POPULATION DENSITY IN FISH SPECIES 271 



Fluctuation from year to year in productivity is connected with different 

 quantities of yolk and fat in the eggs of different fish species. Fish with a 

 small amount of fat usually have greater fluctuations (e.g. herring, cod). 

 Their larvae at the stage of transition from internal to external food can 

 exist without external feeding for a rather shorter time (Soleim, 1942) than 

 those of fish which have eggs with a considerable fat content (sturgeons, 

 roach and some other Cyprinoid fishes). Naturally, the amount of fat in 

 eggs is important not only as a food supply; it can have rather different 

 functions, and in particular fat plays an important role as a hydrostatic organ 

 in many pelagic eggs and fish larvae (Kryzhanovski, i960). 



The conditions of food supply for the parents in the year before the 

 spawning, particularly the abundance of food (the development of plankton 

 and benthos), has been shown by Nikolaev (1958, 1958^) to determine to a 



Table II. Comparison of the fecundity and fatness of fat* and 'lean Baltic herring 

 in different size groups [after L. E. Anokhina, 1959) 



great extent the quantity of the generation of the next year in sprat, Atlantic- 

 Scandinavian herrings, Baltic herring and freshwater smelt. The lack of 

 knowledge of the state of the parental stock and particularly of the quahty 

 of its roe, makes it difficult to understand the causes of fluctuations. In some 

 fish the fecundity of individuals in the population is connected with their 

 fatness; fatter individuals usually appear to be more fecund. This has been 

 noticed, for example, for Siberian sturgeon (Acipenser baeri Brandt) from the 

 Ob' River by Petkevich (195 1) and for Baltic herring (Clupea harengus 

 membras L.) from Riga Bay by Anokhina (1959) • As one can see from the 

 figures presented in Table II fatter specimens of Baltic herring possess a 

 higher fecundity. Hence the food conditions of many species during the 

 preceding year seem to determine to a great extent not only the egg quahty 

 but the population fecundity for the next year. 



It should be noted that some adaptations appear in fish to provide better 

 conditions for gonad development under unfavourable feeding conditions. 



