Bulletin STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION DIVISION OF THE NATURAL HISTORY SURVEY STEPHEN A. FORBES. Chef Vol. XVII. BULLETIN Article VIII. The "Knothead" Carp of the IlUnois River BY DAVID H. THOMPSON PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA. ILLINOIS October, 1928 STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION A. M. SiiELTON. Director BOARD OF NATURAL RESOURCES AND CONSERVATION A. M. Shelton, Chairman William Trelease. Biology Henry C. Cowles. Forestry EnsoN S. Bastik, Geology William A. Noyes, Chemistry John W. Alvord, Engineering Charles M. Thompson. Representing the President of the University of Illinois THE NATURAL HISTORY SURVEY DIVISION Stephen A. Forues, Chief SCHNEPP & Barnes, Printers Springfield, III. 192S 87154—1200 Volume XVII Artule VIII THE "KNOTHEAD" CARP OF THE ILLINOIS RIVER David H. Thompson FOREWORD The investigation reported here was begun in collaboration with Doc- tor Ludwig Scheuring, of the Bayerische Biologische Versuchsanstalt fur F'ischerei and the University of Munich, during his visit to this country in the winter of 192()-li)2T. \Miile the "knnthead" carp of the Illinois River had been known to the writer for some time, no start had been made on the ])roblem because of the lack of such a fund of information on the biologT and pathology of fishes as Doctor .Scheuring was equipped to furnish. During the two months when he took an active part in the work in field and laboratory, most of the important facts were brought out. and since his return to Germany he has given advice and suggestions for the continuance of the investigation and has aided in the prepara- tion of the manuscript. INTRODUCTION Any serious hazard to the carp of the Illinois River is of considerable economic consequence, inasmuch as the carp is by far the most important commercial fish of this stream. Introduced in 1885, out of a .«tock brought to the United States a few years earlier from Europe.' the carp first became an important item in the Illinois River fishery soon after 1890. In 1898. the Illinois Fishermen's Association reported that the carp catch exceeded the value of all other connuercial fishes. Statistics gathered by the United States Bureau of Fisheries- show that in 1908 the total yield of commercial fishes of the Illinois River was 23,896,000 pounds, of which carp constituted 64 per cent, or 15,400,000 pounds. More recentl)\ because of the decreased acreage of water due to drainage of bottomlands for agricultural uses, and because of the southward en- croachment of pollution, the annual yield of the entire river has been re- duced about one-half, and even more in the middle section of the stream. ' Forbes, S. A., and R. B. Richard.son. The Fishes of Illinois. Illinois State Xat- ural History Survey Final Report. Vol. III. Second Edition. 1920. = Fisheries of the United States. Bureau of tiie Census. Special Reports. I'.iO.S. [28.5] 286 Illinois Natural History Sikvey Bulletin o a C8 3 3 2 The "Knothead" Carp of thk Illinois River 2S7 s a .a o 13 288 Illinois Natural History Survey Buli^tix The carp has a greater viabihty under these unfavorable conditions than other fishes and now contributes between 80 and 90 per cent of the total annual catch of commercial fishes. The plant and animal life of the middle Illinois River changed funda- mentally during and immediately following large increases in the load of pollution in the years 1916-1918. One of the changes was a marked alteration in the growth form of the carp. While this alteration varied widely in degree, it was generally obvious in most of the carp and was readily recognized by the fishermen. This change has been found to re- solve itself into two general outstanding differences: retardation of the rate of growth throughout the life of the carp; and marked anatomical abnormalities, especially of the bones. A number of methods have been employed to find out what factors are responsible for these structural and functional changes. Several months of painstaking search gave no evidence of an infection of a kind to have any bearing on this case. How- ever, a variety of information has been accumulated which indicates the presence of developmental and metabolic disturbances similar in many re- spects to those found in rickets among the higher vertebrates, and this has been supplemented with evidence of factors, in both the early and adult food supplies of the carp, apparently capable of inducing diet- deficiency disease. DESCRIPTION OF KKOTHEAD CARP The carp {Cyprinus carpio L.) of the Illinois River are not at all broad or high-backed like the cultivated European races. The malformed carp considered in this paper are, as a rule, still more slender (see Figures 1 and 3) and have a body shape similar to the German wild carp (Bauern- karpfen). The carp of the Mississippi River, on the other hand, tend to have a more fleshy body and a "roach" back and can usually be dis- tinguished readily from Illinois River specimens. This seems not to be an inherent difi^erence but rather a result of more favorable conditions for growth in the Mississippi. Some of the smaller bottomland lakes of the Illinois valley occasionally have their outlets closed during seasons of low water, so that the imprisoned fishes may exhaust the available fish food. Under such conditions of starvation, carp are often found which appear stunted and have the slender form that accompanies the present abnor- mality, but they lack many other readily recognized and more significant characteristics of the abnormal carp described in this paper. "Knothead" is the name that the Illinois River fishermen use most often to designate these abnormal carp, but the terms "lunkhead", "pop- gill", "clam-jaw", "lump-jaw", etc., are also heard. These names all refer to the striking malformation of the head and opercles. The sweeping streamlines of the normal carp are broken up by several irregularities in the conformation of the knotheads. The opercles, instead of being slight- ly convex as in normal carp, are more or less bulged or curled up, with an average curvature about like that of the bowl of a spoon. The skull is The "KxoTitEiAD" Caw of the Illinois River 289 narrowed and the cheek region is conspicuously sunken between the eye and the opercle. Many of the knotheads have a marked lateral constric- tion or narrowness at the pectoral girdle just back of the opercles, and a more or less well-defined wrinkle over the snout. The head appears gen- erallv emaciated, and the sclerotic ring and the sculpturings on the cranial bones can be plainly seen (see Figure 3). On the under side there is usuallv a depression beneath the chin and a bulge in the region of the heart. All these e.xternal peculiarities which are commonly found in the more seriouslv affected individuals, together with a "drooping"' of the fins and general sluggishness of habit, jiresent a very different picture from the trim, sleek, and alert normal carp. Fig. 3. Carp with their opercles removed: a, normal; b. knothead. The knothead malformation is variable, and all degrees of it have been found up to monstrous, emaciated individuals three times as old as normals of the same weight. (See Figure 4.) There seems to be a close correlation in the degrees of malformation of the previously mentioned parts ; that is. individtials with slightlv bulging opercles have all these parts affected slightly ; those of moderate degree have them all moderately affected, and so on. The bulging of the opercle is the most practical criterion of the knothead nialforniation. at least for use in the held, but it is so variable that among any dozen carp from the middle Illinois River one or more are usually difficult to classify as normal or abnormal (see Figure 10a). .All references to the frequency of knotheads in this paper are based on specimens exhibiting obvious malformation; and specimens that were doubtful or only slightly malformed were commonly included with the normals. When a number of living carp from the middle Illinois River are ob- served in a crib or pond or aquarium, the knotheads are seen to be some- 290 Illinois Natural Histoky Survey Bulletin a o a o •a 01 K The "Knothead" Carp of thk Illinois Rivei 291 what darker and less uniform in color ihan the iKirnials, the darkest of them usiiall)' heing the most malformed. The large serrated s])iiies of the dorsal and anal fins are strikingly altered in most knothead carp, being more or less reduced in length, sometimes to vestiges scarcely one-fourth the normal length, and having fewer and shorter serrations, or none at all. The dorsal spine of a normal carp is a little longer than the anal spine, but the reverse is usually true in knothead carp. The spines of knothead carp are often thicker laterally than those from normal carp of the same size, especially in the more distal portions. The right and left halves of these malformed spines are not so closely fused as in the normal S])ines, and in some instances they can be readily pulled apart. The dorsal spines of the knothead carp are not held erect as in normal carp, but are inclined backward, sometimes lying flat. (See Figures 1, 2, and 5.) F^io. 5. Dorsal and anal spines of carp of the same length: a, normal; b, moderate knothead: c, extreme knothead. In each pair the dorsal spine is at the left anil the anal spine at the right. Many of these abnormal carp have the opercle .so highly arched that it fails to reach the posterior margin of the gill chamber; in con- sequence, the opercular membrane does not lap flat on the body wall as in normal fishes, but extends inward or is curled under the edge of the opercle, seriously hampering the mechanics of res]iirations. In ex- treme cases considerable portions of the gills are always exposed, and the ragged and irregular arrangement of the lamellae in some knotheads may be the result of constant irritation by the edges of opercles that fall short of the posterior margin of the gill chamber. In such cases some of the lamellae have club-like thickenings at their tips (see Figure 3) ; less often they are partly grown together and show evidence of branching. 292 Il.LI.XOIS NATI'IiAI. HlSTOliV Sl'KVKY BULLETIN Pig. 6. Frontal sections through carp heads ot the same length: a. normal; b, knothead. MC—mouth cavity; O—opercle; G—gills; OM—opercular membrane; BW—body wall. To ascertain whether the bulging of the opercles might be (kie to an enlargement of the gills as an adaptation to diminished concentration of dissolved oxvgen, counts and measurements of the lamellae on each gill were made on one normal and another mal- formed head, with the result that the lamellae were found to be both smaller and less numerous on the malformed head—averaging 15.5 mm. long on the normal head and 11.0 mm. on the malformed, with numbers of the lamellae to each gill as shown in Table I. More- over, the gill chamber in the knot- head was smaller than in the nor- mal, as can be seen in Figure 6. Table I. NUJIBEKS OF LAMEtLAE OX THE GiLL AliCHES OF NOIillAL AND Knothead Caep First gill arch Second gill arch Third gill arch Fourth gill arch Normal Knothead 138 132 133 122 117 105 118 100 A close study of the opercles and the bones of an abnormal head showed no traces of an infection of the bones, either at present or in the past, which could be responsible for this malformation. Some of the bones of the abnormal head were thicker and heavier than those of the normal head, but this may have been due partly to the greater age of the knothead, which was 6 summers old as against 3 summers for the normal carp. Age determination in knothead carp is more difficult than in normal ones, not only because the rings are more crowded as a result of the greater age, but also because there are numerous secondary rings in the scales. This is also true for the growth rings of the cranial bones and the vertebrae. It is possible, however, according to the criteria given by Hoffbauer,-* to distinguish the winter rings from secondary rings, since at the division line of the scale the annuli of the winter ring '' Hnftbauer, C. Die Altersbestimmuiig de.s Karpfen an seiner Schuppe. Allge- melne Fischerei Zeitung. VoL 23, p, 341. ISdS. VoL 25, pp. 135, 150, 297. IDOO. The "Knothkad" Cari' ok thk Illinois Rivkh 293 diverge, but those of the secondary ring dn not. Most knotliead carp have scales tliat are granular and opaque, and it is often necessary to examine large numbers to get a few showing the rings clearly—a con- dition found also among cultivated carp that have been starved. Dissection and examination of normal and abnormal specimens showed that the skin, gill surfaces, and internal organs were free from infection or parasites; and the color of the gills in the living carp was bright red in both. There is less fat on the mesenteries and pericardium in the abnormal than in the normal ones, but fat is never altogether lacking. The flesh of the knotheads is softer than that of normal carp, although it is not as flabby and watery as in starved car]). Fishermen ^:^r7i '^<^^^^-^i Fui. Microphotograph of gill tissue of knotliead carp. of the middle Illinois River say that it is difficult to dress these carp for the market withotit seriously tearing the flesh and removing portions of it with the scales and skin when they are "fleeced". This softness of the flesh is well known to fish culturists and has been ])roduced arti- ficially in the experiments of Podhradsky and Kostomarov.^ Examination of the gill tissues of knothead carp gave no evidence of an infection, but sections of the gills showed varying numbers of bod- ies larger than ordinary tissue ce'ls ( 10 to 20 microns in diameter) and staining ditiferently. ( See Figure 7 . ) These bodies were observed throughout the epithelium of the lamellae and gill arch and occasionally * Podhradsky. .1., and B. Kostoniarov. Da.=: Wachstum der Fische beini absoUiten Hungern. Arfhi\- fur Eiitwioklungsmechanik und Organi.'^men. V(il. 10.5. p. 587. 1925 294 Illinois Natuhal Histoky Sikvey Bull?:tin- in the connective tissue, but most often in the deeper proximal parts of the lamellae. They were found in each of the several normals and knot- heads examined, but were more abundant in the latter. For a time they were thought to be parasites, with perhaps some connection with the knothead disease, but after considerable study it was seen that they had no characteristic resemblance to microscopic jjarasites of any known kind. They are quite different from mucous cells. They evidently be- long to the tissues of the carp itself, but their place in its histology has not been determined. A knothead carp of moderate degree and a normal one of the same age were selected for comparison from a collection made at Peoria in November, 1927, which was kept alive and under observation several days in large aquaria, in order to make sure that the selected individuals were representative of the two kinds and not obviously injured or af- fected differently by other factors. By taking these precautions, differ- ences due to age, environment, parental stock, etc., were largely elim- inated, so that any observed differences were probably effects of the disease. Data on these two carp are given in Table II. Table II Measurements of Carp Used for Comparison Normal Knothead Age 4 summers 4 summers Length 14.2 inches 12 4 inches Depth (at front of dorsal fln). 5.0 inches 3.9 inches Width (at front of dorsal fin). 2.9 inches 2.5 inches Weight 57 ounces 30 ounces These two carp were anaesthetized, and dorsal and lateral X-ray photographs were made of them by Dr. C. S. Bucher at the Bucher Clinic, Champaign, both being rayed at the same time and on the same sensitized surface in order to maintain conditions as uniform as possible. These X-ray photographs, reproduced in Figures 8 and 9, show many of the malformations already described—the bulged opercles, sunken cheeks, narrow pectoral girdle, etc. Examination of the original negatives by strong light also shows a difference in the thickness of flesh on top of the head. In the normal specimen the median ethmoid, frontals, parietals, and pterotics, are overlaid by flesh 1 to "2 millimeters in thickness, while in the knothead practically none is visible over the frontals, parietals, and pterotics and a layer only Yz to 1 millimeter thick over the median ethmoid. These X-ray pictures, besides confirming observations already made and revealing further malformations, give a clue to the more intimate nature of these malformations ; for, other factors being substantially ecjual, the depth of the "shadow" cast by a bone depends mainly on the thickness of that bone and its degree of mineralization, The "Knothead" Carp of the Illinois River 295 Fig. S. Dorsal X-ray photographs of 4-year-old carp: a, normal ; b, knothead. 296 iLLrxois N.\TrKAL History Si hvey Bili.etix The heads of the two 4-year-old carp used in the above compari- son were heated ahiiost to boiHng, and the flesh was cleaned off the skulls and the bones of the ])ectoral girdle. The right opercle. the right cleithrum. and the right pharyngeal bearing the teeth were selected from each for chemical analysis. These bones were weighed and their per- centages of ash and calcium determined by Mr. O. \\'. Rees, chemist of the Illinois State ^^'ater Survey, and the weights and percentages are given in Table III as an indication of the degree of mineralization. The "Knothhad" Caiu' of the Illinois Riveh 297 Fi(!. 9. Lateral X-ray photographs of 4-year-old carp: a, normal; b, knothead. 298 Illinois Natural Hlstoby Survey Bulletin The relation of the weight of the bones of the head to their depth beneath the surface is well shown in Table III. The opercles are cov- ered by only a thin skin and are taken as representative of the peripheral bones. The ratio of the weight of the normal to. the knothead opercle is 1 : 0.81. The cleithra form part of the pectoral girdle and are of in- termediate depth. The ratio of their weights is 1 : 0.64. The pharyngeals that bear the teeth are fairly large and more easily removed entire than the other deeper-lying bones. Their ratio is 1 : 0.46. This pronounced thinness of the bones of the occipital region of the knothead skull is c|uite obvious in the cleaned skulls. While the knot- head skull is actually smaller than the normal, its foramina for the pas- sage of nerves and blood vessels are almost always actually larger. The two heads had been heated in the same vessel of water and cleaned and treated throughout in the same manner, and the skulls had been air dried in the same box; but the thinner bones forming the brain case of the knothead sktill opened at the sutures, leaving gaps occasionally more than a millimeter in width. These gaps are not due to a general disarticula- tion of the skull, because it is still held firmly together by the heavy bones on top of the head and the parasphenoid. No such gaps appeared in the normal skull as it dried. Those of the knothead skull are appar- ently produced by differential shrinkage of the heavy, dense bones form- ing the roof of the cranium and the thin, lightly mineralized bones lying deeper in the head. X-ray photographs of knotheads of different degrees of malforma- tion, from slight to extreme, made for comparison with the one of mod- erate degree we have been considering, are shown in Figure 10. The extreme knothead (Figure lOd) shows heavier and denser bone at the ossification centers of the peripheral bones of the skull than those in Fig- ure 8b, and these centers of heavy ossification are lacking in the head with only a slight degree of malformation (Figure lOa). Photographs of the skulls from these two heads are reproduced in Figure 11. The difference in conformation is greater than would have been suspected from the appearance of the living fishes or from their X-ray photographs. The areas of heavy mineralization found in the peripheral bones of the knothead are visible in its skull as unusually heavy sculpturing and ridging at the ossification centers. The anterior margins of the malformed opercles have a slightly inflated appearance, and, when freshly cleaned, they had the red color of cancellated bone tissue. There are further alterations in shape and details of structure in every bone of the knothead skull, but a detailed description of them is hardly justified in this account. In proportion to the length of the spinal column, the head in these malformed carp is 2 to 5 per cent shorter than normal. This is under- standable in view of the fact that there is no malformation visible in the vertebrae of the knotheads, either by direct examination of the bones or from their X-ray shadows. The "Knothb:ad" Carp of the Illinois River 299 Fig. 10. Dorsal X-ray photographs of knotheads showing different degrees of malformation: a, slight; h, moderate; c, moderate; d, extreme. 300 Illinois Nati-kal History Si'uvey Bulletin Differences in the intimate structure of the skull bones of normal and diseased carp were studied by means of X-ray photographs made by Dr. G. L. Clark of the University of Illinois by the monochromatic pin-hole method,'' showing the degree of diffraction of a beam of X-rays passed through the bone. The crystals of both were foimd to have a random orientation and to be within the range of size of colloidal particles, those of the knothead being slightly larger than those of the normal—a fact which substantiates previous observations of the greater brittleness of knotheads' bones. DISTRIBUTION OF KXOTHE.'\D C.^RP Carp fingerlings with bvilging opercles were first noticed in the fall of 1918 in the Illinois River at Peoria. Fishermen report that knothead = Clark, G. L,. Applied X-ray.s. JttCiaw-Hill Book Co. 1927. ^^^^^^^^^^^^^^^^^^KA ^^^^^^^^^^^^^^^H The "K-\()Thhm) " Carp of the Illinois River 301 carp bes;an to appear in the catch of market-size tishe? in l!i"3(). and that the percentage of knotheads increased rapidly, reaching" about 50 per cent in U»2"i at several points on the river. In order to find out more definitely the distribution of the malady and the percentage of market-size carp affected, many points on the river were visited during 1926 and 1927, and catches of carp were examined and information was obtained from fishermen. A summary of the data is given in Table IV. and the location of sam])ling stations is shown in Figure 12. These data were all obtained during the winter of 1!I26-192T except the Peoria and Meredosia numbers, which are for the summer and fall of 1927. The rather high percentage of knotheads at Peoria is largely due to the recognition of slight degrees of malformation in individuals such as had formerly been included with the normals. The observed percent- ages are based on hoop-net catches and give a more accurate measure of the proportion of knothead carp than hauls made with seines which have larger meshes and allow many of the stunted and slender knot- heads to go through. In the course of several years work on the fishes of Rock River, less than a dozen, knotheads have been noticed among the many thou- sands of carp taken. Careful examination of catches of carp from various parts of the Mississippi \alley in the Chicago markets has oc- casionally revealed a few individuals with distinctly knothead character- istics. Indirect reports have lieen received of large niunbers of knot- head carp being taken in Lake Pepin on the Upper Mississippi, but no direct information has been available as yet on this point. RELATIOX TO POLLl'TIOX The beginning of this malformation among the carp was coincident with a jieriod of very rapid increase in pollution, due partly to an increase in the population of Chicago and other cities which pour their wastes into the river, but still more to the war-time boom in many industries, particularly those engaged in the packing and manufacture of food prod- ucts. The subsequent fundamental alteration of the character of the bottom fauna of the Illinois River has been described by Richardson in the various publications of the Illinois Natural History Survey. As indicated in Table I\ . knothead carp range from Utica (Starved Rock ) down to the Copperas Creek Dam with only occasional individuals below that point. There are no fishes of any kind above Utica. since, becatise of pollution, the amount of dissolved oxygen in that part of the river is so low throughout most of the year that fishes can not live. As long ago as 1912 Utica was the upper limit of most of the species of fishes in the Illinois.'' The few species that were occasionally found above that point disappeared completely, according to the reports of fishermen, in the vears between 1912 and 1917. ' Forbes. S. A., and U. E. Richard.^on. .Studie.s on the BiologA- of the Upper lUi- nois River. 111. Xat. Hist. Surv. Bull. Vol. IX. Art. X. 191;!. 302 Illinois Natural History Sikvey Bulu;tin In the years since 11)17, pollutional conditions have ]:)revailed over a hundred-mile stretch of the middle Illinois River, and the fishery yield of this section has shown marked reduction as compared with the yield before 1917. The fishes themselves are largely restricted to the cleaner bottomland lakes and backwaters of this section. The carp, being of more tolerant habit than other fishes, have continued to range over a large part of this pollutional area except during periods of severe oxygen depletion Table IV DlSTKIIiUTIOX AND PERCENTAGE OF KXOTHBAD CARP IX THE IlLINOLS RiVER During 1926 and 1927 Stations ' Number going j of carp downstream I examined Observed per- centage of knotheads Information and estimates of local fisliermen Utica LaSalle-Peru Depue Henry Havana Beardstown Meredosia none 23 316 539 3 42 91 75 Chillicothe Spring Bay The "Kxothead" Carp of the Illv.nois Rh-er 303 in summer and under heavy ice in winter. The distribution of knothead carp coincides, both in main features and in detail, with the areas of greatest change in sanitary condition. Below the Copperas Creek Dam the carp are all normal except for occasional knotheads that have straggled down the river. Xatural purification of the sewage load of the upper and middle river is approach- ing completion in midsummer by the time this lower section is reached ; Sketch map of the Illinois River. and there has been no general or serious alteration in the character of the food supply of the carp. The observed percentages of knotheads at various points on the Illinois River indicate that the proportion of ati'ected carp decreases downstream, as is to be expected if the malformation is correlated with pollution. 304 Illinois Nati'iial Histoky Sirvey Bulletin The knothead malformation appears among those races of carp with reduced scalation ( mirror and leather carp ) to the same extent as among the scaled variety. The proportion of these mutant races of carp in the original stock of the Illinois River was quite large, but their numbers soon decreased to the present proportion of one or two per cent of mirror carp and still fewer leather carp. The carp populations of other waters of the Mississippi Valley show these varieties in about the same proportions. The occasional occurrence of a few knotheads in the Rock River and other waters seems to have no relation to pollution. No malformations similar to those of the knothead carp have been found in other species of fishes of the middle Illinois River. During the course of this and other investigations considerable numbers of about twenty species of larger fishes from this secMon of the river were handled without any appearance of alteration in their growth form, but an ex- amination of other cyprinoid fishes (minnows) of this area might dis- cover an abnormality like that of the knothead carp.' CHANGES IN THE FOOD SUPPLY OF THE CARP The following statement by Mr. R. E. Richardson concerning re- cent changes in the natural food supply of the carp in the middle river is based on various field observations and unpublished plankton data in the possession of the Natural History Survey, accumulated between 1910 and 1925. The most significant changes in the plant and animal food supply of the carp in the middle Illinois River in recent years had to do either directly or indirect^ with the plankton and the coarse aquatic vegeta- tion. That the latter usually serves to some extent as green forage for carp above the late fingerling stage can hardly be doubted, though the exact extent to which either the roots or the stems or other parts of coarse aquatic plants enter into the normal food supply of these fishes has not been quantitatively determined. The almost complete extermina- tion of the Potamogetons and other large plants in Peoria Lake and also in the river and in its connecting lakes and sloughs far below that point, between 1915 and 1930, left large numbers of carp practically without any green forage at all for several seasons. Since 1922 or there- about, restoration of the Potamogetons particularly has proceeded rather ' Some young goldfish, spawned in the .summer of 1927 in the University of Illi- nois lily poiid, were brought indoors in October and kept in a large aquarium, which was supplied with running tap-water and aerated with compressed air. They were fed commercial goldfish food occasionally, and there was no other food available except mud on the bottom. There were no algae or other green plants present. The fishes were an inch to an inch and a halt long when they were brought indoors, and they did not grow perceptibly afterwards. In December some of them showed signs of emaciation and an occasional one died. About half of the 50 fishes in the aquarium on December 15 had distinctly arched opercles, most frequent on the emaciated fishes but present also on some of the fleshier ones. Those with arched opercles also showed the sculpturing of the skull bones, the drooping fins, the narrow pectoral girdle, and the general sluggishness characteristic of knothead carp. More recently a few golden shiners with strongly bulged opercles have been found in the Salt Fork River in Champaign County. The "Knothead" Cakp of the Illinois River 305 rapidlv in nianv sections of Peoria Lake as measured by the areas af- fected, though the growths are yet generally very sparse as compared with those of the pre-1920 years, and the areas covered are still decidedly smaller. The distinct change in the general composition of the normal summer plankton in the section of river extending from Spring \'alley to a point in Peoria Lake onlv a few miles above Peoria Narrows, between 1!(13- 1915 and 1920. was probablv no less important in its final etfects than the reduction in the coarse aquatic vegetation as an item in the general reduction of the green forage supply that took place during the period as a result of pollution ; and this has continued substantially without visible relief up to the date of our latest observations. This change in- volved the substitution on a large scale of non-chlorophyll-bearing Pro- tozoa for chlorophvll-bearing kinds, and the replacement of vast numbers of DiATOM.ACEAE and Chlorophvceae by filamentous or non-hlamentous blue-green algae. The change was so marked in the summer of 1920 be- tween Chillicothe and the foot of Upper Peoria Lake that the water, which in former years had shown a normal pale-green tint when rela- tivelv free of silt during the hot season, took on a distinctly blackish tinge, apparent to the unaided eye in bright sunlight. The recovery of the diatoms and Chloroi'hvceae as well as of other green microscopic forms, was quite rapid both that summer and in the summer of 1922 below the upper third or half of Middle Peoria Lake—a fact enabling us to make fairly satisfactory, though rough, colorimetric determina- tion on given days of the location of the boundary line between pre- ponderance of blue-green and green plankton by observing the change in color of the plankton samples in the bottle in the field a few minutes after pouring on the alcohol-formalin mixture used for preservation. A similar boundary line between the greens and the blue-greens of the plankton could be made out in the same way in the summers of 1911 and 1912 at points usually lying only a shon distance below Spring \'alley. These changes in the plankton could obviously affect directly the frv of carp and other fishes depending upon it for their first food. A far more important effect upon the carp between the fingerling and adult stage could apparently be exerted through the medium of the bot- tom, shore, and linmetic organisms of microscopic size, which had fed upon the plankton and had later entered into the carp's rations in the same or adjacent areas. During the same period (1913-1915 to 1920- 1922) the changes in the principal constituent organisms of the small bottom fauna.- though also great in degree, appear to have been in them- selves of no particular importance to the health of the carp and other bottom-ranging fishes. The increased restriction of the diet of the small bottom animals above Peoria to microscopic organisms lacking in chloro- phyll or food elements, such as vitamins, directly or indirectly derived from it can easily be believed, however, to have been capable of caus- s Richaid.son, R. E. The bottom fauna of the middle Illinois River, 1913-1925: its distribution, abundance, valuation, and index value in the study of stream pollu- tion. 111. State Nat. Hist. Surv. Bull. Vol. XVII, Art. XII. (In press.) 306 Illinois Natural History Survey Bulletin ing disturbances both in the growth and metaboHsm of fishes confined to these small bottom animals for food, if the requirements in that re- spect of man and many lower animals above the grade of fishes are to be accepted as in any sense a usable criterion. RATE OF GROWTH OF KNOTHEADS During the winter of 1936-1927, representative samples of carp, obtained from fishermen at various points on the Illinois River, were weighed to the nearest ounce and measured for length and for depth at the front of the dorsal fin, and about twenty scales were taken from the left side of each fish between the dorsal fin and the lateral line. The carp were classified into normals and knotheads, but many with doubt- ful or slight degrees of malformation were included with the normals. The scales were cleaned and age determinations were made by counting the winter rings of at least ten scales from each fish. Scales which have been regenerated, and consequently have smaller numbers of win- ter rings, can usually be recognized by their irregular shape, and avoided. The data on the age and weight of normal and knothead carp at various points on the Illinois River are given in Table V. Differences in age of normals and knotheads of the same size are shown in Table VI, the data of which have been arranged in twelve 5-ounce weight classes and the ages of the two kinds compared in each class. The knotheads average older than the normals in each of the twelve classes. Arithmetical averages (not weighted) of the ages of the two kinds in the first six and the last six classes show that the differ- ence in age between normals and knotheads of the same size is much greater in the heavier classes. Growth curves plotted from the data in Table VI are shown in Figure 13. These curves indicate that the stunting effect of the dis- ease is cumulative and stops growth altogether in the knotheads at the time when normals are growing most rapidly. Since it has been obvious throughout the investiga- tion that growth is retarded in pro- portion to the degree of malforma- tion, it seems probable that carp with slight degrees of malformation have a growth rate intermediate be- tween normals and knotheads, and the inclusion of knotheads of slight ° ' ' ' t^\n'.J^° ' " " "' degree with the normals has prob- P.G.13. Growth curves of normal ably resulted in an appreciable low- and knothead carp of the Illinois ermg of both the curves between River. the third and sixth years. i i. The "Knothead" Carp of the Illinois River 307 Calculated from the data in Table \T, the average individual growth rate of normal carp is about 11 ounces a year^ and that of knothead carp is only about 6 ounces a year. There is no considerable variation in the growth rate of the former from one end of the river to the other, but the growth rate of the latter tends to increase downstream. Thus the lowest rate for knotheads, 3.8 ounces a year is at Depue, while the highest rate, 9.1 ounces a year is at Pekin. The greater growth rate down the river is strikingly reflected in the body contours of the knotheads. A large number of them were seen at Pekin and practically all, instead of being more slender than the normal, were pot-bellied—a condition strikingly illustrated in a sin- gle knothead taken at Meredosia in July, 1!)2T, a drawing of which is shown in Figure 14. An examination of the gonads of this fish showed that it was a spent female. Its scales showed that it was four years old, and the relatively greater width of the last two growth rings indi- cated that in those years growth had gone on at a relatively rapid rate. Pig. 14. Pot-bellied knothead carp from the lower Illinois River. The 30 per cent or so of pot-bellied knotheads seen in catches from the stretch between Pekin and the Copperas Creek Dam may indicate either that the conditions associated with pollution and inducing malformation were of short duration or else that these fishes migrated downstream, perhaps from the Peoria Lake region, after their growth form had been altered by exposure to the knothead-inducing conditions during their early life. The development of the pot-belly seems to indicate that, under non-poUutional and hence more favorable conditions, the viscera tend to grow at a normal rate while the growth of the skeleton and mus- cles of the carp is permanently retarded. ' Doctor Scheuring says that the growth of normal carp in the Illinois River is about as rapid as under fairly good cultivation in Germany. 308 Illinois Natural Histouy Survey Bulletin 3 a •a J3 a . o Co < OJ The "Kxothead" Cakp of thk Illinois River 309 310 Iixixois Natural History Survey Bulletin s o s o S CI The "Kxothead" Carp of the Illixois River 311 O! "^ 312 Illinois Natuhal Historv Survey Bulletin s o o > -<1 _o o ~-«* 00 a a a _o o o o CD CD _M CD •^TP'*-^'*mu5lOm ^^^^-^TJH'^CD CO rP -* m CO CO -