638 



BIOLOGY OF THE SEAS OF THE U.S.S.R 



we examine on food provision and to determine the strength of this effect 

 on individual food groups.' 



The general scheme of inter-relation in feeding of the main breeds of fish 

 in the Northern Caspian is given in Fig. 300. It is clear that fish feed almost 

 equally on all groups of fauna. As has been mentioned before, molluscs and 

 crustaceans are the main groups fed upon. The crustaceans, except for Deca- 

 poda and Chironomidae (especially Cumacea), are relatively the most inten- 

 sively consumed. Among fish the sprat, a small, quickly growing fish living 

 in large masses throughout the Caspian Sea, is consumed in huge amounts. 



AGRAKANSKAYA 



HERRING 



DOLGINSKAYA 

 ERRING 



900?Q 



Fig. 300. Diagram of food correlations between Northern Caspian fish. Only groups 

 constituting no less than 25 per cent of food are given in the diagram (Schorygin). 



Schorygin was the first to evaluate the feeding inter-relation between 

 species. First of all a 'degree of coincidence ' in the diet of two species offish 

 can be determined. This index is obtained (as a percentage of the total amount 

 of food) when the percentage composition of the diets of two fish is compared, 

 as the sum of smaller percentages. The basis of this simple method of calcu- 

 lation can be illustrated by a graph (Fig. 301) where the area of the coin- 

 cidence of the diet of the two species is defined by the smaller ordinates, 

 independently of which kind of feeding the species belong to. This index 

 (food coincidence, denoted Fc) will decrease with the increase of the precision 

 of determination of the specific composition of food. It was found that the 

 nature of vobla diet is nearest to that of Benthophilus and the bullhead 

 Gobius melanostomus affinis (36 to 39 per cent) ; this similarity is much weaker 

 with carp and golden shiner (27 per cent) and with other fish it scarcely exists. 

 Golden shiner diet is much like that of some bullheads (32 to 61 per cent), 

 and least like that of carp (34 per cent). The diet of cyprinoids is usually 

 coincident with that of some bullheads (25-6 per cent) ; however, between 



