THE CASPIAN SEA 



639 



the members of this family this coincidence is even greater (29-3 per cent). 

 The diet of the Clupeidae examined has a high coincidence coefficient (75-9 

 per cent) ; with pike perch the coefficient is 26-2 per cent, with the Acipen- 

 seridae 24-8 per cent; it is low with cypri- 

 noids 6-7 per cent and with Gobiidae 7-5 

 per cent. The various Acipenseridae species 

 differ greatly in their diet (16 per cent), yet not 

 so much as do cyprinoids (14-3 per cent), but 

 more than Gobiidae (17T per cent) and Clupei- 

 dae (24-8 per cent) and so on. 



The food-coincidence coefficient, while 

 giving an idea of the relative similarity between 

 the diets of rival species, does not express the 

 intensity of their competition. Schorygin dis- 

 tinguishes also the amount and intensity of it. 

 The amount of competition is the ratio of the 

 part of their diet for which they compete to 

 their total consumption. The intensity of the 

 competition is the ratio between the demand 

 made, in the shortest possible interval of time, 

 by the rival organisms on the food for which 

 they are competing and the availability of that 

 food. The product of the amount of com- 

 petition by its intensity expresses the force 

 of competition. Comparative food competi- 

 tion of fish in the Northern Caspian as given 

 by Schorygin is shown in Table 272 (in con- 

 ventional units). 



It is clear from these data that intraspecific competition is, on the whole, 

 higher than intrageneric, and the competition between the genera is weaker 

 than between the forms of the same genus. Thus with cyprinoids the intra- 

 specific competition is on the average expressed by 170 conventional units, and 

 the intrageneric one by 121 ; with the Gobiidae it is only 41, with the 



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Fig. 301. Extent of similarity 

 between nature of feeding of 

 Benthophilus macrosephalus 

 and Gobius JJuviatilis pallasi 

 (Schorygin). 



