266 G. V. NIKOL'SKII 



in connection with changes in the conditions of hfe. The changes taking 

 place in the structure of some species populations and in their reproductive 

 rate have not only arisen fortuitously as a result of the effects of external 

 influences on deviations, but are as well the result of the adaptive response of 

 the population to the change of conditions of life. 



Fish, like other organisms, establish adaptations which allow the population 

 to increase its density rapidly with an increase of food supply, and to slow 

 down the tempo of reproduction (or to decrease the population density in 

 some other way) with the decrease of food supply (even when the decrease 

 in food is due to an increase in the population density). The ability to change 

 the intensity of reproduction is expressed in different species in different ways. 

 A change in reproductive ability can be realized in different ways: through 

 a change in rate of growth and in the time of maturity, and through an 

 alteration in fecundity in individuals of the same size. Thus there is achieved 

 either an increase or a decrease in the number of eggs laid with the change 

 of food supply. 



The timing of approach to the spawning grounds and the difference in 

 the quahty of the ova produced by spawners of different sizes and ages is an 

 important adaptation to the regulation of population. Fish possess a diverse 

 system of adaptations for the protection of their population under unfavour- 

 able conditions of food supply. This system includes transition to feeding on 

 their own young, the preservation of a high quality of gonad products under 

 unfavourable feeding conditions, the change of variation amplitude of those 

 characters and properties of the population which make it more eurybiotic, 

 as well as others. 



ALTERATION OF THE GROWTH RATE AND THE TIME OF 



MATURITY 



The growth offish was shown by V. V. Vasnetsov in a series of his investiga- 

 tions (1934, 1947, 1953^, 1953^, 1958) to be an important adaptation, closely 

 connected with the character of the dynamics of the stock. In considering 

 the phenomena in general, one can reveal the following regularities: in fish 

 protected against their predators by various armaments (spines and plates), 

 the growth preceding maturity is as a rule slower than that of fish lacking 

 armament; different populations of one and the same species under conditions 

 of different intensities of predation grow differently. It was shown by V. S. 

 Kirpichnikov (1943), that the young of Aral carp decrease their linear growth 

 under deteriorating food supplies; but during this decrease, reserve substances 

 are accumulating in their organs to provide for hibernation. The young of 

 Amur carp, on the contrary, continue to grow, which growth decreases the 



