 48.9 percent for all fish marked in April to averages of 38.4, 37.9, and 30.5 for fish marked in the months of June, July, and August respectively. When rates of mortality for all fish marked in each pond were averaged (all marking peri- ods combined), mortalities in the add- stock population (44 percent) exceeded those in either companion pond (33.2 percent and 39.6 percent in Zeta and Theta respectively). As in our small- mouth experiments, all mortality rates were abnormally high. The data indi- cated a rather high and fairly uniform mortality due to handling and marking, with a slight increase in mortality with increase in time the fish were marked. Table 22 also reveals that mortalities were quite uniform over the size ranges marked, and not greater for either smaller or larger (younger or older) fish. This was evident from the rather normal and consistently small differ- ences between those average lengths, as well as ranges in lengths, that were de- termined when the fish were marked and when they were recovered. A dispropor- tionately high mortality among older fish would have been reflected by a lesser average length for those fish recovered than for those marked, and greater mor- tality of smaller fish would have had the opposite effect. A comparison of the lengths in Table 22 indicates a range of differences that in most cases would rep- resent normal growth increments. Rates of mortalities of the largemouth population from Unit 1 are more com- pletely known because of greater knowl- I Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 111 c £ -^ o 7=? Sw s:;~ -i "^ So SJcc ;:^^ CO t^ ^ Oi ^1 112 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 edge of their age compositions. When possible, as during a draining census, entire year classes were given distinctive marks. These, plus marks given during normal sampling, and to fish transferred during each year, made it possible at times of census to compute rates of mor- tality for most components of the popu- lations. Known rates of mortality for fishes stocked in or transferred to pond Delta are presented in Table 23. Pond Delta contained 363 Age O bass when censused in October, 1963. This was particularly interesting since our ob- servations had indicated no successful reproduction (and certainly no survival) in this pond in either previous year. We believe that the earlier failures were related to high pH levels, as discussed in the section devoted to spawning suc- cess. Age I was completely missing in the 1963 fall census of pond Delta. The 1962 spawn had been unsuccessful, and the 451 Age O fish transferred from Gamma as 2-4-inchers in late August and September, 1962, had disappeared, probably through predation. As indicated in Table 23, Age II fish (1961 year-class) in the final popula- tion of pond Delta contained four com- ponents for which mortalities ranged from 16.7 to 90.3 percent. Mortalities were higher among transferred than among resident bass and were highest among those fish transferred earliest and when smallest. We suspect, how- ever, that higher mortalities were re- lated more to the size of the fish when transferred than to length of time in the new population. The Age III and older fish also had four separate origins. Mortalities (Table 23) ranged from 16.7 to 82 percent, and were again related to length of time in the population and/or to size of fish when transferred. Known rates of mortality for bass in the control pond Beta are presented in Table 24, and are restricted to the original stock of October, 1960, or to those restocked following the draining census of October, 1961. A total of 1,705 young-of-the-year were recovered in the draining census of October, 1961, of which 892 were marked and re- stocked. The recovery of only 152 of this marked group in the census of October, 1963, indicated a mortality of 83 percent over the final 2-year period. Table 23.— Mortalities of various categories of largemouth bass based on recoveries in the draining census of pond Delta (add-stocl() in October, 1963. Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 113 Table 24. — Mortalities of various categories of largemouth bass based on recoveries in the draining censuses of pond Beta (control) in October, 1961, and October, 1963. Year 114 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 1960, and young-of-the-year were re- covered in all draining censuses. At- tempts to obtain comparative counts of schools of fry in ponds of both Units 1 and 2 were unsuccessful, due primarily to concealment of the fry by either large mats of filamentous algae or sub- merged aquatic weeds. As may be seen in Table 19, the numerical abundance of young-of-the-year in all final stand- ing crops was much higher in the cropped than in the companion ponds. This may have reflected higher rates of spawning success due to lesser densities of breeding stock in the cropped ponds than in the companion ponds, as ob- served in the smallmouth populations, or may have been due to lesser degrees of predation by lesser numbers of older and larger fish in the cropped ponds, or both. In Unit 1, successful spawns were known to have occurred in all ponds in all years, 1961-1963, except in pond Delta in 1961. A series of observations made in 1963 suggested a possible reas- on for the apparent failure in pond Delta in 1961, as well as for limited production of young in that pond in 1962. On May 9 and 10, 1963, adults were observed to be building nests in ponds Gamma and Delta but not in pond Beta. By early June, young-of-the- year had been seen in both Gamma and Delta, but neither young nor spawning activities of any type were observed in pond Beta throughout May or early June. During May we also observed that weeds of various Potamogeton spp., particularly P. crispus L., were becom- ing extremely dense in pond Beta, and considerably more so than in either companion pond. As an apparent con- sequence to the dense vegetation and associated high rates of photosynthesis, the pH of the pond waters rose to levels above 10.2 in pond Beta, and to levels of between 9.5 and 10 in ponds Gamma and Delta. Fish of larger sizes in pond Beta were observed to be extremely listless, with some appearing to be dazed and gasping for air at the surface, while at the same time smaller fish of about 8-inch lengths appeared unaffected. On May 27 approximately 1.25 acre-feet of fresh water were introduced into pond Beta, and the pH receded to about 9.5 and the fish appeared to recover. The treatment proved quite temporary, how- ever, and we decided to eliminate the weeds to reduce the high pH. On June 4, 1963, approximately one-third of the area of each of the three ponds was treated with enough endothal to provide a concentration of about 1 ppm in each treated area. On the basis of total pond volumes, the concentrations were prob- ably between 0.3 and 0.5 ppm. The treatments proved to be effective in all ponds. As the weeds began to die and the pH levels declined, the bass in pond Beta were observed to build nests and commence spawning activities. Nests were first observed on June 19 and 20, approximately 6 weeks later than in either companion pond, and several successful hatches were later recorded. We have attributed this delayed spawn- ing to the high pH during the normal spawning season. From observations made in the three ponds we believe that the critical level of pH was in the range of 9.6-10. These observations agree closely with those made by Jackson (unpublished) in some Wisconsin hatch- ery ponds where he reported no success- ful reproduction by bass in those ponds in which the pH remained at 9.6 or above during the spawning season. While adult bass have been known to tolerate pH levels exceeding 10 for short periods (Doudoroff & Katz 1950), reproduction may be restricted at some- what lower levels. We believe, as did Jackson, that high pH may influence bass production in one or more ways : 1. High pH may inhibit spawning. 2. High pH may kill the eggs or larvae. 3. Emerging fry may starve to death due to elimination of zooplankton by high pH. Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 115 Since levels of 9.6-10.2 were com- mon to our new, poorly buffered ponds in earlier years, we suspect that high pH may have eliminated or seriously reduced spawning of bass in those years. Review and Discussion of Largemou+h Bass Dafa, and Comparison with Smallmou+h Data Brynildson & Truog (1959) have recommended the stocking of Wiscon- sin warmwater ponds with largemouth bass alone because of unsatisfactory re- sults from combinations of bass and bluegill. Brynildson (personal com- munication) has reported fairly good fishing from such single species popula- tions providing the bass are not elimi- nated by winterkill or lack of natural reproduction. Bass alone have been stocked fairly extensively in both Illinois and Oregon. Bennett (1962) has achieved satisfactory results in Illinois ponds by stocking an assortment of bass larger than 10 inches, in addition to 100 fingerling bass per acre. The adults pro- duced young at the first spawning season after stocking, and the fingerlings al- ready present prevented the develop- ment of a dominant new brood subject to stunting. Bond (personal communi- cation) has reported excellent fishing in some Oregon ponds and poor results in others (particularly in small, weedless ponds) due to stunting and limited re- production, and he believes that best results may be obtained from use of ponds with areas of 2 acres or more. In a series of fishing trials. Bond and his associates found bass fishing to be much superior in single species populations than in populations containing both bass and bluegill. Largemouth are known to have been stocked as single species in ponds in other localities, but few such populations have been evaluated, due probably to early contamination by other unwanted species. A search of the literature has pro- vided only six listings of standing crops of bass as a single species (Table 15). The maximum poundage reported was 94.1 pounds per acre in a small, un- fertilized pond in Texas (Brown 1951). However, Mraz (1964) has reported data from a stunted population of large- mouths in a 1-acre pond in Wisconsin where the standing crop appeared to have exceeded 200 pounds per acre. The author reported removing 688 bass 5-9 inches long, and weighing about 100 pounds, in a single seine haul in the fall. The pond was heavily fished by hook and line both prior to this removal and during the following summer. In the following fall an additional 707 bass were removed, having a total weight of 108 pounds and an average length of 7 inches, and which had made practically no linear growth during the year. A substantial population of small bass also remained following this second harvest. The data suggest that both harvests were from the same age and size class of bass, which means that the standing crop at the time of the first harvest must have exceeded 200 pounds per acre. Our largest standing crop was 160.2 pounds per acre in pond Delta in Octo- ber, 1963 (Table 17). However, this population had received heavy additions of stock (100.5 pounds) since the last previous inventory, and the deficit of 56.3 pounds in the "pounds lost or gained" column indicated that the pond's carrying capacity had been sur- passed. The maximum poundage re- covered from a control pond was 126.8 pounds per acre from pond Beta in October. 1961. This pond had received an original stock in the preceding Octo- ber of 87.3 pounds, and upon drainage contained 1,705 Age O fish (55.6 pounds) ranging in length from 3 to 6.9 inches, no bass of Age I, and 122 bass (71.2 pounds) that were Age II or older, ranging from 8.5 to 18.6 inches in length. Additional inventories (Table 18) indicate that from 120 to 130 pounds per acre may have been the maximum carrying capacity of these ponds for largemouths. These compare J 16 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 with maximums of from 170 to 180 pounds of smallmouths earlier recov- ered from ponds in this same unit. Greater production of smallmouths than largemouths was believed due primarily to the superior ability of smallmouths of larger sizes to subsist on an invertebrate diet. As observed in Oregon and Wiscon- sin, largemouth bass as a single species are sometimes self-limiting due to pre- dation of young, or failures to repro- duce. We observed a common tendency toward progressively greater propor- tions of older and larger fish in both smallmouth and largemouth popula- tions, but elimination of young was less pronounced among largemouths than smallmouths in our studies. Table 25 compares the final compositions of 10 populations for each species (4 control, 3 cropped, and 3 add-stock populations). Higher average standing crops of small- mouth than largemouth bass were asso- ciated with greater numbers and greater percentages by weight of fish older than Age O in the final smallmouth than in final largemouth populations. For both species, numbers of fish older than Age O were highest in the add-stock ponds, lowest in the cropped ponds, and inter- mediate in the control ponds. One would suspect that numbers of Age O fish in the final censuses might be in- versely correlated with numbers older than Age O. This was true for large- mouth, but not true for smallmouth populations. Average numbers of young smallmouths were 422 in four control ponds, 582 in three add-stock ponds, and only 23 in three cropped ponds. Comparable figures for largemouths were 511 in four control ponds, 234 in three add-stock ponds, and 4,069 in three cropped ponds. It is not clear whether smaller numbers of Age O smallmouths than largemouths in the final censuses were due to the lesser fecundity of the smallmouths, to greater predation on smallmouth young by their elders, or to more periods of high pH. The greater numbers of older small- mouths could have accounted for great- er predation. However, the fact remains that all populations of largemouths con- tained young-of-the-year at the times of the fall censuses, ranging in individual ponds from 20 to over 10,000 survivors (Table 19), and average numbers of these survivors were higher in large- mouth than in smallmouth populations (Table 25). Evidence therefore exists that largemouths alone also are capable of sustained, year-to-year production in 1-acre ponds, especially when subjected to cropping. Numbers of surviving young-of-the- year in cropped largemouth ponds aver- Table 25. — Comparison of final compositions of 10 populations of largemouth bass with 10 of smallmouth bass on basis of treatments received. Averages are presented for four control, three cropped, and three add-stock populations for each species. Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 117 aged much higher than in companion largemouth ponds, as seems normal and expected, but numbers of surviving young in cropped smallmouth popula- tions averaged much less than in com- panion smallmouth ponds (Table 25). As mentioned previously, the apparent spawning failure of smallmouths in cropped pond Tau in 1963 may have been due to the absence of a breeding pair, or to predation of eggs or larvae by an abundant population of crayfish. We suspect that other failures of small- mouths in cropped ponds were related more to physical or chemical limitations in the environment, as discussed in the sections devoted to spawning success, than to any significant difference in be- havior of the two species. Ponds in Unit 2 provided evidence that under an optimum program of cropping, largemouth populations are capable of a seasonal replacement of fish flesh equal to the poundage that the pond can support. Our most efficient largemouth popu- lations were believed to be those com- prised of large weights of both smaU and large fishes, and this division of sizes was apparently more critical in largemouth than in smallmouth popula- tions. As detailed earlier, standing crops of largemouths in pond Beta declined substantially as the population pro- gressed from an approximate equal divi- sion of weights of fishes of Age O and those of Age II and older to one in which about 90 percent of the weight was comprised of fish longer than 8.5 inches (Age II and older). Populations of smallmouths in these same ponds attained maximum poundages (170- 180 pounds) when over 90 percent of the weights were comprised of fishes that were Age I and older in one instance and Age II and older in two instances. The essential difference apparently lies in the greater ability of smallmouth of larger sizes to make efficient use of in- vertebrate foods. Probably none of our bass populations attained the maximum of which either species was capable. While we have little knowledge of what constitutes an optimum size distribution, and it would probably vary with the type and amount of available foods, we would presume it to be one in which all sizes and ages of fish had optimum representation. Since even our most productive populations had one or more age (and size) classes missing, possibly none achieved maximum efficiency. As noted for the smallmouth popula- tions, carrying capacities for large- mouths were larger in the fall than in the spring. As shown in Table 18, standing crops measured in pond Delta in the spring were 120.6 pounds (drain census, 1962) and 128.1 pounds (esti- mate, 1963), as compared with fall standing crops of 164.5 pounds (esti- mate, 1962) and 160.2 pounds (drain census, 1963) in the same pond. Thus, carrying capacities in pond Delta in the spring were approximately 77 percent as large as those in the fall. Carrying capacities of ponds in Unit 2 showed a marked increase over the period 1958-1960, due primarily to in- creased production of crayfish and other invertebrates, but there was no evidence that the carrying capacities of ponds in Unit 1 increased over the period 1961- 1963. However, carrying capacities were evaluated by differences in standing crops, and these were influenced by dif- ferences in size and age compositions of the populations. Thus, differences in population structures in the different years could have masked any increase in carrying capacities. As in the smallmouth populations, life histories of the largemouths were strong- ly influenced by differences in popula- tion densities. In general, rates of growth and condition were poorest, and rates of mortality highest, in those pop- ulations having the greatest densities of stock. Again, however, fastest growth rates were not always accompanied by highest rates of condition, and compara- tive rates of either were sometimes poor 118 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 indicators of comparative rates of the other. Comparative rates of reproduc- tion were poorly known, but numbers of young-of-the-year surviving through their first growing season were consis- tently by far the highest in the cropped populations. YELLOW PERCH We will present data obtained over 2-year (1958-1959) and 1-year (1960) periods from the single pond Alpha, earlier devoted to smallmouth bass, and a 3-year period (1961-1963) in ponds Zeta, Theta, and Iota (Unit 2), which earlier contained largemouth bass. The original stock of perch was transported from the Green Bay section of Lake Michigan where the fish had been trapped by personnel of the Wisconsin Department of Conservation. Pond Alpha was first stocked with perch on April 30, 1958, receiving 79 males and 4 females, all adults of un- known age, having a total weight of 19.4 pounds. In spite of a loss of eggs, in transportation, by one or more of the 4 females a substantial spawn was obtained. Cropping in 1958 consisted of only 20 young weighing about 1 pound. Total cropping in 1959 was 78.7 pounds, including 57.2 pounds re- moved by summer hook-and-line fish- ing, and an additional 21.5 pounds taken by wire traps in October. Following the October draining census in 1959 the entire population was replaced in the pond. This included two of the original adults, 1,531 young-of-the-year, and 464 yearlings, representing a combined total weight of 123.4 pounds. In addi- tion, 21 adults from a local lake were added to augment the breeding popula- tion, which brought the weight of the original stock for the 1960 experiment to 125 pounds. The pond was cropped of a total of 88.3 pounds in 1960, and when censused in October of that year had a standing crop of approximately 192 pounds. Perch obtained from the 1960 census of pond Alpha were im- mediately restocked in ponds Zeta, Theta, and Iota, with each pond receiv- ing 3,771 young-of-the-year and 225 of mixed Ages I and II for total weights in each of about 52 pounds. These popula- tions were sampled and observed through three growing seasons (1961- 1963), and final censuses were accom- plished in October, 1963. Pond Zeta was cropped, Theta received additional stock, and Iota was maintained as a control. Standing Crop Data Table 26 presents standing crop data for six populations of perch together with such pertinent supplementary data as pounds of original stock, pounds added to or removed from the popula- tions during the course of the experi- ments, and the net gain or loss in pounds of fish flesh by the termination of the experiment. Table 27 presents numbers in each size or age group and percents of total numbers and weights of each group recovered in each draining census. Standing crops of perch ranged from a low of 51.3 pounds per acre in the cropped pond Zeta in 1963 to a high of 191.9 pounds per acre in the cropped pond Alpha in 1960. Differences in standing crops of perch were due in part to differences in pond fertilities, differences in pond treatments, and dif- ferences in size and/or age compositions of the populations. Fertilities of the three ponds in Unit 2 were believed to be quite similar, but all were clearly less fertile than the single pond Alpha. The degree of differences is indicated by the fact that the cropped pond Alpha had a larger standing crop in 1960 than did pond Theta in 1963 when it received quite large additions of stock. The differences in standing crops in pond Alpha in 1959 and 1960 were not as large as indicated (Table 26) be- cause 21.5 pounds were cropped from Alpha just prior to the October draining census in 1959, and this poundage is Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 119 included under the heading "pounds re- moved." Without this October removal the standing crop in 1959 would have been approximately 144 pounds rather than 123.4. However, the "pounds gained" would have remained the same at 183.7 in 1959, regardless of how the standing crop was computed, which illustrates the usefulness of the pounds gained statistic. Remaining differences in standing crops and in pounds gained or lost in pond Alpha in the two periods have been attributed to differences in sched- ules of cropping and in population com- position. In 1959 cropping was poorly distributed over the growing season with a total of only 32.5 pounds removed by late July. Since average length increment of the dominant yearlings was only about 0.5 inch from May through Sep- tember, 1959, and since coefficients of condition steadily declined from 4.25 to 3.78 during the same period, the re- moval apparently was too little to per- mit increased growth by survivors. In 1960 we wanted to determine if more intensive cropping of pond Alpha earlier in the season might have a more measur- able effect on the survivors, and tend to increase total production. In 1960 24.5 pounds were removed in April, 22 in May, and a total of 77.2 pounds, or about 88 percent of the total season's cropping had been accomplished by July 20. Table 26 shows a marked in- crease in production during the second and shorter of the two periods. From an original stock of 19.4 pounds the first population showed a pounds gained fig- ure of 183.7 pounds in two seasons, whereas the second population, with a much larger original stock (125 pounds), showed 155.2 pounds gained in only one growing season. The actual mechanisms were a greater production and/or sur- vival of young, and increased growth by survivors of the original stock. While the accelerated cropping surely increased production in 1960, there were also important differences in popu- 3 s (S-^ S 5 o 00 »o m -- -H c^l^ rt „ C-J ++++++ CN Ol OlO 00 lO < (N lO -t lO »C lO CD O CD CO CD Oi Oi Oi O^i Oi Oi •^•s g-^ aS. S S-S S. d. p. ti t^ _L d. o o 9 sis o oo oo<:o 1 I C^C^IMC^ 120 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 § s Oh S E-<6q .1^ S ^ § S o-r ira CO '* ^ CS CO o |>. O CO CO Oi ^H o oo . CO CO ; 00 (M o+l Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 121 lation compositions. The first relates to the increased abundance of breeding stock in 1960, which resulted in an in- creased production of young-of-the- year. The only mature females present in 1959 were survivors from the origi- nal stock of four, since females of the 1958 brood would not have matured by 1959. In 1960, however, the 1958 brood was in its third year, and is known to have spawned. Weight of Age O perch in the final censuses increased from 15.7 pounds in 1959 to 96.2 pounds in 1960. A second important diiference in composition of the two populations in pond Alpha was in their size distribu- tion. In the 1959 census, in excess of 99 percent of the population, both by weight and by numbers, was contained within a length range of only 3.3 inches (from 4.1 to 7.4). Concentration into such a small size range undoubtedly in- tensified competition for food and great- ly restricted predation of the smaller by the larger perch. In contrast, when cen- sused in 1960 the population was quite normally distributed over a range of 7.2 inches (from 2.5 to 9.7), which could have permitted substantial cannibalism and a more efficient utilization of total food resources. Age O fish grew more slowly in 1960 than in 1959 because of their much greater abundance, but both growth and condition of fish older than Age O improved in 1960. When this pond was devoted to small- mouth bass an initial stock of 16.16 pounds of small breeders produced a standing crop of 64.7 pounds after one and one-half growing seasons. The standing crop of perch in 1959 approxi- mately doubled, and the 1960 crop ap- proximately tripled this previous pro- duction of smallmouths. Standing crops of perch in Unit 2 ponds ranged from 51.3 pounds in cropped Zeta in 1963 to 183.7 in the add-stock pond Theta in the same year (Table 26). The control pond Iota was drained and censused at the end of only one growing season (October, 1961), and was restocked and censused again after two additional growing seas- ons (October, 1963). Standing crops were similar at 94.7 and 100.3 pounds in 1961 and 1963 respectively. One might therefore conclude that the car- rying capacity of this pond for perch was approximately 1 00 pounds per acre. It seems likely, however, that produc- tion may have been inhibited, and stand- ing crops depressed by poor production of young in both census years. For reasons unknown no young were pro- duced, or none survived, in 1961, a strong year-class was produced in 1962, and there was another complete failure in 1963. Thus neither the 1961 nor 1963 census contained young-of-the- year. The presence of young would have increased total poundages and would have provided food for older, cannibal- istic individuals. The carrying capacity of this pond (Iota) may therefore have exceeded the poundages obtained from the two censuses. It should be noted that the maximum standing crop of largemouth bass in this pond had been 71.7 pounds when it was used as an add-stock pond in 1960. As might be expected, pounds gained were lowest and standing crop highest in the add-stock pond, and pounds gained were highest and standing crop lowest in the cropped pond. The difl'erentials reflect an increased productivity associ- ated with cropping. For example, with the addition of stock the pounds gained figure in pond Theta amounted to only 19 pounds, compared with 220.8 pounds gained where cropping was heavy. Here pounds gained was almost equal to the total of pounds removed, indicating a complete turnover of the population. Effects of Cropping on Fish Production The influences of cropping on perch populations were perhaps less obscured by environmental differences than in ex- periments with either of the basses. We 122 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 have already noted the increased pro- duction in pond Alpha in 1960, over that of the 1958-1959 period, due at least in part to accelerated cropping in the latter year. The same was equally evident in Unit 2 ponds where the fig- ures for both pounds gained and growth (length of the fish) were greater in the cropped than in either companion pop- ulation (Tables 26 and 28). Table 27 Growth and Condition In the marking experiments in pond Alpha in 1960 we gained useful informa- tion on rates of seasonal growth. Table 29 includes figures indicating the per- cent by which each monthly lot increased its average length between time of marking and time of the October cen- sus. The greatest average length incre- ment was 17.6 percent made by fish Table 28.— Growth of yellow perch in ponds used for carrying capacity experiments on the McGraw Foundation grounds, along with sinnilar data on yellow perch in several other waters. Group Year Treatment Average Length in Inches (at Ends of Numbered Growing Seasons) with Number of Fish Used to Establish the Average in Parentheses 3rd I Alpha 1959 Cropped Alpha 1960 Cropped Unit 2 Iota 1961 Control Unit 2 Iota 1963 Control Unit 2 Theta 1963 Add-stock Unit 2 Zeta 1963 Cropped Other Waters Author Lake Chautauqua, 111. Lake Erie Red Haw Lake, Iowa East Lake, Iowa Starrett and Fritz (1965) Jobes (1952) Lewis (1950) Lewis (1950) 4.4 (100) 2.7 (100) 3.9 (15) » 4.3 (200) Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 123 Table 29. — Averages and ranges In total lengths of yellow perch marked in pond Alpha in spring and summer months indicated in I960, and recovered in the tall draining census, with percents of mortality and length increments fcr each mcnthly lot. Marked (During periods indicated, in 1980) Recovered (Nov. 7-9, 1960) . Percent Percent Length Range Average Range Average Mortality Incre- Date Number in Length, Number in Length, ment Length, Inches Length, Inches Inches Inches April 5-6 May 10-13 June 1-2 July 6-8 Aug. 29 Sept. 14-15 4.2-6.1 4.2-6.3 4.6-6.9 5.2-6.9 5.5-8.2 5.5-8.1 5.03 5.18 5.44 5.90 6.83 6.85 5.2-6.8 5.4-8.1 5.5-8.5 5.4-8.1 6.0-9.7 5.7-8.7 5.90 6.29 6.43 6.40 7.19 7.20 28.3 14.7 18.3 17.6 8.3 15.4 23.3 8.5 26.9 5.0 12.5 4.9 posed of ripe, freely milting males, and the few females caught at that time were retained for stocking a different pond. Fish marked and released in April are therefore known to have been all males. Spawning occurred in April, and fish marked in May and June were not sexed, but were believed to have had a more normal sex ratio. We therefore believe that the smaller increment made by the exclusively male sample marked in April reflects a poorer growth made by this sex. These data seem to be somewhat at variance with those of Herman et al. (1959) who showed that among Age I and Age II perch from Green Bay and Lake Mendota the males were of similar or slightly larger average size than fe- males, but that the females were larger by Age III and thereafter. Growth data presented for compari- son with data from other waters (Table 28) are restricted to fish recovered in the draining censuses, and for which ages were known (not calculated). Data presented in the growth curves (Fig. 6 and 7) are based on samples collected on the dates indicated. Some such samples were collected on a single day and others represent a composite of small collections over periods which rarely exceeded 10 days. Perch samples were collected by wire traps, hook-and- line, and by boat shocker. Ages of fish were known by presence or absence of marks, or by distinct separations in size. First-year growths compared favorably with those from other waters (Table 28), except when the age class was un- usually large, as in pond Alpha in 1960 in the presence of abundant, protective vegetation. Good first-year growth was probably related to heavy predation by older perch. Growth by perch older than Age O in our single-species popula- tions was very poor, and poorer than for perch in most other waters with which they were compared. For example, growth increments of the 1960 year- class over a 2-year period (Ages II and III ) in our control and add-stock ponds (Unit 2) were only 1.24 and 0.69 inch- es respectively. Concurrent growth in our cropped pond was higher, but still only 2.26 inches. The influences of cropping and addi- tions of stock are apparent in Fig. 6 and 7 which present growth and condition curves for the 1960 and 1962 year- classes throughout their existence in the populations. For the dominant 1960 year-class (Fig. 6) growth was fastest in the cropped pond, slowest in the add- stock pond, and intermediate in the con- 124 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 t <^ 1/ -I 1 1 ' 1 1 r to cvl in —r^ o to >-. ;;; '^ o I.I aoiovj NouiaNoo S3H0NI Nl H±9N3n HVlOl Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 125 5.0 4.0 3.0 2.0 6.0 -| 126 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 Table 30. — Averages of total lengths, weights, and coefficients of condition of various com- ponents of Age I and Age III yellow perch In the final census of the add-stock pond Theta. 1963, in relation to their origin or date of transfer. Coefficient Length, Weight, of Inches Pounds Condition Original Stock 4.67 0.037 3.63 Transferred in 1962 4.54 0.034 3.63 Transferred in 1963 5.75 0.073 3.84 Age III Original Stock 5.07 0.046 3.53 Transferred in 1961 4.89 0.040 3.42 Transferred in 1962 6.94 0.130 3.89 Transferred in 1963 7.73 0.200 4.33 the first year in which each was trans- ferred (1962 for Age I perch, 1961 for Age III perch). We know that in all cases fish transferred from the cropped to the add-stock population were of larger average size at time of transfer than perch of the same age among the original stock in the add-stock popula- tion. Since transferred fish received no harsher treatment than fish of the origi- nal stock which were sampled, weighed, marked, and returned to the add-stock population, the retardation of growth would have been due more to stresses encountered in the new environment than to handling. Progressively greater average sizes of perch transferred at later dates reflects the growth advantage obtained before transfer from the cropped population. Comparative data on condition are generally consistent, with fish transferred latest having the highest coefficients of condition. Mortalities In 1960 we were able to compute total mortality of perch older than Age O in pond Alpha. From an original stock of 2,017 "older" fish, 1,089 were removed by cropping and 764 were re- covered in the final census, leaving an unaccounted for loss of 164, or about 8.1 percent of the original stock. In 1960 we also attempted to obtain rates of seasonal mortalities, along with rates of seasonal growth. From April through September a number of perch of Ages I and II were caught and re- leased with a distinctive mark for each month. As in earlier, similar experi- ments with smallmouth and largemouth bass, the data provided little useful in- formation on rates of seasonal mortality and emphasized the difficulties of such an experiment. For example, lowest mortalities were among fish marked in June, and mortalities for fish marked in July and August were much higher than for those marked in June and little dif- ferent from mortalities of those marked in April (Table 29). From marks given to various year- classes, and to fish transferred to pond Theta, we have information on mortali- ties of many components of the popula- tions from ponds Zeta, Theta, and Iota. Our most complete information is from i pond Theta, which received additions of stock. Among the 141 Age O fish re- covered in the final census, 126 were survivors of the 408 transferred from pond Zeta in July and August, 1963 (69.1 percent mortality) and only 15 were survivors from the 1963 brood spawned in this pond. These returns in- dicate a rather heavy predation of young by older perch. Recoveries in the draining censuses and computed mortalities for various categories of perch are presented in Table 31 for the control pond Iota, in Table 32 for the cropped pond Zeta, and in Table 33 for the add-stock pond Theta. Mortalities were generally quite irregular and not clearly density-related, i.e., not notably or consistently higher in the add-stock population than in the cropped or control populations. This indicates that density probably was not a critical factor among our populations. However, mortalities were higher among fish transferred to a new population than Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 127 among members of the original stock already resident in that population. For example, for three groups of perch, all of the 1960 brood, mortalities by Octo- ber 1963 were 96.1 percent among 462 perch transferred in 1962 and 85.1 per- cent among 1,309 transferred in 1961, while mortality of original stock over the 3-year period was only 74.4 percent (Table 33). While fish densities did not appear to be critically high, trans- ferred fish encountered stress in the new environment which increased their mortality. As mentioned before, the act of marking and transfer was not in itself cause for greater mortality because un- transferred fish were also marked and handled. Table 31. — Mortalities of various categories of yellow perch in the 1961 and 1963 October draining censuses of the control pond lota, approximately I I months and 24 months after stocking. Recovery in Oct., 1961 of Fish Stocked in Nov., 1960 Recovery in Oct., 1963 of Fish Stocked in Oct., 1961 Number Removed by Other Than Natural Mortality Percent Unaccounted for: Natural Mortality 1960 128 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 Spawning Success Variations in spawning success and in year-class strengths were large and of particular interest in the perch popu- lations. Adequate breeding stock was present in the form of Age III perch in all ponds in 1961, but all three popula- tions failed to either spawn or produce a surviving year-class in this first spawn- ing season in the Unit 2 ponds. In the control pond Iota a strong year-class was produced in 1962, having 3,079 surviv- ors in the fall of 1963, and this was fol- lowed by an extremely light year-class in 1963, of which none survived to the end of that year. Theta, the add-stock pond, produced no surviving young in 1961, and there was extremely light reproduction (or survival) in both 1962 and 1963. Zeta, the cropped pond, after its 1961 failure, produced a large year-class in 1962. However, this class was dras- tically thinned by mortality of unknown origin. Over a 2-week period in early June, 1962, the 1-to 2-inch fingerlings could be seen at the surface periodically immobile and "belly-up" for a few sec- onds, from which they would suddenly recover as if under some sudden stimu- lation and swim rapidly away, only to repeat the process a few minutes later. While no mortalities were observed at the time, subsequent sampling revealed a greatly reduced population of the Age O fish. Only 230 survived until October, 1963. Fish older than Age O appar- ently were unaffected. The heavy 1962 spawn was followed by a weak brood in 1963, of which only 752 were recovered in October of that year. A normal spawn was anticipated in 1961 because both ripe and spent fe- males were observed, and developing eggs were found on wire traps. When the failures became apparent we specu- lated that the cause may have been due to the absence of submerged vegetation in ponds Zeta, Theta, and Iota, and that successful spawning in Alpha may have been facilitated by an abundance of Chara. Starrett & Fritz (1965) attribu- ted poor spawning success of yellow perch in Lake Chatauqua to the lack of submerged vegetation. Though moder- ately thick growths of pondweeds (Po- tamogeton spp.) developed in the sum- mer months in our ponds, none were present during the spawning period. As a spawning aid in 1962 and 1963 multi- branched tree limbs were placed in the ponds. Eggs were deposited upon the limbs in both years, and eggs trans- ferred from the limbs to the laboratory made normal development. Reproduc- tion was heavy in Zeta and Iota in 1962, and light in Zeta and Theta in 1963, but no young were recovered from Iota in 1963. Obviously, some additional factor was limiting. Our data indicated that high pH was not a limiting factor in the reproduction of perch, and that water quality was otherwise satisfactory. We suspect that under conditions of normal water qual- ity, failures of reproduction are due less to failure of the adults to produce normal eggs and young than to a failure of the young to survive. Fluctuations in year-class strength are common to perch populations (Herman et al. 1959), as to most fish species. In the present in- stance we believe the most probable cause was predation on eggs or young. Failures occurred when either weight or numbers of companion perch (principal- ly Age I) were large, and successes oc- curred when they were small (Table 27). Though the populations were not large by normal standards, they occurred in the absence of protective cover for young. Abundant spawns in pond Alpha could have been facilitated by the pro- tective cover provided by the beds of Chara, but no such protection existed in the ponds in Unit 2. Predation of eggs or larvae by large populations of cray- fish and tadpoles may have augmented that by perch. Both forms were of un- usual abundance in 1962 and 1963 be- cause of the absence of bass or other large predators. iMar. 1 970 Buck & Thoits: One-Species Populations of Fishes 129 Related Environmental Factors Although quantitative data are lim- ited, some observations can be made of such related ecological factors as abun- dance of associated plants and animals, and of the physical and chemical char- acteristics of the pond waters. Of par- ticular note were the unusual abundances of tadpoles and crayfish, presumably because of the absence of bass or other more efficient predators. Table 34 pre- sents weights of these forms, together with weights of perch collected during the final censuses. As indicated in the table footnote, poundages of crayfish were conservative, because many failed to be flushed from the pond with the water and were not available for weigh- ing. Poundages presented, however, are believed quite representative of differ- ences in weights which occurred. Tadpoles were primarily those of the bullfrog, Rana catesbeiana, crayfish were identified as Arconectes virilis. There are several noteworthy associa- tions in Table 34. 1. The largest poundages of perch were associated with the smallest poundages of crayfish, and vice versa, indicating some degree of control of crayfish by perch. 2. Tadpoles were abundant when crayfish were scarce, and vice versa, indicating some degree of control of tadpoles by crayfish. 3. At the time of the three 1963 drainages, weeds (principally Anacharia sp.) covered an esti- mated 90 percent of the bottom of pond Theta, which had the fewest crayfish, while the bottom was almost completely bare of weeds in pond Zeta with its com- paratively great weight (231.2 pounds) of crayfish. Neither weeds nor crayfish were abundant at the two drainages of pond Iota. Actually, weeds in all three ponds had been quite drastically thinned by treatments with endothal in June, 1963, but the weeds had again become abundant where the crayfish were less abundant and had become increasingly scarce where the crayfish were most abundant. Nymphs of the large burrowing may- fly, Hexagenia limbata, were abundant in ponds Iota and Zeta, but were absent from pond Theta at the time of drainage in October, 1963. At the time, the bot- tom of pond Theta contained thousands of small "craters" from 1 to 3 inches in diameter, and from 1 to V/i inches deep, often with subsurface cavities larger than the openings. Since these craters were not present in the com- panion ponds having abundant burrow- ers, it suggests that the larger population of perch in Theta may have eliminated the mayflies by burrowing or sucking them out of the mud. Concurrently, however, scuds, phantom larvae, smaller mayflies, dragonflies, and assorted other Table 34.— Standing crops of yellow perch, crayfish, and tadpoles, and relative abundances of rooted, subnnerged aquatic plants in ponds Zeta, Thota, and lota at times of fall draining censuses. 130 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 invertebrates were of the same general abundance in Theta as in the companion ponds, suggesting no inordinate preda- tion pressure on these forms. Throughout most of each of the three growing seasons weekly observations were made of the surface pH, Secchi disc transparency, and the temperature profile of each pond. Because of differ- ences in treatments of the populations, no interpretations can be made of the influence of these factors upon fish pro- duction. However, the data warrant the following observations. Of the three years, thermal stratifica- tion was greatest, and transparency low- est in all ponds in 1963. The decrease in average transparency was negligible in the add-stock pond, but quite marked in the companion ponds, particularly the cropped pond Zeta (from 72 inches in 1961 to 20 inches in 1963). One there- fore suspects that decrease in transpar- ency was related to increase in the cray- fish population, which was notably high in pond Zeta. Increase in thermal strati- fication was in turn probably related to decrease in transparency of the pond water. DiiTerences in pH were not believed to have had any important influence on fish production. Generally lower pH's in 1963 were probably related to the reduction of weeds by chemical treat- ment in all ponds in June, 1963. A less- er average pH in pond Zeta was prob- ably related to the influence of crayfish in reducing vegetation in that pond. Review of Perch Data, and Comparison with Largemouth and Smallmouth Data Perch were maintained in ponds pre- viously occupied by either smallmouth or largemouth bass. In Unit 2 ponds the maximum standing crop obtained from a control population of largemouths was about 50 pounds per acre, while that for a population of perch receiving the same treatment was about 100 pounds. The largest standing crop obtained from an add-stock population of largemouths in this unit was about 72 pounds, while that from the add-stock population of perch was about 184 pounds. We have already noted that in pond Alpha the standing crop of perch in 1959 more than doubled, and that of 1960 approximately tripled, the stand- ing crop of smallmouths earlier pro- duced in this pond. We believe, how- ever, that the differences in efficiencies in food utilization or conversion were less than indicated. Smallmouth bass probably did not attain their potential maximum in pond Alpha because of the relative immaturity of the pond when occupied by smallmouths, and because the population had developed over only one and one-half growing seasons and contained only two year-classes when censused. Perch are believed to be more de- pendent upon an invertebrate diet than either of the basses, and their greater efficiency is probably related to their lower trophic level. However, in our data the yellow perch of intermediate size and age appeared to be less efficient in utilizing invertebrate foods than were smallmouth bass of the same category. The largest standing crops of perch were those having the largest numbers of young-of-the-year, and where young were scarce, older fish seemed incapable of taking up the slack or of making effi- cient use of the food that would have been eaten by the young-of-the-year. On the other hand, our largest standing crops of smallmouth bass were those dominated by fish older than Age O, and production in these populations seemed very little inhibited by an ab- sence or scarcity of young-of-the-year smallmouths. It may therefore be that in the absence of vertebrate food the smallmouth is a more efficient insecti- vore than the yellow perch. Data from the add-stock population in Unit 2 indicate that the perch may be less versatile or adaptable in its food habits, and that there may be relatively Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 131 less competition for food between dif- ferent sizes and ages of perch than be- tween different sizes and ages of small- mouths. We have seen, for example, that in the 1963 census of pond Theta, 2,861 perch of the 1962 brood had an average length only about 0.4 inches less than that of 728 perch of the 1960 brood. While the younger perch had grown almost 5 inches in 2 years, the older perch had grown less than 1 inch over the same period. Thus the young- er, more abundant, faster-growing perch obviously utilized foods not taken by the older fish, which permitted them to lit- erally grow into the size range of the older perch. BLUESILLS Bluegills were studied in ponds Kap- pa, Lambda, and Rho (Unit 3) over a 3-year period, 1961-1963. This unit of ponds had been constructed in 1957, and for the period 1958-1960 was stocked with white crappies. Following the final census of crappies in October, 1960, each pond was stocked with 4,080 Age O bluegills from the state hatchery at Spring Grove, Illinois. Over the peri- od May 11-June 23, 1961, each pond received an additional stock of 104 adult bluegills obtained by boat shocker from Fox Lake, Lake County. Illinois. The combined total weights of young and adults stocked in each pond were similar, ranging from 34 to 38.3 pounds (Table 35). Standing Crop Data Table 35 presents standing crop data for four bluegill populations together with such pertinent supplementary data as pounds of original stock, pounds added to or removed from the popula- tions during the course of the experi- ments, and the net gain or loss in pounds of fish flesh by the termination of the experiments. Table 36 presents numbers in each size or age group and percentages of total numbers and weights of each group recovered in each I : i i :S=^ o t^ C-. o t^ lOO« CO CO CO cc CI CI o o 2 S m O. a c"2 o OQ<;o o o a g 132 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 1 ^-^ S-s 11 "S e t^ QOI^iO 00+ I 1—< CO CO CO CD CD CD CD Ol OS Oi OS draining census. Small poundages were removed from both control and add- stock ponds in all years, primarily for study purposes. The total of 141.3 pounds shown removed (Table 35) from the cropped pond Lambda is a mini- mum figure. Of this total 16.9 pounds of mixed Age I and adults were cropped in 1961, of which 15.5 pounds were transferred to pond Kappa. Known cropping from Lambda in 1962 con- sisted of mixed Age I (37.4 pounds), Age II (17.3 pounds), and original adults (17.5 pounds) for a total of 72.2 pounds. However, a significant part of this cropping was achieved on May 31, 1962 by spraying rotenone over a 30- to 40-foot width of approximately one- half of the shoreline. We hoped to achieve an early-season kill of a large proportion of the abundant Age I fish. A 3-day pickup of all visible dead in- cluded 4,616 bluegills (12 adults, 115 Age II, 4,489 Age I) weighing 15.9 pounds, which are included in the total of pounds removed (Table 35). Many fish (particularly Age I) were observed to sink to the bottom and be attacked by an abundant crayfish population, and were thus lost from the records. This unknown weight is believed to have been something less than that recovered and recorded from the treatment. Thus, the true pounds cropped, and pounds gained, were in excess of those totals listed in Table 35. In 1963 a total of 52.1 pounds was cropped from pond Lambda including 10,954 young-of-the-year weighing 16.9 pounds, removed in August, and 719 Age II fish weighing 35.2 pounds, re- moved in June, July, and August. Of this total, 626 Age II fish weighing 30.7 pounds were transferred to pond Kappa. Standing crops of bluegills in pounds per acre ranged from 99.8 in the con- trol pond Rho in 1963 to 173.3 in the add-stock pond Kappa in the same year. As in our previous experiments, varia- tions in production by the separate pop- ulations were influenced by differences I Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 133 in 1) treatments received, 2) popula- tion compositions, and 3) pond fertilities; however, the relative importance of each factor was difficult to assess. We may first note that standing crops (pounds) were highest where numbers of both Age O and older bluegills were largest (Table 36). There is evidence, how- ever, that the add-stock pond had the additional advantages of greater fer- tility and a larger carrying capacity. It seems significant that even with the much larger standing crop in the add- stock pond, its pounds gained figure was also greater than in the control popula- tion (Table 35). Thus, pond Kappa made a substantial production of fish flesh in addition to that transferred from the cropped pond. This suggests that the additions to pond Kappa had not exceeded the pond's carrying capacity. Since the numbers of both small and large fishes contained in pond Rho ap- peared sufficient to utilize available food resources, we must suspect that the carrying capacity of pond Rho was much smaOer than that of pond Kappa in 1963. We have evidence that the carrying capacity of the control pond Rho de- clined quite substantially between 1961 and 1963. Table 35 shows that in this pond both final standing crop and pounds gained figures were substantially higher in 1961 than in 1963. We may note (Tables 35 and 36) that final pop- ulation structures were comparable, having similar abundances of both small and large bluegills, that original stocks were similar, and that differences in pounds removed were not significant. We thus have evidence that the carrying capacity of pond Rho had been attained in its first growing season (1961), that a greater weight of fish was produced in the first than in either of the two follow- ing seasons, and that the productivity of this pond declined quite substantially between 1961 and 1963 in spite of mod- erate fertilization in 1962. It also seems significant that final standing crops of both crayfish and tadpoles were also higher in Rho in 1961 than in 1963. Pounds of crayfish in 1961 (173.2) were more than triple the amount of 1963 (55.6), and pounds of tadpoles in 1961 (113.9) more than doubled the standing crop (55.9) of 1963. The final population in pond Rho in 1961 contained 75,424 young-of-the- year which comprised about 42 percent of the total weight of the standing crop. The 1963 census of Rho also contained abundant Age O fish (67,453), but these comprised only 12 percent of the total weight due to their smaller average size. This smaller size suggests that spawning in 1963 may have been later than in 1961, or that food for young was in shorter supply. With regard to date of spawning, our field notes indi- cate that spawning occurred in late June or early July in both years, with young of similar sizes first found in early July in both years. Our sampling provided no evidence of a second and larger spawn in late 1963. We believe that the young-of-the-year simply grew more slowly in 1963. in spite of their lesser abundance. We have no comparative data on abundance of benthos or zooplankton in the two years. However, data on mean seasonal transparencies (Secchi disc) show that pond Rho maintained a richer bloom in 1963 than in 1961. Means of weekly readings from April through October were 19 inches in 1963 and 45 inches in 1961. Higher turbidity in 1963 than in 1961 was not caused by crayfish since the standing crop of cray- fish in 1963 was only about one-third as great as that in 1961 when turbidities were lowest. These ponds received very little erosion silt, and never appeared muddy. Turbidities were due primarily to phytoplankton. The greater plankton turbidity in 1963 indicated a condition favorable to fish production in that year, a fact not borne out by the standing crop data. It seems apparent that the princi- pal food utilized by the bluegills was not 134 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 dependent upon phytoplankton abun- dance. Mortalities were frequently high and quite variable, but were not believed to have had an important influence upon size of final standing crops. Dead fish were rarely seen, and no large or sud- den "kills" were observed just prior to the censuses, or at any other time. The most probable effect of mortalities on the table data was in some instances an artificial depression of the pounds gained figure. For example, of the 626 Age II fish transferred to pond Kappa in 1963, only 102 were recovered in October of that year. A major portion of this mor- tality was probably due to handling, with a large proportion of the fish dying so soon as to have had a minimum impact upon the existing population. In our computations of pounds gained, how- ever, the total weight of transferred fish was treated as though all had survived. Since this was a minus (subtracted) quantity in the calculations, the figure of pounds gained was artificially de- pressed, and production of fish flesh in the population was higher than indi- cated. This factor was probably more critical among bluegills than among other species studied because in our ex- perience the bluegiU was the most sus- ceptible to injury or death due to mark- ing and handling. Effects of Cropping on Fish Production Increased production gained by crop- ping was probably more clearly evident in the bluegill study than in those for other species. The key figures were those of pounds gained (Table 35). The figure of 204 pounds gained in pond Lambda was a minimum one due to un- known losses from the use of rotenone, yet it was more than twice that of either companion pond. It is significant that even with the cropping of 52.1 pounds from Lambda in 1963, its standing crop in October slightly exceeded that of the control pond Rho. It is also noteworthy that numbers in the final censuses were less than one-sixth as great in cropped Lambda as in the control population in pond Rho. A total of 10,954 young-of- the-year were cropped from Lambda in 1963, and the smaller number of 8,644 remaining in the final census represented 29 percent of the total weight of the population, while the much greater num- ber of 67,453 Age O bluegills censused in pond Rho represented only 12 per- cent of the total weight censused in that pond. The fewer fish had grown much faster, indicating that total numbers are in themselves not an overriding factor. With no evidence of a greater inherent productivity in pond Lambda than in pond Rho, greater production of blue- gill flesh in pond Lambda must have been stimulated by cropping. Growth and Condition Growths of bluegills in our single- species populations through their first 3 years of life were in most cases slower than for bluegills from normal, mixed populations in Illinois reservoirs with which they are compared (Table 37). Again, our growth data were from large samples of known-age fish as collected in the final censuses at the ends of the growing seasons. Failure of certain age classes in our cropped pond to show faster growth than in the companion ponds was due in some instances to dif- ferences in abundance. For example, the average length after three growing seasons was greater in the control pond Rho (4.9 inches) than in the cropped pond Lambda (4.3 inches) (Table 37), because the group in question (Age Group II, Table 36) numbered only 346 individuals in the final census in pond Rho, as compared to 1,438 in pond Lambda. At the same time, total lengths of 3-year-old fish were little dif- ferent in the cropped and add-stock populations even though the add-stock pond contained the greater number of 3-year-old fish (1,973, as compared to 1,438) and total population pressure of fish older than Age O was much greater Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 135 Table 37. — Growth of bluegills in pcuds used for carrying capacity experiments on the McGraw Foundation grounds, along with similar data on bluegills in other Illinois waters. 136 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 JUN JULY AUG SEP OCT 1961 MAY JUN JULY AUG SEP OCT 1962 Fig. 8.—Growth and condition (C) of bluegills of the I960 year-class In the control pond Rho (rectangles), add-stock pond Kappa (dots), and cropped pond Lambda (circles) during the 1961 and 1962 growing seasons. Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 137 showed a regular continuous trend, the source of variation was clearly in weights. A part of such variations could be attributed to differences in fullness of stomachs at times of measurement and a part to unusual sampling varia- tion, but a principal source could have been a loss of weight by spawning fe- males. It seems evident, however, that for a variety of possible reasons condi- tions can vary widely in fishes having similar patterns of linear growth, and that comparative rates of condition may be poor indicators of comparative rates of linear growth. Fig. 9 presents similar data for the 1961 year-class in 1962 and 1963. Due to their greater numerical abundance, fish in the cropped pond grew the least in 1962, and their growth was little different from growth of those in the add-stock pond in 1963. However, growth curves climbed quite regularly and uniformly upward in all ponds be- fore leveling off in August, 1963, indi- cating little sampling variability. Condi- tion curves were again extremely irregular in 1962, but quite regular and comparatively stable throughout 1963. The difference in the two years may have been related to differences in feed- ing habits or in availabilities of food. When younger, in 1962, these fish may have been largely dependent upon zoo- plankton, with frequent, large fluctua- tions in condition corresponding to normal variations in the abundance of such food. When the fish were older and larger, in 1963, foods taken may have been more diversified, and of a more stable abundance, resulting in more stable rates of condition. We have noted in Fig. 8 and 9 that comparative strengths of year-classes were reflected in rates of growth. We referred to comparative abundance of the same year-class in different ponds. It is of further interest to consider the influence of differing strengths of year- classes in the same pond. Dominant year-classes in our ponds derived from the original stock of 4,080 bluegifls of the 1960 brood in each pond, and from the large broods spawned in all ponds in 1961. We should then remember that reproduction failed in all ponds in 1962. We may thus consider comparative growth of Age I fish in the presence or absence of the younger year-class. Based on our bimonthly sampling from April into October, Age I fish in all three ponds grew more in 1962 than 1961 (Fig. 8 and 9). The average length in- crement of the Age I fish in all three ponds in 1961 in the presence of abun- dant Age O fish was only 1.29 inches, compared to an average increment of 2.29 inches for Age I fish in 1962 in the absence of Age O fish. A large part of this difference was probably due to differences in degrees of competition be- tween the two year-classes in the two years. Mortalities On the basis of returns of bluegills bearing distinctive fin clips given at the time of stocking or transfer we were able to compute mortalities for various age groups in the control pond Rho (Table 38) and the add-stock pond Kappa (Table 39). Mortalities for blue- gills in the cropped pond Lambda could not be calculated because of the un- known loss occasioned by the use of rotenone in 1962. Mortalities were frequently high, but quite erratic, and tended to be some- what higher among those fish trans- ferred to pond Kappa than for the origi- nal stock in either population. Highest mortality was among 626 Age II blue- gills transferred to pond Kappa in June, July, and August, 1963. The recovery of only 102 in October represented a loss of 83.7 percent over a fraction of a growing season. These fish were handled in larger quantities than most groups sampled and/or transferred, and we be- lieve that the longer period required for weighing, measuring, and marking these larger groups contributed to their higher 138 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 Table 38. —-Mortalities of various categories oi bluegllls in the 1961 and 1963 October draining censuses of the control pond Rho, approxinnately I I months and 24 months after stocking. Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 139 Table 40. — Numbers of bluegllls marked by a distinctive fin clip and released in each month of the growing season in pond Rho in 1961, numbers recovered in the draining census of October, 1961, and percents of mortality. 140 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 pond in Kentucky (Turner 1959). The highest standing crop reported in the Kentucky study for bluegills when in a mixed population was 547 pounds per acre. Both of these rather large pound- ages in Kentucky ponds were by esti- mates based on the return of marked fish when the pond was treated with rotenone. The highest bluegill pound- age that we have seen recorded by Swingle (1950) was one containing 464.7 pounds per acre in a fertilized pond which also contained 49.4 pounds of largemouth bass. Bennett's (1954) highest recovery of bluegills per acre in his draining censuses of Ridge Lake was 193.3 pounds in 1947. Our highest standing crop of bluegills was 173.3 pounds per acre in the add-stock popu- lation in 1963, and the average of our four standing crops was 125.3. The comparative figures reveal the low fer- tility of our bluegill ponds. Since the bluegill ponds were previ- ously devoted to white crappies, we have some basis for comparing the relative efficiencies of the two species. In both instances Rho was the control pond, and fish were cropped from Lambda for transfer to Kappa. Tables 35 and 44 show that in all cases standing crops of bluegills exceeded those for white crap- pies. Maximums were 89.6 for crappies in the control pond Rho, and 173.3 for bluegills in the add-stock pond Kappa. A more legitimate comparison, how- ever, might be that of the control ponds: 89.6 for crappies and 127.1 for blue- gills. Greater productivity for the blue- gill might be expected due to its greater fecundity and lower trophic level. It seems possible that the true differences may have been greater than indicated. There is strong evidence that the pro- ductivity of pond Rho declined over the period 1961-1963. If an equal decline occurred over the period 1958-1961, the differences in efficiencies of the two species would have been greater than indicated. BROWN BULLHEADS According to Forbes and Richardson (1920), the brown bullhead had an early but rather brief popularity as a commercial species in European ponds. Swingle (1957) developed procedures for its culture in Alabama ponds, but in later studies found the channel catfish to be more efficient and less subject to disease. Cross (1967) noted that the brown bullhead was propagated and dis- tributed for use in ponds in Kansas in the 1950's but that the introduction was not a success. The species has appar- ently found small favor, and seems to have been very little studied. Brown bullheads were used in the present study because thriving populations of these fish of desirable size m post- glacial lakes near our study area indi- cated that they might have some value for sport fishing. Ponds Phi, Chi, and Psi (Unit 5) were completed in the fall of 1960 and first filled with water in the early spring of 1961. Between May 11 and June 23, 1961, each pond was stocked with 28 adults obtained by boat shocker from Fox Lake, in Lake County, Illinois. Weights stocked in each pond were simi- lar, ranging from about 19 to 21 pounds. The control pond Psi was drained and censused at the end of the first and third growmg seasons, and the companion ponds only at the end of the third. Although several interim esti- mates were made by conventional esti- mating procedures, some were so strong- ly in error that none have been used as standing crop data. No fish were cropped in 1961, and when drained in October the control pond showed a "pounds gained" of 21.2 pounds and a final standing crop of 40.6 pounds. The pond was restocked with 19 adults and 7,882 young-of-the- year having a combined total weight of 33.5 pounds. In 1962 the cropped pond Phi was harvested of 61.9 pounds, of which 33.5 were transferred to pond I Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 141 Chi. In addition, both ponds Chi and Psi were cropped in 1962 to thin what were believed to be excessive numbers of small bullheads. From pond Psi, 1,670 Age I fish (12.7 pounds) were removed in May and June, and 562 Age O (3.5 pounds) were removed in July and August. From pond Chi, 3,031 Age I fish (16 pounds) were removed in May and June, and 2,458 Age O (9.6 pounds) were removed in July and August. The only additional cropping was 35.4 pounds removed from the cropped pond Phi in 1963, of which 33 pounds were transferred to the add- stock pond Chi. As in adjacent Units 3 and 4, this unit was lightly fertilized in 1962 (Table 2). Sfanding Crop Data Table 41 presents the standing crop totals for four populations of brown bullheads, together with pounds of original stock, pounds gained, and other pertinent supplementary data. Table 42 presents the numbers in each age group and the percents of total numbers and weights of each group recovered in each draining census. The cropped pond Phi had the small- est standing crop (16 pounds), but the highest pounds gained figure (91.9); the add-stock pond had the largest standing crop (90.2) and the lowest figure for pounds gained (29.8); the control pond ranked intermediate in both respects. Data from the control pond Psi indicates that this pond had a carrying capacity of between 40 and 50 pounds per acre. We may note that its final standing crop was 40.6 pounds when consisting of only two age groups in 1961, but increased to only 49.3 pounds when having the full comple- ment of four age groups 2 years later (1963). At the same time, however, the final standing crop of the add-stock pond was almost twice that of the con- trol pond, and the fact that a substantial weight of fish flesh was produced in 3 ° ^ -a Oh OS s i I --< CO ^ (N 1—' 00 C^ CO 142 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 ^1" ^6q "^ i CD O ^ O O" rt r-T 1«> O >0 t^ Oi ^ Tt^ Oi o »0 CD '^ O 1963, in spite of the addition of stock, indicates that the pond's carrying capa- city had not been exceeded. The princi- pal gains were made by bullheads of Ages O and I, most of which had been spawned in the pond, while practically no growth was made by fish of Age II and older which were the primary ages transferred into this population. Thus the limiting level of competition created among the larger and older fishes (Ages II and above) had little apparent influ- ence upon the younger and smaller fishes. Average growth of Age O bull- heads in 1963 was in fact substantially larger in the add-stock than in either companion pond, due, presumably to a lesser abundance than in the companion ponds (Table 42). The add-stock pond was probably somewhat more fertile than either companion pond, but we believe that the combination of greater total numbers and a more optimum dis- tribution of sizes of bullheads contribu- ted substantially to its larger standing crop. Growth and Condition Table 43 shows growth of brown bullheads of known age in our ponds as determined by actual measurements at the times of the fall draining censuses. Growth of bullheads in general, and brown bullheads in particular, seems to have been very little studied. Our growths were quite similar to those few records available for this species from New York and Illinois, as summarized by Carlander (1950). Our brown bull- heads had growths similar to the black bullheads in Clear Lake, Iowa (Forney 1955), but grew more slowly than black bullheads in Oklahoma waters (Houser & Collins 1962). Rate of growth was closely related to abundance. For example, first year growth in the cropped population in 1963 was less than in the add-stock pond because, in spite of cropping, the group numbered 1,065 in the final cen- sus in the cropped pond as compared to only 497 in the add-stock population. Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 143 Table 43.— Growth of brown bullheads in ponds used for carrying capacity experiments on the McGraw Foundation grounds. Average Length in Inches (at Ends of Numbered Growing Seasons) with Number Censused in Parentheses Group 144 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 "MAY JUN JULY AUG SEP OCT" 1962 MAY JUN JULY AUG SEP 1963 Fig. 10.—Length and weight increments and condition (C) of the 1961 year-class of brown bullheads in the control pond Psi (rectangles), add-stock pond Chi (circles), and cropped pond Phi (dots) during the 1962 and 1963 growing seasons. due to gonadal development, lengths would not have been so affected. Such overwinter growth was observed only among bullheads in our studies, and may be peculiar to this species. The fact that the bullheads had increased their lengths and weights, and were at their seasonal peak of condition in early spring suggests that they had found abundant food over much of the fall-to- spring period. Such a period of feeding and growth could conform with the seasonal fluctuation in abundance of some benthic fauna. As has been previ- ously observed (Ball & Hayne 1952; Eggleton 1931; and others) invertebrate abundance (primarily midge larvae) typically increases in the fall to a mid- winter peak, and is followed by a con- tinuous decline throughout the spring and summer and again increases in the fall. Exploitation of such a fall-to-sprini: abundance of invertebrate food could explain the pattern of growth by our bullheads. The spring-to-fall decline o\ the invertebrate population, coupled Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 145 with increased competition from an abundant new year-class of bullheads, could explain the summer decline in growth and condition. It further implies small utilization or preference for the abundant crayfish, and little cannibalism by the larger and older bullheads. Crayfish were abundant throughout 1962 and in the spring of 1963, and standing crops in pounds per acre in October, 1963 were 5.2, 6.2, and 38.8 in Phi, Chi, and Psi respectively. We had expected that the larger bullheads would make efficient gains on these abundant crayfish, and excretions by sampled fish indicated that the crayfish were commonly eaten. However, the fact that average size and condition of the original adult breeders actually de- clined over the 3-year course of the ex- periment indicates that crayfish were not heavily eaten, or were of little food value. It further suggests that competi- tion for food among adults and Age II fish in 1963 may have been severe. Mortalities As in our previous presentations, mortality data are restricted to those groups for which both original and final numbers were exactly known. For bull- heads we have data from only two year- classes— that of the original stock of 28 adults, and the 1961 brood, which was the only additional group present at drainage of the control pond in 1961 and the only group from which trans- fers were made. Because of their pau- city, these data were not tabled but are presented here in the text. Of the original 28 adults in the cropped pond Phi, 2 were removed in 1963 and 6 were recovered in the final census, representing a reduction of 20 individuals over the 3-year period, with a mortality rate of about 77 percent. Of the 28 adults stocked in the control pond Psi in May-June, 1961, 20 were recovered the following October, for a mortality rate of about 29 percent. For the 19 individuals restocked, mortality over the following two growing seasons was about 53 percent. Mortality was about 68 percent for 28 adults in the add-stock population over the full 3- year period, and 100 percent for the two adults transferred to this pond on August 13, 1963. Mortalities among three groups of the 1961 brood by October, 1963 ranged from 6.5 percent for 200 bullheads transferred to pond Chi over the sum- mer of 1963 to 94.8 percent for the 7,882 returned to the control pond Psi following the 1961 census. Mortality among 616 fish of this brood trans- ferred in 1962 was 63.6 percent by October, 1963. The large mortalities recorded for the 1961 brood occurred during their second and third years when the fish ranged from 2 to 7 inches in total lengths. The absence of large cannibals among the 1961 brood, and the negligible growth and poor condi- tion of the adult bullheads, indicate that cannibalism was not a major source of mortality. Since no large die-offs were observed, and sampled bullheads appeared to be healthy and vigorous at all times, the large mortalities among the 1961 brood appear to have been due to natural causes. Spawning Success New broods were produced in all ponds in all years, and reproduction was probably restricted to survivors of the original stock of adults. Table 42 shows that 10.547 Age O bullheads were recovered in the control pond in October, 1961, while recoveries of Age O bullheads in October 1963 ranged from a low of 497 in the add-stock pop- ulation to 1,065 in the cropped pond. The numbers of Age O fish were highest where numbers of older fish were lowest, indicating a relationship to total density. Review and Discussion of Brown Bullhead Data No records of standing crops of brown bullheads could be found for comparison with ours, but we found 146 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 four records from populations of single species of black bullheads which are listed in Table 15. All were from small Iowa ponds for which the standing crops were estimated by mark and re- capture procedures or from the return of marked fish among those killed with rotenone. These estimated standing crops ranged from 128 to 653 pounds per acre, with an average for four popu- lations of about 300 pounds per acre. It should be noted that this average was more than three times the maximum standing crop of brown bullheads pro- duced in our ponds. It is also of interest that bullhead production ranked lowest for any species maintained in our ponds. We must therefore conclude that the Unit 5 ponds were not only extremely infertile, but probably afforded extreme- ly poor habitat for this species. Forbes & Richardson (1920) found that Illinois brown bullheads showed a preference for glacial lakes, lowland lakes, and larger rivers, in that order, and that they grew best in weedy ponds or quiet rivers. Trautman (1957) found Ohio specimens occur- ring most commonly in deep, clear ponds having some vegetation, preferring deeper water than either the yellow or black bullhead, and being less tolerant of turbid waters than the black bull- head. Limiting factors in our ponds probably included their small size and shallowness, the complete absence of weeds, and their moderate turbidity, which most commonly ranged 15-25 inches of Secchi disc visibility. WHITE CRAPPIES White crappies, although not one of the five principal species studied over the 3-year period 1961-1963, were used in two short-term experiments in ponds Kappa, Lambda, and Rho (Unit 4) in the period 1958-1960. The Unit 4 ponds had been completed in 1957, and in May of 1958 each pond was stocked with six male and five female white crappies in breeding condition. Lengths ranged from 7.2 to 9.3 inches, and total weight of the 1 1 fish released in each pond was about 3 pounds. All were collected by boat shocker from Fox Lake in Lake County, Illinois. By early July of 1958 we became convinced that no fish remained alive in pond Lambda. Both ponds Lambda and Rho had been treated with rotenone on May 20, 1958 to remove green sun- fish. Tests with live fish in cages indi- cated that both ponds were free of rote- none on May 27, and they were stocked with crappies on May 28. Despite evi- dence to the contrary, sufficient rotenone must have remained in pond Lambda to kill the breeding stock. Therefore, be- tween July 7 and 22 1,750 young-of- the-year and 3 adults were transferred to Lambda from each companion pond so that Lambda received 3,500 young and 6 adults, having a total weight of about 4 pounds. There were no addi- tional stock transfers, and all ponds were drained and censused in October. All ponds were kept dry over the winter of 1958-1959 while a blanket of clay was spread over the base of the dams to help reduce seepage. Between May 7 and 14, 1959, each pond was stocked with 85 yearlings and from 10 to 12 adults, for a total weight in each pond of about 7.5 pounds. The year- lings were from our 1958 stock and the adults were new stock also obtained from Fox Lake. Over the 1959 and 1960 growing seasons pond Rho was maintained as a control and pond Kappa received crappies cropped and trans- ferred from pond Lambda. Cropping was moderate in both years: 8.2 pounds in 1959, of which 7.9 pounds were re- leased in Kappa and 22.8 pounds in 1960, of which 19.6 pounds were re- leased in Kappa. All three ponds were drained and censused in October, 1960. Standing Crop Data Table 44 presents the standing crop totals for six populations of white crap- pies, together with pounds of original Mar. 1970 Buck & Thoits: One-Spf.cies Populations of Fishes 147 CO CC COCC 00 00 OsQS e S g -++ CO Tf CO CO CO 00 ^plK^fM C^ (M COGOOOO OO lO lO lO CO CO CO Oi 05 O Oi Ci O 2 S S o. S Stock, pounds gained, and other perti- nent supplementary data. Table 45 pre- sents the numbers in each age group and the percents of total numbers and weights of each group recovered in each draining census. Actual standing crops in 1958 were 37, 43.4, and 35.5 pounds of crappies in ponds Kappa, Lambda, and Rho, re- spectively. However, since pond Rho had a greater loss of water through seep- age, and an increasingly smaller pond area, its actual production in terms of pounds per acre was probably quite similar to that of pond Lambda. Of special interest is the fact that pond Lambda, restocked in mid-July, produced the largest standing crop. From final census data it was clear that both weight and numbers of fish stocked in Lambda in July were only small per- centages of those existing in the two companion ponds at that date. It is also clear that this smaller midsummer stock had only about one-half of the growing season in which to surpass total produc- tion in the companion ponds. Three possible explanations may be suggested: 1 . Pond Lambda was much the most productive; or 2. The lesser abundance in pond Lambda permitted a more efficient conversion of available food; or 3. In the absence of fish there had been an accumulation of food which imparted a marked advan- tage when fish were stocked in mid-July. Since no environmental differences were observed which would indicate a greater productivity for pond Lambda, we are inclined to favor the second and/or third of these possibilities. It may have been a combination of the two, either of which could have im- portant connotations. Also of interest is the fact that 3,140 of the mid-July stock of 3,500 in pond Lambda were recovered in October. This was a survival of about 90 percent which seems unusually high for fish of 148 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 "S- s ft. § 5 s oo>raoo(McoO * ^ O5C0^ CO cc t^'^w~(>r -f "O -f to CO o O O O O 1^ CD O O O -f* Cl CO -rocoooira O-f cO T-" o oocoi--'" o oo + I o CO CO tx o o o lO iO »0 CO CO CO C; O C: O Ci O this age. A high availability of accumu- lated food may have influenced survival. Over the period 1959-1960 the actual production of fish flesh (pounds gained) was lowest (35 pounds) in the pond which received additions of stock, but only slightly higher in the cropped pond (87.5 pounds) than in the control pond (82.1 pounds). At the same time stand- ing crops were similar in the cropped and add-stock ponds at 64.5 and 69.7 pounds respectively, but considerably the highest (89.6 pounds) in the control pond. These data suggest that 1 ) ponds Kappa (add-stock) and Lambda (cropped) were similar in their poten- tials, and that "pounds gained" were high (87.5 pounds in Lambda) or low (35 pounds in Kappa) in order to at- tain the presumed carrying capacity of 65-70 pounds, and 2) the control pond Rho, with its relatively large pounds gained figure and a final standing crop higher than that of the add-stock pond Kappa, had a considerably higher pro- ductivity than either companion pond. While both of these observations may be true, there were differences in total densities and in the size and age struc- tures of the populations that probably had some influence upon production. All populations produced strong year- classes in 1959, but in 1960 young were recovered only from pond Rho. The low production (pounds gained) of crap- pies in pond Kappa was associated with the largest total number of fish (4,200) but the poorest distribution of sizes and ages, with over 98 percent by numbers and 91 percent by weight concentrated in Age I over a length range of only about 2.5 inches (2.5-5). The high pro- duction in pond Rho was associated with the smallest total number of fish in the final census (2,613), with the fish more evenly distributed in three age groups (O, I, and II) over a length range of about 6.3 inches (1.7-8). We may note also in Table 46 that growth of all ages of crappies was fastest in the control pond Rho. Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 149 Table 46. — Growth of white crapples In ponds used for carrying capacity experiments on the McGraw Foundation grounds, along with sinnilar data on white crappies in other Illinois waters. Illinois Natural History Survey Bulletin Vol. 30, Art. 2 150 This contamination was limited to 2 adult and approximately 800 young green sunfish (total weight, 7 pounds) recovered in the draining census of pond Kappa in 1960. There is some question, of course, as to whether the presence of the green sunfish may have increased or inhibited production of crappies. In this case the contaminants were small enough to have provided forage for the crappies, but too small to have preyed upon crappies. It is doubtful that competition was great, or that absence of the 7 pounds of green sunfish would have permitted production of 7 additional pounds of crappies. It seems likely that the rather small weight of green sunfish had little influence upon crappie production. There is little known basis for evalu- ating the influence of differing weights of tadpoles and crayfish upon fish pro- duction. Both would probably contribute to the production of largemouth bass, but might inhibit production of such less predatious forms as bluegills or small crappies. While small tadpoles or cray- fish might be eaten by bluegills or crap- pies, larger forms might offer competi- tion. Although the competition might not be direct, one must suspect that pro- duction of a large weight of crayfish, and/or tadpoles must divert energy that might otherwise contribute to fish pro- duction. Crayfish might have a secon- dary influence when so abundant as to eliminate weeds or algae. Evaluation may be additionally complicated by the sudden death or disappearance of these forms. We have observed large die-offs of crayfish for reasons unknown. Tad- poles may transform sooner in one pond than in another so that ponds yielding greatly disparate weights at time of cen- sus may have had similar weights one week earlier. So the importance of these forms is difficuh to evaluate in the light of present knowledge. However, there are several associations to be noted in Table 47. Densities of vegetation (chara) in these ponds at the time of final census in 1963 were inversely cor- related with standing crops of crayfish about 128 pounds of crayfish with no chara, 69 pounds with "medium" chara, and only a trace of crayfish where chara was heavy. The degree of control seemed related to density of crayfish, and such control could have consider- able influence upon a pond's ecology. In this instance the highest production of crappies was associated with a scarcity of crayfish and the densest stand of chara. In the 1958 census of these ponds neither crayfish nor vegetation of any type were present. In that year the high- est production of fish was associated with the greatest standing crop of tad- poles. Growth and Condition White crappie broods were not given distinctive fin clips as were those of our other species, and ages were assigned on the basis of length frequency distribu- tions. Thus, ages of crappies were less precisely known than those of our other species, but probably sufficiently so for the comparisons offered in Table 46. These data show that growth of white crappies as single species in our infertile ponds was slower than growth made by crappies from mixed populations in other Illinois waters with which they are compared (Table 46). Totals cropped and transferred were too small to alter the original differences in densities. The control pond, Rho, with the fewest fish older than Age O in the final census of 1960 (Table 45), had the largest 2- and 3-year old fish in that year, and pond Kappa, with the largest numbers, had the smallest fish of these ages (Table 46). This is further shown by compara- tive length increments made by Age 1 fish in 1960. Based on average lengths of samples collected in June and Octo- ber, length increments were highest (1.59 inches) in the pond having the least density of fish (Rho) and lowest (1.09 inches) in the pond having th« highest density of stock (Kappa). Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 151 Samples taken during the growing season were too few and too small to provide seasonal curves for growth and condition for these populations, but we may compare conditions of a few fish from each pond at the time of final cen- suses: 21 from Kappa ranging from 4 to 1 1 inches in length, 25 from Lambda from 6 to 10 inches long, and 80 from Rho from 6 to 12 inches long. Condi- tions in the add-stock pond Kappa aver- aged low (3.5), the cropped pond high (4.3), and the control pond intermedi- ate (3.9). These rankings, however, did not conform with those of rates of growth, which were intermediate in the cropped pond with its denser population and high in the control pond with the lower population density (Table 46). As with some of our earlier species, rates of condition would here be poor indicators of comparative rates of linear growth. K/lortalities We have already mentioned the re- markably low mortality among the Age O crappies that were transferred from ponds Kappa and Rho to pond Lambda in mid-July, 1958. When censused on October 1, 3,140 of the original 3,500 were recovered, a mortality of only about 10 percent over an approximately 2 Vi -month period. Mortality of adults over the same period was 3 of 8, or about 37 percent. The crappie broods were not identi- fied by distinctive clips as were those of our other species, and data for crappies is therefore limited. Our only addition- al data were derived from an attempt in 1960 to measure seasonal rates of mor- talities for Age I crappies given a dis- tinctive fin clip for each of the three summer months. Approximately 100 were marked in each pond in June, July, and August, and returns were counted in the October census. The results were extremely erratic and indicated that mor- talities from handUng and marking were sometimes large and subject to large variations. For example, mortahties by October of Age I crappies were 77, 67, and 47 percent for those marked in June, 98, 100, and 43 percent for those marked in July, and 98, 100, and 93 percent for those marked in August, in ponds Kappa, Lambda, and Rho re- spectively. Spawning Success Reproduction was successful in ponds Kappa and Rho in 1958 and was absent in pond Lambda due to the elimination of brood stock by rotenone. Recoveries in October of young spawned in these ponds were 37,006 from pond Rho and 8,904 from pond Kappa. We have no knowledge of what caused such a wide difference in numbers of young pro- duced in 1958. Numbers of Age I crappies recovered in the 1960 census (Table 45) show that all three populations produced mod- erately large broods in 1959. In 1960, however, young-of-the-year were recov- ered only from pond Rho. Environmen- tal factors which also showed marked variations in these ponds in 1960 were the abundances of older crappies, and the abundances of crayfish and chara. Age O crappies were present 1 ) where numbers of older crappies, particularly those of Age L were least abundant (Table 45), 2) where crayfish were ab- sent, and 3 ) where chara was the most abundant. Thus, the absence of Age O crappies in the two companion ponds may have been due to greater predation by greater numbers of older fish in waters having less chara and less pro- tective cover. Predation of eggs or lar- vae of crappies by the abundant cray- fish populations might also have been a factor. Review and Discussion of White Crappie Data The white crappie has been found to be undesirable in small ponds (Hall et al. 1954; Jenkins 1958; and others) be- cause of overcrowding and slow growth. So far as is known it has not previously 152 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 been used as a single species, but has produced large poundages in mixed populations. Jenkins (1958) reported an average standing crop of 72 pounds per acre of white crappies in mixed populations in 18 Oklahoma ponds, and a maximum standing crop of 205 pounds per acre when combined with black bullheads (107 pounds per acre) and mixed sunfishes (15 pounds per acre). The species finds its greatest value as a sport fish in larger lakes and reservoirs where catch rates are sometimes quite high. Buck & Cross (1951) reported a large catch from a 5,000-acre Oklahoma reservoir where the crappies were at- tracted by a temperature gradient. Over a 69-day period in late winter the hook- and-line catch totaled 626,897 crappies weighing 194,338 pounds which com- puted to approximately 39 pounds per acre for the entire 5,000 acres. A ma- jority of this total was removed from a single cove in which the entrance of warmer water was creating a mild temperature gradient. Total catch from this 3-acre cove over the 69-day period was at a rate of over 22 tons per acre, practically all from a single year-class. In an earlier section we noted that subsequent stocking of bluegills in these same ponds (Kappa, Lambda, and Rho) produced considerably larger standing crops than had been obtained from white crappies. Maximum standing crops in pounds per acre were 89.6 for crappies and 173.3 for bluegills. Com- paring only control ponds, the pound- ages were 89.6 for crappies and 127.1 for bluegills. Such a difference indicates a lesser efficiency for white crappies, due probably to their subsistence at a higher trophic level. As earlier observed for bluegills when stocked in the same unit of ponds, high- est total production of crappies was not made in the pond containing the most lish. This encourages the speculation that there is an optimum density for maximum production. This optimum number would include enough individu- als of proper sizes to obtain efficient utihzation of the pond's resources, but they would not be so abundant as to stunt their own growth. We must also recognize that maximum production of fishes is dependent on high production of their invertebrate foods. Maximum production of invertebrates would occur under an optimum rate of exploitation. The optimum density of fishes might in some cases be that which provides opti- mum cropping and maximum production of the invertebrates used as food. Such optimum synchronization would result in maximum fish production. Variations in the degree of synchronization would produce variations in fish production. GENERAL DISCUSSION Table 48 summarizes standing crop data presented in earlier sections. For each species it includes an average standing crop for all ponds receiving each treatment, and in the last column an average for all populations in all seasons, regardless of treatment. In this last column of the table we can observe that standing crops of bluegills narrow- ly exceeded those for yellow perch, which were followed by smallmouth bass, largemouth bass, white crappies, and brown bullheads in that order. We must emphasize, however, that because of differences in environments this order of rank does not necessarily reflect the relative efficiencies of these species. Its principal value is to indicate the poten- tial for variation in standing crops of the individual species. Direct compari- sons between species have been limited to those instances where standing crops of two different species were measured in the same ponds in consecutive years. As detailed earlier, such comparisons in- dicated that smallmouth bass were more efficient than largemouth bass, that yel- low perch produced larger poundages than either largemouths or smallmouths, and that weights of bluegills were larger than those earlier produced by white crappies in the same ponds. Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 153 Table 48. — Average standing crops (rounded) in pounds per acre for the six species oi varnnwater fishes studied as single species in l-acre ponds, 1957-1963. Standing Crop for All Ponds Species Receim 154 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 1 1-inch individuals, and these larger fish seemed as efficient in utilizing the small invertebrates as were the young-of-the- year. In contrast, standing crops of largemouth bass declined progressively as the populations became increasingly dominated by largemouth of these same intermediate sizes. The smallmouth was unique in that high production was not dependent upon large numbers of both small and large fish. Data from Unit 1 indicated that when sufficient numbers were present to make efficient use of an abundance of invertebrates, size of the bass was relatively unimportant, and the absence of young, small fish might cause little or no loss in total production of fish flesh. When larger fish (about 11 inches and above) were present, how- ever, their growth was inhibited when neither crayfish nor smaller bass were available as prey. Standing crops of smallmouth bass measured in the fall exceeded those esti- mated in the spring. For the control and add-stock populations in Unit 4, three standing crops estimated in the spring ranged from 55.4 to 64.5 pounds per acre, averaging 58.8 pounds and three measured in the fall ranged from 84.9 to 95.5, averaging 91.2 pounds. Thus the spring standing crops averaged only about 64 percent as large as those measured in the fall. When numbers of bass were large, as in the add-stock populations, reductions to the spring levels were brought about by large over- winter mortalities, chiefly among the older and larger individuals. Where fish were less abundant, as in the control pond, the low spring level was due more to a failure of the older individuals to add flesh, in spite of large reductions made in their numbers, and their having eaten major portions of the smaUer members of the population. Carrying capacities were higher in Unit 4 ponds in the fall of the third season than at any previous time due to improved fer- tility and greater food production in the ponds. Higher standing crops at this time were believed also due in part to a more optimum distribution of sizes and ages in the population than when former- ly dominated by older and larger indi- viduals of the 1958 brood. Manipulations of stock produced dif- fering levels of population densities, and these influenced the populations in a number of important ways. In general, rates of growth and condition were poor- est, rates of reproduction were lowest, and rates of mortality were highest in those populations having the greatest densities of stock. Fastest rates of growth, however, were not always ac- companied by highest rates of condition, and comparative rates of either fre- quently were poor indicators of compar- ative rates of the other. Our largest standing crops, and pre- sumably our most efficient populations of largemouth were those containing large numbers of both small and large fish. We believe that when largemouth populations contained few or inade- quate numbers of young-of-the-year, the pond's carrying capacity was not at- tained and much of the pond's resources remained unutilized. Our data from Unit 2 ponds provided evidence that when cropped at a proper rate largemouth populations are cap- able of a seasonal replacement of fish flesh equal to the poundage that the pond can support. Although we ob- tained no tangible evidence of such a replacement potential among our small- mouth populations, we believe that such failure was due primarily to our experi- mental procedures. The replacement potential of smallmouth in our ponds was not adequately tested by proper cropping, but the large standing crops and large gains made in uncropped ponds indicated that such a potential did exist. We suspect that the replace- ment ability of cropped populations of smallmouth would be at least equal to that of the largemouth. The largemouth bass has been quite widely used as a single species in ponds, Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 155 and probably considerably more so than the smallmouth. When so used, the largemouth has sometimes failed to sustain itself due to failures of repro- duction, or of survival of young, and this is believed to be a common failing. In the present experiments, however, young-of-the-year were present in all of 10 largemouth populations and 10 of 11 smallmouth populations at the time of the final censuses. We believe that our data show both species to be cap- able of a high sustained production as a single species in 1-acre ponds, and especially so when subjected to crop- ping. Since bluegills are also known to frequently overpopulate and to eliminate reproduction among coexisting bass, we believe that the chances for a sustained production of bass are as good when stocked alone as when mixed with blue- gills. Absence of a forage species for either of the basses has certain disadvantages. In our populations coefficients of condi- tion were generally poor, fish were never fat, and growth of fish older than Age O was usually slower than for bass in mixed populations. On the other hand, our bass populations indicate that total weights of bass actually could be larger when a bass species is alone than when it is in a mixed population. In addition to their greater total weight and number, bass as single species would, in most cases, be more readily caught than when better fed through preying upon a com- panion species. As mentioned before, Bond and his associates in Oregon (per- sonal communication) found bass fishing to be much superior in ponds where the bass were alone than when they were associated with bluegills. If a pond owner is primarily interested in bass, use of either bass species could prove rewarding. In our experience, however, we would have a slight preference for the smallmouth because it better with- stood marking and frequent handling, suffered fewer mortalities, and was more able to subsist on an invertebrate diet. So far as is known, the yellow perch has not previously been stocked as a single species, and no standing crop data are available for comparison with ours. The species can, however, attain large numbers and weights when in com- bination with other species in fertile waters. In 1920, the 10,000-acre Lake Mendota in Wisconsin was estimated to contain 15,000,000 adult perch (Hasler & Wisby 1958) which, using the aver- age weight of adults caught at that time, projects to a standing crop of about 1 69 pounds per acre of adult perch only. Moyle et al. (1950) have recorded esti- mates as high as 184 pounds per acre in Minnesota game fish lakes, although the mean for 41 such lakes was only 32.1 pounds per acre. Our highest standing crop was 192 pounds per acre as a single species in pond Alpha, which only slightly exceeds the maximums esti- mated for perch in multispecies popula- tions. These data suggest that the perch may be particularly well adapted to its own ecological niche, and relatively little affected by competition from companion species. Perch growth was generally fastest in our cropped population and slowest in our add-stock population, and closely related to density of stock. There was strong evidence, however, that intra- brood density was more critical or con- trolling than total population density, with little apparent competition between perch of different ages or sizes. For ex- ample, perch of the 1962 brood in the add-stock population grew faster than their more numerous counterparts in the control population in spite of the much greater total weight (all age groups combined) of perch in the add-stock population. It was further notable that at the time of the final census in 1963, 2,861 perch of this 1962 brood had an average length (4.7 inches) only about 0.4 inch less than that (5.1) of 728 perch of the 1960 brood that had sur- vived from the original stocking of October, 1960. The 1962 brood of 156 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 perch had made an average length incre- ment of about 3.5 inches in its first year and 1.2 inches in its second year, while the 1960 brood had made a length incre- ment of less than 1 inch over the entire 2-year period. It should be pointed out that num- bers of perch older than the 1962 brood were being continuously augmented by perch transferred from the cropped pond Zeta. However, the total of all perch older than the 1962 brood in the 1963 census numbered only 1,012 as com- pared to the 2,861 in the faster-growing 1962 brood. While the older perch of the 1960 year-class had apparently at- tained a growth limit due to intraclass competition for some element within the environment (probably food), they ob- viously were offering little competition to the much more abundant and faster- growing perch of the 1962 brood. We had at the same time a small group of "cannibal" perch of the 1960 year-class that made excellent growth due to their cannibalistic tendencies. So we had within the add-stock population three distinct groups of perch, probably with different food habits, and which offered relatively little competition to each other: 1 ) the large, fast-growing canni- bals of the 1960 brood, 2) the more abundant perch of the 1960 brood that had attained their growth plateau, obvi- ously subsisting upon an invertebrate diet, but of a different composition than that utilized by the younger perch, and 3) the younger, most abundant and fastest-growing group in the population, which obviously utilized foods not taken by the older fish, and which permitted them to literally grow into the size range of perch that were 2 years older. Some studies have shown the food habits of perch to change to some degree with increase in size. Tharratt (1959) found that immature insects were promi- nent as food for all size groups (2.5 inches and up) of perch in Saginaw Bay, but that mayfly nymphs, which were the largest insects available (pre- sumably Hexagenia sp.), were found only in the stomachs of perch longer than 4.7 inches. Tharratt also found that copepods were the chief constitu- ents in the food of young-of-the-year, but were absent from the stomachs of perch more than 5.4 inches long. On the other hand, Pearse & Achtenberg (1920) and Herman et al. (1959) have observed that adult perch commonly feed on cladocerans, utilizing gill rakers adapted to straining these small forms from the water. It therefore seems quite remarkable that in our studies one age group of perch could find sufficient food to grow well, while an older group of only slightly larger average size ap- peared to have done little more than subsist. We have previously noted that the nymphs of the larger burrowing mayfly, Hexagenia limbata, were appar- ently eliminated from the add-stock pond, while remaining abundant in the companion ponds. Possibly the 1960 brood in pond Theta had developed the habit of feeding almost exclusively on these mayflies and the perch were un- able to change their feeding habits when this food was exhausted. The senior author observed a somewhat similar oc- currence (unpublished) in which a large population of white bass in a large southern reservoir literally starved to death in a year when its principal food (gizzard shad) had a spawning failure, although other forage fishes of similar size were available. The brown bullheads were used only in Unit 5 where they produced the low- est standing crops recorded in this study. We believe that this was because the Unit 5 ponds were the least fertile of our ponds, and because the brown bull- heads were poorly adapted to our weed- less, moderately turbid, moderately shal- low ponds with their soft clay-silt bot- toms. This poor adaptability was em- phasized by the fact that survivors of the original stock of 28 adults failed to increase their average size over the 3- year period of study. There was no evii Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 157 dence of cannibalism in these popula- tions. Although crayfish were usually abundant, and were known to be eaten by the larger bullheads, they obviously did not provide an adequate diet. With no cannibalism, food for all sizes of bullheads was limited primarily to inver- tebrates. It is therefore interesting that there was little apparent competition for food between the Age O fish and those older than Age O. As in the yellow perch populations, competition among bullheads was greater within than be- tween age classes. In this instance Age O bullheads in the add-stock pond (most of the bullheads added were older ones) grew faster than their counterparts in the companion ponds because of their lesser abundance, even though the pop- ulation of fish older than Age O was much larger in the add-stock than in either companion pond. We earlier noted that production of bluegills was greater than that of white crappies earlier maintained in the same ponds. Our standing crops of bluegills were low by common standards (Table 35) due to the low fertility of the ponds in which they were stocked. However, the bluegills produced the highest aver- age standing crops, and were probably the most efficient of any of our six species. Both bluegills and white crappies were stocked in Unit 4, and both ex- hibited the phenomenon whereby a smaller number produced a greater weight than a larger number of the same species. We had observed that our cropped pond Lambda had a larger standing crop (weight) of bluegills than the control pond Rho even though the total population numbered less than one- sixth that of Rho at the time of census. Also, the same association was apparent in the white crappie populations in both 1958 and 1960, with that of 1958 being the most distinct. We may recall that in 1958 the population in pond Lambda was inadvertently destroyed by rotenone, and the pond was restocked in mid-July with a smaller weight and number than were present in either companion pond. By the time of the census in October this smaller number had produced a larger poundage than either companion pond in only about one-half as much time. The differences in standing crops could have been caused by differences in pond fertilities. If so, however, in the case of the crappies pond Lambda would have been enormously more fer- tile than pond Rho in one period and slightly less fertile in the other. Differ- ences in age structures of the popula- tions could also have had some influ- ence. We recognize that there may be other possible explanations, one involv- ing the concept of an optimum density. The effects of an optimum density might be asserted in at least two ways: 1. The density of fishes would be in optimum relation to that of their prey if feeding activities maintained the prey species at population levels at which they most efficiently replaced their own numbers. Thus the maximum food sup- ply would be provided for the predator fishes. Too few fish would make ineffi- cient use of the prey species, whereas too many would cause overexploitation and a temporary collapse of both pred- ator and prey populations. 2. There may be an optimum density which contains enough fishes to make efficient use of available foods, but not so dense as to be self-inhibiting. We know that some fishes attain densities at which they may inhibit their own growth and reproduction. Even when the food supply is not a limiting factor, it may be possible for a smaller number of fish to attain a greater total weight than a larger population so dense that it created an inhibitory barrier. Such a barrier might be either physiological or psychological, and could be more lim- iting upon a species maintained alone than when the fish are in a mixed cul- ture. It it known that certain combina- tions of species may be extremely com- patible and that one may actually en- 158 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 hance production of the other. Data reported by Rose (1959) suggest the engaging possibility that when two unre- lated or distantly related species are combined, one may have the effect of metabolizing away self-inhibiting prod- ucts produced by the other. The influence of total fish densities, of differing densities by size or age group, and the influence of such associ- ated organisms as tadpoles and cray- fish upon fish production are difficult to measure and evaluate in ponds, but may be of great importance. This may be especially true in single species popula- tions. We have seen that in our control pond Rho the standing crops of blue- gills, crayfish and tadpoles were all much higher in October, 1961 than in Octo- ber, 1963. We must therefore wonder if the carrying capacity of pond Rho declined so markedly over that period, or if the greater production of bluegills in 1961 was in some way enhanced by the greater numbers of crayfish and/or tadpoles. Whether it may or may not be influ- enced by an inhibitory factor, an opti- mum density will exist for each popula- tion. In its simplest terms, the amount of new flesh produced in a population is dependent upon three things: 1) the carrying capacity of the pond for that species, 2) the number and weight of fish already present, and 3) the pounds being cropped (or otherwise removed) that are subject to replacement. If the carrying capacity has been attained, and no fish die or are otherwise removed, the fish cannot grow and no new flesh can be produced. On the other hand, the more pounds cropped the more that can be produced in replacement. The most efficient and productive arrange- ment will be one in which the optimum number is cropped, maintaining the pop- ulation continuously below carrying ca- pacity but not reducing the number of fish below the level that can by growth and recruitment efficiently replace those removed, as was done in two of our smallmouth populations. As indicated by studies of smallmouth bass (Bennett & Childers 1957) and by our large- mouth experiments in Unit 2, under an optimum rate of cropping it may be possible to remove more pounds in a season than the pond can support at any time. For each pond there must exist an optimum population density, and an optimum rate of cropping, and these will differ and fluctuate as the pro- ductive potential of individual ponds must also differ and fluctuate. The rates of production to be obtained will be the result of the interaction of these complex and unstable forces, and the efficiency of production will be deter- mined by how nearly the optimum rates of cropping and density can be main- tained. A primary purpose of this investiga- tion was to consider the relationship of carrying capacity to standing crop. In doing so we should again clarify our usage of the two terms. Standing crop is universally recognized as the quantity of organisms present at the particular time of measurement, and is so used here. As defined by Krumholz et al. (1957), carrying capacity was con- sidered to be that quantity surviving through the least favorable environ- mental conditions over a stated interval of time. By such terms it automatically becomes a minimum quantity. There are, however, many practical reasons for measuring maximum rather than mini- mum levels of abundance. Many re- searchers and commercial fish growers refer to their maximum standing crops as the carrying capacity of the unit in- volved. They interpret this to mean the maximum poundage that the pond can produce, or support, of the number and type of organisms involved, and under the environmental conditions that exist- ed or were maintained. This usage is now very widely accepted. For our present purposes we have recognized what was termed a temporary carrying capacity by Edwards & Fowle (1955). Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 159 We have measured standing crops in both spring and fall and have evaluated these in terms of carrying capacities, recognizing that the poundage of fish supported by a pond in late winter or early spring may be quite dilTerent from the poundage that the same unit might support in late summer or fall. In evaluating carrying capacity it is important to come to an understanding of its relative stability. Carrying capacity is sometimes conceived as being a rela- tively stable quantity, changing little from year to year. This is tantamount to saying that the environment itself is not subject to change. We recognize a num- ber of factors other than changes in the environment as being capable of influ- encing the standing crop, but we believe that our data have shown that the pond environments were relatively unstable, and that variations in standing crops were frequently due to changes in the environments and in the ponds" carry- ing capacities. Changes in carrying capacities of the ponds were believed to have had two principal causes: 1) changes in such physical elements of the environment as inorganic fertility, turbidity, or type and density of vegetation, and 2) changes in densities of associated invertebrates, in- cluding those utilized as food, such as scuds or mayfly nymphs, or those hav- ing a potential for either inhibiting or enhancing the production of fish, such as crayfish. In working with our data, standing crops frequently were judged to have been either larger or smaller than the pond's carrying capacity. When larger, it was because of excessive additions of stock, and the excess was indicated by a loss in the pounds gained or lost column. We could not, of course, know when the point of equilibrium was reached and when additional losses would or would not occur. However, when a loss had occurred, and when additions of stock had been quite continuous up to near the time of census, we assumed that equalization was still in progress and that the standing crop probably still exceeded the pond's carrying capacity. It was more difficult to make a judg- ment as to when the standing crop was at or below the pond's carrying capacity. If the standing crop was medium to large, the pounds gained not overly large, and the population structure reas- onably normal, we felt that the standing crop probably approximated carrying capacity. If the pounds gained figure was unusually large the question arose as to whether the carrying capacity was also unusually large and may not yet have been attained. We suspected our stand- ing crops to be below carrying capacity when they were small and when total numbers of fish were small, or when one or more size or age groups of the popu- lation were inordinately small, or absent, due either to heavy cropping, excessive cannibalism, or reproductive failures. Among smallmouth bass, however, we found that absence of Age O fish was of little importance if a sufficient num- ber of intermediate sized smallmouth were available to make efficient use of the invertebrate food supply. While the standing crop is not neces- sarily a measure of carrying capacity the two are intimately related, and the former is limited by the latter. Both must be considered dynamic quantities dependent upon the interaction of all forces within the environment. As an expression of these forces, carrying capacity must also ffuctuate, and can have no greater stability than that of the environment. As a finite quantity, carrying capacity may be extremely diffi- cult to measure exactly, and might only by chance be the same at any two points in time. Standing crop might be con- ceived as the "tail on the kite," always tending to follow, either up or down, the movement of the controlling body. LITERATURE CITED Ball. Robert C. and Don W. Hayne. 1952. Effects of the removal of the fish popula- tion on the fish-food organisms of a lake. Ecology 33(11:41-48. Beckman. William C. 1949. The rate of growth and sex ratio for seven Michigan fishes. American Fisheries Society Trans- actions for 1946, 76:63-81. Bennett, George W. 1948. The bass-blue- gill combination in a small artificial lake. Illinois Natural History Survey Bulletm 24(3):377-412. . 1954. Largemouth bass in Ridge Lake, Coles County, Illinois. Illinois Nat- ural History Survey Bulletin 26(2) :217- 276. -. 1962. Management of artificial lakes and ponds. Reinhold Publishing Corpora- tion, New York. 283 p. -, and William F. Childers. 1957. The smallmouth bass, Microptems dolo- mieui, in warm-water ponds. Journal of Wildlife Management 21(4):414-424. Brown, William H. 1951. Results of stock- ing largemouth black bass and channel cat- fish in experimental Texas farm ponds. American Fisheries Society Transactions for 1950, 80:210-217. Brynildson, Clifford L., and John R. Truog. 1959. Fish management of Wis- consin farm ponds. Wisconsin Conserva- tion Bulletin 24(11 ):l-4. Buck, D. Homer, and Frank Cross. 1951. Early limnological and fish population con- ditions of Canton Reservoir, Oklahoma, and fishery management recommendations. Report to the Oklahoma Fish and Game Council, reproduced by The Research Foundation, Oklahoma Agricultural and Mechanical College, Stillwater. 174 p. and Maurice Whitacre. 1960. A new method and a new material for screen- ing fish. Progressive Fish-Culturist 22(3): 141-143. and Charles F. Thoits III. 1965. An evaluation of Petersen estimation proced ures employing seines in 1-acre ponds. Journal of Wildlife Management 29(3): 598-621. Carlander, Kenneth D. 1950. Handbook of freshwater fishery biology. Wm. C. Brown Company, Dubuque, Iowa. 281 p. and Robert B. Moorman. 1956. Standing crops of fish in Iowa ponds. Iowa Academy of Science Proceedings 63:659- 668. Cooper, Edwin L., Herbert Hidu, and John K. Andersen. 1963. Growth and produc- tion of largemouth bass in a small pond. American Fisheries Society Transactions 92(4):391-400. Cross, Frank B. 1967. Handbook of fishes of Kansas. Museum of Natural History, University of Kansas, Lawrence. Miscel- laneous Publication 45. 357 p. Durham, Leon.\rd. 1955. Ecological factors affecting the growth of smallmouth bass and longear sunfish in Jordan Creek. Illi- nois Academy of Science Transactions for 1955, 47:25-34. Doudoroff, Peter, and Max Katz. 1950. Critical review of literature on the toxicity of industrial wastes and their components to fish. I. Alkalies, acids, and inorganic eases. Sewage and Industrial Wastes 22(11):1,432-1,458. Edwards, R. Y., and C. David Fowle. 1955. The concept of carrying capacity. North American Wildlife Conference Transactions 20:589-602. Eggleton, Frank E. 1931. A limnological study of the profundal bottom fauna of certain fresh-water lakes. Ecological Mono- graphs 1:231-331. EiPPER, Alfred W. 1964. Growth, mortality rates, and standing crops of trout in New York farm ponds. Cornell University Agri- cultural Experiment Station Memoir 388. 67 p. Forbes, Stephen Alfred, and Robert Earl Richardson. 1920. The fishes of Illinois. Second ed. Illinois Natural History Survey, Urbana. 357 p. Forney, John L. 1955. Life history of the black bullhead, Ameiunis melas (Rafines- que), of Clear Lake, Iowa. Iowa State College Journal of Science 30(1):145-162. Hall, Gordon E., Robert M. Jenkins, and Joe C. Finnell. 1954. The influence of environmental conditions upon the growth of white crappie and black crappie in Oklahoma waters. Oklahoma Fisheries Re- search Laboratory Report 40. 56 p. Hansen, Donald F. 1951. Biology of the white crappie in Illinois. Illinois Natural History Survey Bulletin 25(4) :21 1-265. Hasler, Arthur D., and Warren J. Wisby. 1958. Perch and lake research on Mendota. Wisconsin Conservation Bulletin 23(3): 1-5. Herman, Elmer, Warren Wisby, Lawrence WiEGERT, and Milton Burdick. 1959. The yellow perch, its life history, ecology and management. Wisconsin Conservation De- partment Publication 228. 14 p. 160 Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 161 Hopkins. Cyril G.. J. G. Mosier. E. Van Alstine, and F. W. Garrett. 1917. Kane County Soils. University of Illinois Agri- cultural Experiment Station Soil Report 17. 60 p. HousER, Alfred, and Charles Collins. 1962. Growth of black bullhead catfish in Oklahoma. Oklahoma Fishery Research Laboratory Report 79. 18 p. Isaac, Gary W., and Carl E. Bond. 1963. Standing crops of fish in Oregon farm ponds. American Fisheries Society Trans- actions 92(l):25-29. Jenkins, Robert M. 1958. The standing crops of fish in Oklahoma ponds. Pro- ceedings of the Oklahoma Academy of Sci- ence for 1957, 38:157-172. , and Gordon E. Hall. 1953. The in- fluence of size, age, and condition of waters upon the growth of largemouth bass in Oklahoma. Oklahoma Fisheries Research Laboratory Report 30. 43 p. Jobes, Frank W. 1952. Age, growth, and production of yellow perch in Lake Erie. [U.S.] Fish and Wildlife Service Fishery Bulletin 70, 52:205-266. Krumholz, Louis A. 1948. Variations in size and composition of fish populations in recently stocked ponds. Ecology 29(4): 401-414. , Robert W. Darrow, Oliver H. Hewitt, and Edward L. Kozickv. 1957. Glossary of wildlife terms. Journal of Wild- life Management 21(3) :373-376. Lewis, William M, 1950. Fisheries investi- gations on two artificial lakes in southern Iowa II. Fish populations. Iowa State Col- lege Journal of Science 24(3 ) :287-323. Moyle, John B., Jerome H. Kuehn, and Charles R. Burrows. 1950. Fish-popula- tion and catch data from Minnesota lakes. American Fisheries Society Transactions for 1948, 78:163-175. Mraz, Donald. 1964. Observations on large and smallmouth bass nesting and early life history. Wisconsin Conservation Depart- ment Research Report 11 (Fisheries). 13 p. Pearse, a. S.. and Henrietta Achtenberg. 1920. Habits of yellow perch in Wisconsin lakes. [U.S.] Bureau of Fisheries Bulletin, Document 885, 36:297-366. Price, O. M. 1966. A summary of eleven years of fishery management on Red Hills Lake. 1953 to 1963. Illinois Department of Conservation Special Fisheries Report 12:34-45. Regier, Henry Abraham. 1962. Some as- pects of the ecology and management of warm-water fish in New York farm ponds. Ph.D. Thesis. Cornell University, Ithaca, New York. 419 p. Ricker, W. E. 1954. Stock and recruitment. Journal of the Fisheries Research Board of Canada 11:559-623. 1958. Handbook of computations for biological statistics of fish populations. Fisheries Research Board of Canada Bulle- tin 119. 300 p. Rock, Leo F. 1966. A summary of nine years of fishery management on Johnson Saulk Trail Lake, 1956 to 1964. Illinois Department of Conservation Special Fish- eries Report 12:2-14. . 1966. A summary of eight years of fishery management on Siloam Springs Lake, 1955 to 1962. Illinois Department of Conservation Special Fisheries Report 12:15-23. Rose, S. Meryl. 1959. Population control in guppies. American Midland Naturalist 62 (2):474-481. Ruttner, Franz. 1953. Fundamenta's of Limnology. University of Toronto Press. 242 p. Snow, Howard. Arthur Ensign, and John Klingbiel. 1960. The bluegill, its life history, ecology and management. Wiscon- sin Conservation Department Publication 230. 14 p. Starrett, William C. and Arnold W. Fritz. 1965. A biological investigation of the fishes of Lake Chautauqua, Illinois. Illinois Natural History Survey Bulletin 29(1):1-104. State of Illinois. 1958. Atlas of Illinois Re- sources. Section I, Water Resources and Climate. 58 p. Stinauer, Rudy. 1966. A summary of ten years of fishery management on Ramsey Lake, 1953 to 1962. Illinois Department of Conservation Special Fisheries Report 12: 24-33. Stroud, Richard H. 1948. Growth of the basses and black crappie in Norris Reser- voir, Tennessee. Journal of the Tennessee Academy of Science 23(l):31-99. Swingle. H. S. 1949. Some recent develop- ments in pond management. North Ameri- can Wildlife Conference Transactions 14: 295-312. 1950. Relationships and dynamics of balanced and unbalanced fish popula- tions. Alabama Polytechnic Institute Agri- cultural Experiment Station Bulletin 274. 74 p. 162 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 . 1952. Farm pond investigations in Alabama. Journal of Wildlife Manage- ment 16(3):243-249, . 1957. Commercial Production of red cats (speckled bulUieads) in ponds. Conference of Southeastern Association of Game and Fish Commissioners Proceedings for 1956, 10:156-160. Swingle, H. S., and E. V. Smith. 1943. Factors affecting the reproduction of blue- gill bream and largemoulh black bass in ponds. Alabama Polytechnic Institute Fx- periment Station Circular 87. 8 p. TuARRATT, Robert C. 1959. Food of yellow perch, Perca fiavesccns ( Mitchill i in Sagi- naw Bay, Lake Huron. American Fisher- ies Society Transactions for 1959, 88: 330-331. THOMrsoN, David H., and George W. Ben- nett. 1939. Lake management reports. 3. Lincoln Lakes near Lincoln, Illinois. Illi- nois Natural History Survey Biological Notes 11. 24 p. Trautman, Milton B. 1957. The fishes of Ohio. Ohio Slate University Press. 683 p. Turner, William R. 1959. The standing crops of fishes in twenty-two Kentucky farm ponds. Kentucky Department of Fish and Wildlife Resources, Federal Aid to Fisheries, Project F-IO-R Final Report 21 p INDEX "Add-stock" pond, 73 Anachaiis canadensis, 73 Aquatic plants, 73 control of by crayfish, 129, 150 control of with endothal, 114 density of, 72-73 influence of on spawning success, 128 variations in occurrence, 72-73 Arconectes viiilis. 129 Area of study, 71 B Bass, largemouth. 69, 98-118 biological efficiency, 103-104 biological efficiency compared with that of^smallmouth, 79, 115-117, 152-154 biological efficiency compared with that of yellow perch, 121, 130 condition, 105-110 effects of cropping on, 99, 103 fishing success for, 155 growth, 105-110 in Oregon, 115, 155 in Wisconsin, 1 15 mortalities, 104, 110-113 population compositions, 116, 154 production, 103 spawning, 113-1 15 standing crops, 97-101, 103-105, 115-116 stocking as single species, 154-155 stocking for present studies, 98 variations in standing crops, 99-101, 104-105 Bass, smallmouth, 69, 75-97 biological efficiency, 79, 153, 155 biological efficiency compared with that of largemouth, 90, 115-117, 152-154 biological efficiency compared with that of yellow perch, 121, 130-131 condition, 83-85 counts of fry in nests, 90-92 counts of nests, 91 counts of successful broods, 90-92 eft'ects of cropping on, 83-84 foods, 92-96, 153 growth, 84-85 mortalities, 87-89 population compositions, 79-80, 82, 116, 153 spawning, 90-92, 153 standing crops, 76-83, 96-97, 115-116 stocking as single species, 154-155 stocking for present studies, 75 variations in standing crops, 77-78 Bluegills, 69, 131-140 biological efficiency, 140 biological efficiency compared with that of white crappies, 140, 152, 157 condition, 134-137 effects of cropping on, 134 growth, 134-137 mortalities, 134, 137-138 production. 133 spawning, 139 standing" crops, 97, 131-133, 139-140 stocking for present studies, 131 Brown bullheads (sec bullheads) Bullheads, black, 97, 146 growth, 142 standing crops, 97, 146 Bullheads, brown, 69, 140-146, 156-157 condition, 143-145 effects of cropping on, 141 growth, 142-145 growth of in winter, 143-145 mortalities, 145 population compositions, 141 spawning. 145 standing crops, 141-142 stocking for present studies, 140 Cannibalism among fish bluegills, 139 bullheads, 145 largemouths, 104, 112-113, 116 smallmouths, 79, 93, 96, 116 white crappies, 151 yellow perch, 123, 126, 128, 156 Carrying capacity changes in, 82-83, 104-105, 117, 133, 154, 159 definition, 69-70, 158 evolution of concept, 69-70 exceeding of, 81, 101, 104-105, 115-116 factors influencing, 117, 133 in relation to standing crop, 69-70, 158- 159 in the fall. 81-82, 104-105, 154 in the spring, 81-82, 104-105, 154 maximum for largemouths, 115-116 maximum for smallmouths, 77-78 stability of, 70, 159 Censusing fish populations, 74 invertebrates, 94 Chain pickerel, 73, 82 Chubsuckers, lake, 82 Competition bluegills. between year classes, 137 bullheads, between size groups, 142, 145, 157 fish versus crayfish, 150 fish versus tadpoles, 150 largemouths, between size groups, 104 smallmouths, between size groups, 79 yellow perch, in populations, 125, 155- 156 163 164 Illinois Natural History Survey Bulletin Vol. 30, Art. 2 Condition of fish (see also iiiuler individual fish species), 74 as indicator of growth. 151. 154 curves. 85-86. 108-109. 136, 144 decline in. 83, 89 fluctuations in, 109, 143 improvement ot, 84-87 relation of to density, 85-86, 105-109, 123-126, 134-137, 151 Contamination by unwanted fishes, influence of, 98. 149-150 "Control" pond. 73 Crappies, white, 69, 146-152 biological eflfjciency, 152 biological efficiency compared with that of bluegills, 152, 157 condition. 150-151 effects of cropping on. 148-150 fishing success for, 152 growth. 150-151 in an Oklahoma reservoir, 152 mortalities, 151 optimum density of, 149, 152 population compositions, 148 spawning. 151 standing crops, 146-150 stocking for present studies, 146 Crayfish, 82, 129, 132-133, 145, 149-150 control of by yellow perch, 129 depredation of fish eggs by, 82, 92, 151 effect of on water transparency, 130, 133 influence of on fish, 150, 158 weed control by, 129, 150 "Cropped" pond, 73 Cropping of fish effects of (see also under individual fish species), 86, 103, 105, 108, 119, Mi- ne. 134-135, 154 excessive rates, 82, 104 optimum rate, 105 "Current carrying capacity," 70 Daclylogyrus, 98 Dingell-Johnson program, 71 Drift organisms, 78, 84 Energy, flow of, 70 Experimental design of study, 73-75 Experimental treatments, effects of, 153-154 on bluegill populations, 134 on brown bullhead populations, 141 on largemouth populations, 99, 103 on smallmouth populations, 83-86 on white crappie populations, 148-150 on yellow perch populations, 119-120, 123-125 Fertilization of ponds, 73 Fin clips for marking fish, 74-75 Fin 'n Feather Club, 75 Fishing success, 152 Food(s) eaten by smallmouths, 79, 92-96 of brown bullheads. 157 of yellow perch. 156 Food requirements of largemouth bass, 104, 109 Fry, fish collection, 91 number of per nest, 90-92 production, 90-92 Green sunfish, 149 Growth curves. 85-86, 108-109, 136, 144 of bluegills, 134-137 of brown bullheads, 142-145 of largemoulhs, 105-110 of smallmouths, 84-86 of white crappies, 150-151 of yellow perch, 122-126 relation of to fish density, 84-86 110, 117, 137, 142, 150, 155 relation of to sex, 122-123 H Hexagenia limbata, 129, 156 I Index of condition (sec condition) Inhibitory products, 91-92, 157-158 Invertebrates, 92-94, 129-130, 154 fluctuations in abundance, 129 Lake chubsuckers, 82 Lake Mendota, 155 Largemouth bass (see bass) M McGraw Foundation, 70 McGraw Hydrobiological Laboratory, 70 Marking by fin clips, 74—75 Method of operation, 73-75 Mortalities, fish (see also under individual species) due to handling and marking, 75, 88, 110-112, 134, 138 due to transfer, 88, 110-112, 126-127, 134, 137, 145 overwinter. 104 rates of, 87-89, 110-113, 126-127, 134. 137-138 relation of to age of fish, 110, 138 relation of to density of fish, 87, 89, 117, 127 seasonal, 88-89, 110, 126, 138, 151 Myriophyllum exalbescens, 73 N Najas flexilis, 73 North American Wildlife Foundation, 70 Mar. 1970 Buck & Thoits: One-Species Populations of Fishes 165 Optimum density of fish. 149, 152. 157-158 Optimum rate of cropping fish, 158 Optimum size distribution of fish, 118, 142 R Rana culesheiana. 129 Replacement of fish flesh, 96, 154 Reproduction [see spawning) Rotenone, use of, 98. 132, 146, 151, 157 Parasites. 98 Perch, yellow, 69, 118-131 biolcgical efficiency compared with that of largemouths, 121, 130 biological efficiency compared with that of''smallmouths,'l21, 130-131 condition, 122-126 effects of cropping on, 119, 121-122, 155 foods, 130. 156 growth, 122-126 mortalities, 126-127 population compositions, 119-121 predation by, 123, 126 production, 155 spawning, 128 standing crops, 118-121 stocking for present studies, 118 variations in standing crops, 118-121 Petersen estimation procedures, 74 reliability, 74 use, 74, 77, 81-83, 104 pH, 72. 73. 130 changes in, 1 14 control of, 114 influence of on spawning, 114-115 Pickerel, chain, 73, 82 Plan of study, 73 Ponds construction, 71 description. 71-73 physical and chemical characteristics, 7U73 treatments, 73 Populations, fish composition, 79. 82, 116, 120, 153-154 density, 85, 157 manipulation, response to, 86-87 structure, changes in, 103 structure, imbalance in, 104 thinning. 81 Potamo!>elon. 73 crispus, 73 foliosus, 73 peclinutus, I'i Pounds lost or gained (discussed), 75 Predation, 79, 104, 123, 126 of fish eggs by crayfish, 92, 128 Productivity changes in, 82-83, 133, 140 factors influencing, 78, 158 variations in, 79-81, 152-153 Sampling procedures, 73-74 Smallmouth bass {see bass) Spawning (see also under individual fish species ) artificial aids for in fish, 128 influence of temperature on, 139 failures by bluegills, 139 failures by largemouths, 114-115, 155 failures by smallmouths, 82, 91-92 failures by white crappies. 151 failures by yellow perch, 128 relation of to fish density. 91-92, 114, 128, 145. 151 success of (see under individual species) Standing crops, fish (see also under individual species). 152-153 exceeding carrying capacity in pond, 81, 101, 104-105, 115-116 factors causing variations of, 77-79, 99, 132-133, 153 relation of to size composition in popu- lations. 79, 116, 154 variations of, 77-78, 97, 99-100, 118- 121, 132-133, 139-140. 154 variations from spring to fall, 154 Standing crops, invertebrates, 94 Stomach contents in fish, 79, 92-96 Surplus production of fish, 75, 83 Tadpoles, 129, 133, 149-150 control of by crayfish, 129 influence of on fish, 158 Temperature gradient, influence of, 152 Thermal stratification, 130 Transfer of fish effects of. 86-88, 110, 112, 126-127, 134. 137-138, 145 Turbidity of pond water, 133 w Water exchange, 78 White crappies (see crappie) Yellow perch (see perch) Zannichellia palustris, 73 Some Publications of the ILLINOIS NATURAL HISTORY SURVEY BULLETIN Volume 28, Article 2.—The Fishes of Cham- paign County, Illinois, as Affected by 60 Years of Stream Changes. By R. Weldon Larimore and Philip W. Smith. March, 1963. 84 p., frontis., 70 fig., bibliogr., index. Volume 28, Article 3.—A Comparative Study of Bird Populations in Illinois, 1906-1909 and 1956-1958. By Richard R. Graber and Jean W. Graber. October, 1963. 146 p., 4 frontis., 32 fig., bibliogr., index. Volume 29, Article l..^A Biological Investi- gation of the Fishes of Lake Chautauqua, Illinois. By William C. Starrett and Ar- nold W. Fritz. March, 1965. 104 p., frontis., 40 fig., bibliogr., index. Volume 29, Article 2.—Stocking and Sport Fishing at Lake Glendale (Illinois). By Donald F. Hansen. July, 1966. 54 p., fron- tis., 9 fig., bibliogr., index. Volume 29, Article 3.—Hybridization of Four Species of Sunfishes (Centrarchidae). By William F. Childers. September, 1967. 55 p., frontis., 2 fig., color plate, bibliogr., in- dex. Volume 29, Article 4.—The Thrips, or Thysa- noptera, of Illinois. By Lewis J. Stannard. May, 1968. 338 p., frontis., 310 fig., bib- liogr., index. Volume 30, Article 1.—^Largemouth Bass and Other Fishes in Ridge Lake, Illinois, 1941- 1963. By George W. Bennett, H. Wick- liffe Adkins, and William F. Childers. Sep- tember, 1969. 67 p., 10 fig., bibliogr., in- dex. BIOLOGICAL NOTES 57.—Man's Effect on the Fish and Wildlife of the Illinois River. By Harlow B. Mills, William C. Starrett, and Frank C. Bellrose. Jime, 1966. 24 p., 16 fig., bibliogr. 58,—The Life History of the Slough Darter, Etheostoma gracile (Pisces, Percidae). By Marvin E. Braasch and Philip W. Smith. June, 1967. 12 p., 8 fig., bibliogr. 59.—Tables for Estimating Ages and Birth Dates of Cottontail Rabbits with Sugges- tions for Handling Lenses. By William R. Edwards. December, 1967. 4 p., 2 fig. 60.—Use of Feather Minerals as Biological Tracers to Determine the Breeding and Molting Grounds of Wild Geese. By Har- old C. Hanson and Robert L. Jones. Feb- ruary, 1968. 8 p., 2 fig., bibliogr. 61.—Waterfowl Migration Corridors East of the Rocky Mountains in the United States. By Frank C. Bellrose. June, 1968. 24 p., 6 fig., bibliogr. 62.—Nightlighting: Its Use in Capturing Pheasants, Prairie Chickens, Bobwhites, and Cottontails. By Ronald F. Labisky. Octo- ber, 1968. 12 p., 8 fig., bibliogr. 63.—Selected Minerals in Soils, Plants, and Pheasants: An Ecosystem Approach to Un- derstanding Pheasant Disribution in Illinois. By Robert L. Jones, Ronald F. Labisky, and William L. Anderson. December, 1968. 8 p., 1 fig., bibliogr. 64.—The Value of In Vitro Fungicide Tests. By Dan Neely, January, 1969. 8 p., bibliogr. 65.—Trends in Pheasant Abundance in Illi- nois: 1958 to 1968. By Ronald F. Labisky. May, 1969. 8 p., 4 fig., bibliogr. 66.—Tree and Shrub Hosts of Verticillium Albo-atrum. By E. B. Himelick. July, 1969. 8 p., bibliogr. CIRCULAR 39.—How to Collect and Preserve Insects. By H. H. Ross. November, 1966. (Eighth printing.) 71 p., frontis., 79 fig. 46.—Illinois Trees: Their Diseases. By J. Cedric Carter. June, 1964. (Third Printing, with alterations.) 96 p., frontis., 89 fig. 49.—The Dunesland Heritage of Illinois. By Herbert H. Ross (in cooperation with Illi- nois Department of Conservation). August, 1963. 28 p., frontis., 16 fig., bibliogr. 51.—Illinois Trees: Selection, Planting, and Care. By J. Cedric Carter. August, 1966. 123 p., frontis., 108 fig. 52.—Fertilizing and Watering Trees. By Dan Neely and E. B. Himelick. December, 1968. (Second printing.) 20 p., 9 fig., bibliogr. 53.—Dutch Elm Disease in Illinois. By J. Cedric Carter. October, 1967. 19 p., fron- tis., 17 fig. List of available publications mailed on request No charge is made for publications of the Illinois Natural History Survey. A single copy of most publications will be sent free to anyone requesting it until the supply becomes low. Costly publications, more than one copy of a publication, and publications in short supply are subjects for special correspondence. Such correspondence should identify the writer and explain the use to be made of the publication or publications. Address orders and correspondence to the Chief, Illinois Natural History Survey Natural Resources Building, Urbana, Illinois 61801