268 G. V. NIKOL'SKII 



ishchan infsp. gegarkuni) acclimatized in Lake Issyk-Kul in Tien Shan (Luzhin, 

 1956). The growth rate of both accHmatized species accelerated and the time 

 of maturity became earlier. The change of reproductive ability was observed 

 in a herring stock in the southern part of the North Sea, as a result of the 

 alteration of the time of maturity in post-war years (Gushing & Burd, 1957; 

 Nikol'skii & Belyanina, I959)- 



There are many examples of the alteration of reproductive abihty in 

 populations of Cyprinoid fishes. There is a direct correlation between the 

 density of North Caspian bream, its rate of growth and its time of matura- 

 tion. Of course, the growth rate and the rate of maturity of Caspian bream, 

 as well as those of other fish, are connected not only with the density of its 

 population. They depend to a considerable extent on the change of the 

 biomass of its food organisms, which in its turn, is related to the character 

 of the river flow. This interrelation between the stock density and the 

 biomass of food benthos, is shown rather clearly (Zemskaya, 1958). The 

 number of individuals of Caspian bream attaining maturity can be doubled 

 in succeeding years. The same was observed for the Caspian roach by N. L. 

 Chugunov (1928). 



The increased rate of maturation and younger age composition of maturing 

 recruits under favourable conditions of development, results in a rapid 

 increase of the population density. At the same time the wider age range of 

 the series of matured individuals provides greater stability in the size of 

 recruitment and decreases the influence of poor brood years. 



In fish with a short life cycle, e.g. in lake smelt {Osmerus eperlanus eperlanus 

 morpha spirinchus) the changes in the rate of maturity can also be rather 

 considerable and can be connected with a serious alteration in the stock 

 structure (Lapin, i960). In unfavourable years the development of lake smelt 

 can be delayed to such an extent that no spawning takes place. Under 

 favourable conditions of food supply, however, the whole generation can 

 attain maturity in a year, which ensures a rapid increase in stock, the develop- 

 ment of the eggs and young being successful. 



We have just considered that type of alteration to the reproduction rate 

 of a population when the size of individuals attaining maturity remains more 

 or less the same, but the age of maturity and the age composition of recruit- 

 ment undergo change. The second form of response of a population to a 

 change in the conditions of hfe — a lower food supply, and apparently a 

 higher mortahty rate — is the change in the size at which maturity is 

 attained. 



In this case fish attain maturity at a smaller size and at an earher age. Such 

 an alteration in stock structure provides a more rapid restoration of the 

 population, which can therefore exist under more variable biotic conditions 



