522 U. ?. BUREAU OF FISH-ERIES 



INDIVIDUAL VARIATIONS 



It has already become evident that the composition of fish flesh 

 is greatly influenced by such factors as the time of year when caught, 

 relation to maturity, and locality. Of course, the variation due to 

 locality is somewhat complicated by the fact that this may be due 

 rather to the unusual prevalence of food in or off the mouths of 

 certain rivers like the Rogue, Colum])ia, and Yukon. 



There is another type of variation which should be considered, and 

 that is what we may call "individual" variation, for which we can 

 find no other explanation than that it is due to the individuality of 

 different fish. It is a well-known fact that no two biological indi- 

 viduals, whether of plant or of animal origin, are exactly alike. The 

 amount of food they eat, the amount of energy they transform, and 

 other conditions often seem to vary greatly for no apparent reason. 

 This may be the cause of the so-called "individual" variation in fish 

 that may be caught in the same school, where the conditions of 

 time, place, school, and sex are apparently identical. Dill (1921) 

 has analyzed a large number of individual California mackerel by 

 groups caught at the same time, often of the same sex, and approxi- 

 mately the same size. 



It takes only a very brief glance at Table 15, where Dill's results 

 are tabulated, to bring out how great this factor of individual varia- 

 tion may be. For instance, in three individual mackerel caught on 

 October 25 there was a variation in fat content from a minimum 

 figure of 0.85 per cent to a maximum of 7.88 per cent. Again, at a 

 later date — namely, December 10 — five of the California mackerel 

 varied individually in the percentage of fat from a minimum of 

 7.77 to a maximum of 17.78, or, when expressed in calories, from a 

 minimum fuel value of 729 calories to a maximmn of 1,145 calories 

 per pound. The variation in the percentage of protein and ash is 

 not very great, the main difference being in the percentage of fat, 

 with a corresponding inverse change in the percentage of moisture. 



VARIATION IN THE COMPOSITION OF DIFFERENT PARTS OF THE FLESH 



OF THE SAME FISH 



Everyone who cats fish habitually has probably noticed that slices 

 taken from different parts of the same fish will show a different degree 

 of richness. This is also A^ery well known to canners or preservers 

 of fish. Many people, for instance, are very fond of the so-called 

 "cheeks" of salmon and halibut. These portions of flesh on each 

 side of the head are unusually rich and have a flavor of their own. 

 Other parts of the body vary, also, as is well shown in Table 16, 

 in which the results of Shostrom, Clou^h, and Clark (1924) on the 

 analyses of different parts of the same saLmon and Dill's work on the 

 yellowtail and skipjack are presented. 



