THE CASPIAN SEA 641 



from our calculations by determining the extent of the variations in the pro- 

 vision of food, by the method used to determine changes in the nature of the 

 diet. The ratio of the second value to the first is the index of the extent of the 

 elasticity of the diet, regardless of the variations in provision of food. These 

 indices are given in Table 273. 



Table 273 



The elasticity in the diet of sturgeon and starred sturgeon is high ; that of 

 golden shiner is the lowest. It should be noted also that in the Northern 

 Caspian the variability of the nature of the diet is greater than that of the 

 provision of food. Finally Schorygin has introduced one more new con- 

 ception — the feeding activity of fish, meaning the capacity of the organism 

 to maintain its peculiar type of nutrition. The feeding activity and elasticity 

 of the sturgeon, pike perch, and to some extent of the starred sturgeon, are 

 high. Pike perch has a high activity but a low elasticity and vobla, on the 

 contrary, a high elasticity with low activity. In his later work A. Schorygin 

 (1948) has compared the results of his observations in 1935 with those of 

 1941. During that time the edible fauna had decreased by 56 per cent and the 

 changes in benthos had brought about a change in the composition of fish 

 diet. However, the force of competition for food between the six fish chosen 

 (three species of bullheads, vobla, golden shiner and sturgeon) has remained 

 practically unchanged. This is explained by the high elasticity of fish diet and 

 is achieved by : (7) a separation of the feeding grounds of different species, 

 (2) the divergence in the nature of their diet, and (5) by a more even utiliza- 

 tion of food provided. The former strong competition was weakened, while 

 the weak food fink grew stronger (Fig. 302b). Seven or eight years after 

 Schorygin's observations his method of quantitative examination offish food 

 competition was repeated by Ya. Birstein (1952), and it was found that 

 competition for food in 1948-49 was considerably weaker than in 1941 

 (Fig. 302c). It was clear from his detailed examination of fish nutrition over 

 these years that the slackening of competititon for food between the fish- 

 benthophages is due to a huge development of Nereis which took place at 

 that time, and which provided the fish with some millions of centners of 

 supplementary foodstuffs of high calorific value. 



2s 



