COMPOSITION OF FISH 



341 



factors are not primary causes for composition variation. The primary 

 causes are ones such as the feed intake of the fish and the degree of 

 energy expenditure. Thus salmon caught during the late summer and 

 fall months are found to have much lower oil content than those taken 

 during the spring. This is due to the fact that salmon taken during the 

 late summer and fall seasons have not been feeding and that they have 

 been migrating during their late summer and fall spawning season which 

 has required unusual expenditure of energy. This energy, of course, is ob- 

 tained from depot deposits of oil or fat which were stored up at an earlier 

 period when the fish were feeding. Variations in oil content of fish taken 

 in different geographical areas are often due to differences in availability 

 of feed in the different areas. 



Because of the wide variation in the proximate composition of fish, 

 average values have little meaning. It is better to classify species of fish 

 according to one or more of several fairly well-defined categories into 

 which most species fall. Table 26.3 lists five such categories. 



Table 26.3. Types of Fish 



Category A is the most common type. Fish in this category are charac- 

 terized by a high protein content between 15 and 20 per cent and a low 

 oil content of less than 5 per cent. Many species in category A have oil 

 contents near the bottom of this range, i.e., nearer to 1 or 2 per cent 

 than to 5 per cent. 



Category B is the second most common type. Here the fish retain 

 high protein (15 to 20 per cent) but have a medium oil content in the 

 range of 5 to 15 per cent. 



Category C represents a much less common type of fish having a high 

 oil content (over 15 per cent). When the oil content is high, usually the 

 protein content is lower (under 15 per cent). 



Fish in category D have low oil content (under 5 per cent) and very 

 high protein content (over 20 per cent). Although not many species fall in 

 category D several which do (e.g., most tuna and halibut) are quite im- 

 portant commercially. 



