636 



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



vary greatly for different fish — from 26 for starred sturgeon to 368 for B. 

 stellatus. Mollusc-eating fish have the highest index of repletion (from 107 to 

 368) ; it varies from 75 to 21 1 for worm eaters. When fish is the basic diet the 

 repletion indices fall to 26 to 120 and, finally, with a diet of crustaceans the 

 indices range from 27 to 79. In general indices of repletion are inversely pro- 

 portional to the calorific value of food, as illustrated by the following data: 



Gammaridae 

 Corophiidae 

 Chironomidae 

 Sprat 



3-92 cal/g Vobla 1-00 cal/g 



2-34 cal/g Bullheads 0-76 cal/g 



2-34 cal/g Benthophilus 0-63 cal/g 



1-47 cal/g Dreissena polymorpha 0-63 cal/g 



This regularity is somewhat broken only by worm-eating fish, since for 

 food of high calorific value the indices of repletion are high. 



Fig. 298. Vertical distribution of fish feeding grounds in the Caspian Sea (Schorygin, 

 1952). 1 Knipowitschia longicaudata; 2 Bubyr caucasicus; 3 Sturgeon; 4 Benthophilus 

 marmoratus ; 5 Golden shiner ; 6 Hyrcanogobius bergi ; 7 Carp ; 8 Gobius flimatilis pal- 

 lasi; 9 Gobius melanostomus affinis ; 10 Vobla ; 1 1 Benthophilus stellatus ; 12 Sturgeon ; 

 1 3 Casp. Alosa ; 1 4 Starred sturgeon ; 1 5 Pike perch ; 1 6 Casp. braschnikovi ; 1 7 Casp. 

 sphaerocephala {agrakhanskaya) . 



A fish's choice of food is to a considerable degree correlated with its man- 

 ner of life : fish living in a definite horizon use mainly organisms adapted 

 to this horizon. If food organisms are divided into pelagic and benthonectic, 

 epifauna and infauna, we get a basic adaptation of each fish to a certain 

 horizon (Fig. 298). Typical predators, pike perch, and the dolginskaya and 

 agrakhanskaya herrings feed mainly on pelagic organisms. Morover, pike 

 perch feed mostly on bottom-living fish (bullheads and vobla), and herring 

 on pelagic fish (sprats). Starred sturgeon and Caspialosa saposhnikovi eat 

 benthos (both the epifauna and the infauna), although pelagic organisms are 

 predominant in their diet. 



Carp, vobla and some bullheads feed mainly on epifauna, while sturgeon, 

 Pomatoschistus caucasicus and Knipovitschia longicaudata prey mainly on the 

 infauna. 



The change of diet with age of the sturgeon is interesting (Fig. 299 IV). 

 When less than 50 cm long it feeds almost exclusively on Gammaridae, passing 

 first to Corophiidae as it grows and then to river crustaceans. Simultaneously 

 fish and Nereis acquire more and more significance in its diet, comprising 

 40 per cent of its food when the sturgeon is 60 cm long. However, with further 

 growth the sturgeon does not remain on a diet of fish ; it begins to eat more 



