RAINBOW TROUT {SALMO IRIDEUS) PRODUCED IN FINLAND 111. Seasonal variations in rainbow trout Jorma J. Laine, Elina Varesmaa & Fritz P. Niinivaara University of Helsinki, Institute of Meat Technology Received April 14, 1967 Seasonal variations in fish are complex and dependent on many factors. The Atlantic sardine e.g. contains 2 % of lipids in the spring and over 8 % in the fall (Jacquot, 1961).There can also be remarkable changes in the protein content in this same species, namely 16 % in March and 20.6 % in July (Del Riego 1948). Investigations on cultivated fish reveal big seasonal changes. According to Jacquot (1961), with improved dietary conditions the percentage of edible parts in cultivated carp increases from 55 to 67 %, the lipid content from 0.4 to 4.9 %, and the protein from 14.8 to 19.5 %. The seasonal changes in the growth, the chemical composition, and the micro- biological and organoleptic qualities of rainbow trout (Salmo irideus) produced in Finland were studied in the present work. Changes in the chemical and microbio- logical qualities of the cultivating waters were followed. Material and methods Experiments were carried out with 2-year old trout cultivated in four geo- graphically different places of growth (I), (II), (III), and (IV) (Fig. 1). The sampling days were May 11th, July 18th, September 26th in 1966, and February 2nd in 1967. Except on the last sampling day all the fish were taken alive into the labora- tory. Each sample consisted of six individual fishes. At the same time samples of the cultivating waters were collected into sterilized bottles. Each sample con- sisted of 500 ml of water. In the laboratory the fish was killed and weighed. Before weighing micro- biological samples were taken. In this case, the amount of the microbiological sample was added to the weight of the fish. Gutted weights were taken from the fish used in organoleptic evaluation, in the chemical analyses of gutted fish and from the fish used in microbiological analyses. Chemical analyses were carried out with whole and with gutted fish. In both cases the fishes were first ground and then homogenized. From the homog- https://www.c-info.fi/en/info/?token=5FdwJph1AfyGsTWW.czafqSMZEq6wLvxy2h8Hrw.XPeR0z5eE51DExMJuEYp3GJkeGGsTfZZv12vo2ZpWw8IlSfbBakWo6GYfbkIDhDt5vkqWBUE9QI4qPAK5WgHbMbUIdLCReVIZnmynuY_l9Rv2rpVn_TSkhe1jQDV_SYzlkkhdpIzFDumn6HkZOkbCsxITtU6Mg9NraxXKrsdd0hwxL8DqPtlzx8VXsMEJcNc3jILvPjInCaGudiVZOhchCt- enized sample the following determinations were made: pH, water content (AOAC 1965) fat content (Pohja et ah, 1956), protein content (Cocks and van Rede, 1966) and ash content (AOAC, 1965). Straight samples from the water samples were taken and the fol- lowing determinations were made: pH, total hardness, color, iron, and KMn0 4 - consumption (Haase 1954). Microbiological samples were taken from the dorsal side of the fish (skin and flesh) and from the ventral side of the fish (skin, fins, and intestines). Samples from the dorsal side are considered as gutted fish and those of ventral side as whole fish. For each sample 11 grams of fish was aseptically weighed in 99 ml of 0.9 % NaCl-solution. The sample was then homogenized and the necessary decimal dilution series was prepared. Samples were tested for total viable counts on SPC-agar (Orion) and for total coliforms on VRB-agar (Orion) and using the MPN- technique (American Public Health Association, 1958). Incubation was the same as in the previous study (Niinivaara et ah, 1966). The same microbiological determinations were made from the water samples as from the fish. The organoleptic evaluation was made by the same methods and with the same taste panel as in the previous study (Niinivaara et ah, 1966). Results Differences in weights between the sampling points depended upon the type of operation and food used. Number II was an experiment station and the fish were fed with natural food. In the cases of I, 111 and IV, the enterprises were com- mercial fish producers using high energy foods. However, in the IVth case efficient feeding was stopped at the end of the summer and the fish were not sold until the following year. This commercial point of view interfered somewhat with the ex- periment. Fig. 1. Geographical situation of the four sampling places. Table 1. Mean weights of the whole fish (g). In brachets the percentile part of the gutted trout (=considered edible proportion of trout). Place of growth Date I II 111 IV • g- % g- % g- % g- % May 11 227 (93.0) 301 (88.9) 341 (87.8) 186 (89.3) July 18 200 (89.8) 333 (84.8) 495 (83.4) 242 (88.6) Sept. 26 217 (82.9) 384 (80.6) 723 (81.8) 194 (81.3) Febr. 2 260 (83.0) 348 (85.9) 760 (80.5) 250 (84.5) Fig. 2. Results of the approximate chemical analyses of fish. Protein and ash contents. X X I, whole fish X X I, gutted fish □ □ 11, whole fish □ □ 11, gutted fish A A 111, whole fish A A 111, gutted fish O O IV, whole fish O O IV, gutted fish 135 136 Fig. 3. Results of the approximate chemical analyses of fish. Water and fat contents. X X I. whole fish X X I, gutted fish □ □ 11. whole fish □ □ 11, gutted fish A A 111, whole fish A A 111, gutted fish O O IV, whole fish O O IV, gutted fish Chemical analyses. Results from the approximate analyses are pre- sented in Figures 2 and 3. The results revealed that there were differences bet- ween different sampling places and times of the year. Differences also existed between whole and gutted fish. In the spring, the fat content was 1 to 2 % higher in the southern sampling points (Fig. 3) and remained so throughout the experiment. There was also more fat in whole than in the gutted fish. As to protein, the content was higher in gutted than in whole fish. Changes in the water content were reversed to those of fat, and the sum of these two components remained stable throughout the experiment. The water content was highest in the spring and decreased during the summer. The results from the water analyses are presented in Table 2. Basically all the waters were ground waters with an acid reaction. Though they all belonged to very smooth waters (total hardness less than 4°dH) rather large differences existed in the total degree of hardness. The amount of organic matter in the water was highest in all sampling places in July. The chemical composition of the cul- tivating water seemed to influence both the organoleptic and the bacteriological quality of the fish. Microbiological experiments The totalviable counts are presented in Fig. 4. The greatest differences between the different sampling points occurred in July. The ventral side samples had bacterial counts of 113.000/gram in sampling point 11, 43.750/gram in IV, 9500/gram in 111, and 3500/gram in I, respectively. The dorsal side samples were generally lower than the ventral side samples. The coliform counts reached their maximum likewise in July (Fig. 5). In some Fig. 4. Total bacterial counts of whole and gutted fish on SPC-agar. X X I, whole fish X x I, gutted fish □ □ 11, whole fish □ □ 11, gutted fish A A 111, whole fish A A 111, gutted fish O O IV, whole fish O O IV, gutted fish Fig. 5. Total coliforms of whole and gutted fish using the MPN-method. X X I, whole fish X X I, gutted fish □ □ 11, whole fish □ □ 11, gutted fish A A 111, whole fish A A 111, gutted fish O O IV, whole fish O O IV, gutted fish 137 138 cases they were unexpectedly high but generally speaking they remained at a low level and never exceeded 100/gram in any dorsal sample. Bacterial counts for the cultivating waters were independent from those obtained with the fish samples (Figures 4 and 5). Seasonal variations indicated that surface water had mixed with the ponds in the spring and fall. Total counts did not exceed 104/gram in any sample in July or in May, and in September the maximum values were between 105 to 108/gram (Fig. 6). The corresponding values for total coliforms lay between 0 to 120/gram (Fig. 6). Table 2. Results of the chemical analyses from the cultivating waters. Culti- Total Color Iron KMn04 - vating hard- Pt mg/1 mg/1 consump- water Date pH ness °dH tion mg/1 I May 11 6.05 0.56 10