640 



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



Acipenseridae 7, and with pike perch 3-3. With Acipenseridae intraspecific 

 competition is on the average 133, intrageneric 101 ; with pike perch 44, with 

 cyprinoids 7, and with Gobiidae 5 (conventional units). 



By May the competition between golden shiner and vobla becomes less 

 intensive and the two diets become more similar. In June the feeding of the 

 two fish becomes more intensive and the competition is more acute, while the 

 coincidence of the diet is much lessened. By August the intensity of the com- 

 petition continues to grow, and both the amount and the intensity are increased. 



Schorygin gives (Fig. 302a) a general picture of the dynamics of the food 

 relationship between the two forms of fish. By the summer feeding and 

 intensity of competition increase, but the two diets become less similar, since 



Fig. 302a. Diagram of food correlation of two 



fish (Schorygin) : 1 Feeding ground coincidence ; 



2 Competition intensity ; 3 Competition tension ; 



4 Volume of competition. 



the two fish have by then driven each other away to feed on different organ- 

 isms. The force of competition remains practically the same, with a decrease 

 in the coincidence of the diet and a corresponding growth of competition. 

 At the same time, although the two species begin to feed in different areas, the 

 force of the rivalry between them begins to grow. Then, if the intensity of com- 

 petition still increases its force begins to grow rapidly and a complete (forced) 

 divergence may take place both as regards food and feeding grounds; 

 following a decrease in intensity of competition the reverse process may take 

 place. 



The degree of elasticity in the diet of different breeds of fish plays an 

 important part in the course of these changes. Schorygin has also tried to 

 evaluate this latter. The degree of stability of the diet of a definite fish in 

 different seasons and areas can be determined from the indices of food coin- 

 cidence (Fc), and the mean value of this can also be found. Taking the value 

 complementary to 100 of the mean thus obtained, we shall have the index of 

 variability of diet. The results obtained in this way for six fish are given in 

 Table 273. The elasticity of diet develops with increase of the regional and 

 seasonal variations in the composition of the food. This effect can be excluded 



