POPULATION DENSITY IN FISH SPECIES 275 



purposes by the fish-culturists, who store the cadavers of the spawners and 

 feed them to the young that are reared in hatcheries (Smirnov, 1954). 



CHANGES OF AMPLITUDES OF VARIABILITY OF CHARACTERS 

 AND PROPERTIES IN A POPULATION 



As it has been shown recently by a number of workers, the ampHtude of the 

 variabiUty of the characters and properties in a population changes adaptativly 

 with changes in the conditions of hfe, including food supply (Nikol'skii, 

 1955; Nikol'skii & Pikuleva, 1958; Polyakov, 1958; Yurovitskii, 1957). 



It also holds for the variabihty in size in a fish population of the same age. 

 Under favourable conditions the growth of individuals follows a similar 

 course, and the range in size in a population is comparatively small. With 

 the deterioration of food conditions, including an increase of population 

 density, the variability of fish size in the population increases. These changes 

 in size range with deterioration of food conditions can be exemplified by 

 the data of G. D. Polyakov on a common carp population, reared in ponds 

 with different food conditions. 



Upon the deterioration of the food supply the range of variabihty in the 

 size of the fish increases, the modal size shifts to the left part of the curve, 

 but, at the same time, the growth of the largest fish is even faster and their 

 size larger, than in fish under more favourable conditions of rearing. Such 

 extension of the size range within a population of the same age (when some 

 of the fish sometimes pass to the next developmental stage) is an important 

 adaptation, which extends the types of food eaten. Larger fish change to 

 feeding on food other than that taken by the younger ones. In populations 

 reared under good feeding conditions no such deviation in the size and 

 composition of food consumed is observed; all the fish feed on more or 

 less similar food. Knowledge of such changes in the size composition of 

 the fish in a population of the same age allows us, by the amplitude of the 

 variability and by the value of the coefficient of the asymmetry, to judge the 

 conditions under which the fish are being reared, as has been shown by 

 G. D. Polyakov (i960). 



An analysis of the year to year changes of amplitude in population varia- 

 tion shows in which year the population has had better or worse feeding 

 conditions. Thus, for example, in bleak of the Uchinsk reservoir, the fish of 

 the age of i + are less well provided with food than other age-groups. In 

 fish of this age the range of size is greatest. Thus, one of the most important 

 adaptations that provides for the survival of the population under unfavour- 

 able food conditions is the widening of the amplitude of the size variability. 



This adaptation is present in marine fish populations as well. Shatunovskii 



19 



