340 THOROLF LINDSTROM AND NILS-ARVID NILSSON 



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Stockholm, 7, 135-40. 

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DISCUSSION 



M. Graham: Is the taxonomy of the species described secure, and are 

 the differences between them marked ? 



T. LiNDSTROM : Identification of species is based on gillraker structure, 

 there being no other significant character to use. I am convinced that they 

 are species as described by Svardson in the papers cited above. 



G. V. Nikol'skii: The data presented are a very good example of the 

 general rule that in the same faunistic complex, the different fish species 

 present take different food, at least in the adult stage. But if we wish to 

 determine the nature of their interrelations, or competition, we must con- 

 sider not only the composition of the diet but also the rate of growth and 

 the stomach index. The variabihty of the food resources in the Arctic is 

 reflected in the variability of food in the stomach contents. From north to 

 south in the northern hemisphere, it is found that the northern forms have 

 more varied stomach contents than the southern ones. This phenomenon is 

 connected with differences in the stabihty of the food organisms. There is a 

 need for the quantitative estimate of food demand. In sparse feeding times 

 there are several foods : in times of rich food specialization may occur. 



E. D. Le Cren: I agree entirely. We certainly need really quantitative 

 estimates of food consumed. For example, when I estimated the consumption 

 of food by pike of different ages, it appeared that the pike population as a 

 whole may consume as much plankton as fish. 



I. A. McLaren: Does the nature of the food organism affect the number 

 of gillrakers in larval fish? 



T. LiNDSTROM : No evidence of this for young fish is available. There 

 is a suggestion that the number of gillrakers affects the food of the adults. 



W. E. Frost : I had understood from previous work that each plankton 

 animal is taken individually in the adults of these species, as it is in char. The 

 gillrakers are therefore unlikely to have much connection with planktonic 

 feeding. 



