POPULATION DENSITY IN FISH SPECIES 267 



influence of predators, as the Amur predators (especially those belonging to 

 the Chinese lowland complex) feed chiefly on small fish. The absence of the 

 reserve fat products in the organs of young Amur carp makes them feed in 

 winter too. 



Thus, the decrease in the growth rate itself usually leads to a decrease in 

 the population, as a result of the increased influence of predators; in the case 

 of the increase in the growth rate, the effect of the predators weakens, which 

 results in increased survival of the population. 



V. V. Vasnetsov showed (1934, 1947) that the time of maturity in most 

 fish (excluding the formation of dwarf forms), is connected with the achieve- 

 ment of a defmite length. The maturity is correlated to a greater extent with 

 the size than with the age of the fish. This thesis is confirmed by the 

 observations of Monastyrskii (1940) on the Caspian roach and by ample 

 data of Aim (1959) on many fish species. 



There are many well-known examples of an alteration of the time of 

 maturity as a result of an alteration in the growth rate under the influence 

 of food supply, including the cases when population density changes under 

 the influence of a fishery. An increase of the growth rate and earlier maturity 

 can be observed in species acchmatized in new water bodies. In the latter 

 instance it ensures a more rapid increase of the population density and its 

 naturalization in the new basin. When the acclimatized population reaches a 

 certain population density, the rate of the fish growth becomes slower, the 

 maturity is attained later and the rate of reproduction decreases. 



Apparently, in most species of commercial fish the changes in the popula- 

 tion density are coimected with changes in the growth rate of individuals 

 and with the time of maturation. This has been noted for sturgeons, herring, 

 salmon, cod, carp and many others. So, according to the data obtained by 

 Chugunov (1927), as a result of the fishery decrease during the war of 1914- 

 19, the growth of sturgeons slowed and the rate of their reproduction 

 decreased. 



The growth rate of sterlet and of some other fish of the European part of 

 the U.S.S.R. considerably altered during the period of two most recent 

 millermiums, which is, undoubtedly, connected with the intensification of 

 fishery (Nikol'skii, 1958; Lebedev, i960) and the increase of their food 

 supply. 



In the case of the salmon, the data on the relation of the time of maturity 

 to the stock density are presented by I. B. Birman (1951) for the autumn 

 form of Chum salmon of the Amur. 



Some interesting data were obtained on the change of the growth rate and 

 the time of maturity of small whitefish {Coregomis alhula L.) acchmatized in 

 some Ural lakes; and also on the change of growth rate of Sevan trout {Salmo 



