POPULATION DENSITY IN FISH SPECIES 269 



and thus ensure its reproduction under a poor food supply. If under condi- 

 tions of poor food supply a population attained maturity at a size larger than 

 that typical of the given species, then it could not be able to provide for its 

 survival and would perish (Lapin & Yurovitskii, 1959; Aim, 1959). 



This case was analysed for the first time in Russian hterature by Vasnetsov 

 (1947), who exemplified it by slowly growing populations of a common 

 crucian carp. To this group of phenomenon belongs the formation of dwarf 

 lake and river forms of salmon (both Salmo and Oncorhynchus) (Kozhin & 

 Protasov, 1959; Moiseev, 1957; Smirnov, 1959 et ah) and the formation of 

 freshwater populations of smelts (both Osmerus and Hypomesus). Among the 

 Cyprinoids, as well as the crucian mentioned above, is the formation of slow- 

 growing precocious forms of semi-migratory roach (Rutihis rutilus caspicus 

 Jax.) and bream {Abramis brama L.) in the lakes of Uzboi. Unfortunately as 

 yet the physiological mechanism of these changes in stock structure is not 

 exactly clear. It is undoubtedly connected with metabolic changes, but 

 evidently not only with the quantitative aspect of food supply, but also 

 with the alteration of its quahty, which, in its turn, somehow alters hormone 

 activity (Aim, 1959). Within these slowly growing populations the same 

 mechanism as that described above continues to act: a rapid growth and an 

 earUer maturity under a better food supply, and a slower growth and later 

 maturity under a poorer food supply. 



FECUNDITY ALTERATION IN THE INDIVIDUALS OF 

 A POPULATION 



An alteration in the growth rate of fish and the time of their maturity is 

 usually connected with a change in their fecundity. As is shown by many 

 investigators (Svardson, 1949; Nikol'skii, 1950; loganzen, 1955; Yurovitskii, 

 1958 et ah) fecundity of the individuals of a population varies adaptively in 

 relation to changed conditions of life. Many examples are known when a 

 change of food supply is associated not only with a change of growth and 

 the time of maturity but also with an alteration in fecundity in fish of the 

 same size. A similar phenomenon has been noticed in one population of the 

 Sakhalin herring {Clupea harengus pallasi) (Piskunov, 1952), in the North Sea 

 herring (Nikol'skii & Belyanina, 1959), in the North Caspian semi-migratory 

 roach (Chugunova, 195 1), and in thelSForth Caspian common carp (Sokolov, 

 1933). In all these cases the improvement of food supply leads to an increase 

 in fecundity in individuals of the same size. This fecundity increase can be 

 reached either by an increase in the volume of the gonads or by a decrease in 

 egg size. In the latter case, if the density of the yolk and the amount of fat 

 in the yolk do not increase, some decrease of food supply for the developing 

 organism takes place during its feeding on the yolk. 



