|Effects of biological factors (sex,seasonal races,spawning, migrations) on fat, protein and water, their distribution in sea-trout|
Kukucz, J.T. (1962). Effects of biological factors (sex,seasonal races,spawning, migrations) on fat, protein and water, their distribution in sea-trout, in: Heen, E. et al. Fish in nutrition. pp. 76-77
In: Heen, E.; Kreuzer, R. (1962). Fish in nutrition. Fishing News (Books): London. XXIII, 447 pp., more
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VLIZ: Biological Resources 
The effects of some biological factors (sex, spawning migrations, seasonal races) on some components of edible parts of fish and its nutritional value were investigated in 1958-59. Chemical composition of many species of fish has been the subject of numerous investigation but the relationship between the biological stages of fish and such components as fat, water and protein are not sufficiently known in the majority of species. The object of studies was the sea-trout (Salmo trutta L.) from the Vistula River, known on the international markets under the name Vistula Salmon. There exist two populations within this species: (1) the winter population, which undertakes a 1.000 km. long migration and spawns in tributaries of the upper Vistula one year after entering the river, and (2) the summer population, which spawns in the tributaries of the lower part of the main river in the year of entering. The fat, water and protein content of specimens belonging to winter and summer populations caught in the year cycle 1958-59 was examined. The winter population shows an adaptation to the conditions of long spawning migration in the form of enormous accumulation of stock material in the body of the fish. This influences the technological properties of the fish. It has been stated that the fat content of specimens of the winter population was on the average twice as high (fem. 17,40 %, male 13.82 %) as that of the summer population (fem. 7.91%, male 6.86 %). The females of both seasonal populations have a slightly greater amount of fat than the males. The following distribution of fat has been found: The accumulation of fat is considerably greater in the ventral than in the dorsal part of the body. There is less fat in the posterior part of the ventral lobes than in the anterior part; conversely, the fatcontent increases in the dorsal part towards the tail. A distinctly small amount of fat is accumulated in the tail itself. The statements concerning the distribution of fat in the different parts of the body might be important from the technological point of view. The comparison of fat content in winter populations at the beginning of the spawning migration (fem. 17.04 %, male 13,8 %) with the content of fat in spent fish at the end of the migration (fem. 4.05 %, male 2.85 %) shows that sea-trout lose about 80-85 % of their fat supply during their spawning migration. The water content is in inverse proportion to the amount of fat. The differences in protein content are smaller (fem. 20.26 %, male 20.53 % in summer population; and fem. 21,31 %, male 21.76 % in winter population). The protein content decreased considerably during spawning migration (about 15 %: at the beginning of the migration, rem. 20.26 %, male 20.53 %; at the end of the migration, fem. 16.62 %, male 17.30 %). Besides the changes in the amount of single components as mentioned above, the whole weight and the condition factor in spawned fishes decrease considerably.