POPULATION DENSITY IN FISH SPECIES 277 



What is the adaptive importance of the different sizes of eggs under 

 unfavourable conditions of food provision. It has been shown that the 

 embryos from different sizes of eggs hatch and change to external feeding 

 at different times. Besides that, populations of embryos hatching with a 

 range of different yolk reserves survive better under unfavourable conditions. 

 This is clearly demonstrated by Nikitinskaya (1958) in the development of 

 larve of Sakhalin herring {Clupea harengus pallasiY2^.). The same phenomenon 

 was observed by A. V. Morozov (195 1) in Acipenser stellatus Pall. The 

 different rhythms of feeding at the stage of mixed feeding are connected 

 with the differences in the reserve of yolk in the larvae when they hatch. 



As was shown by Pikuleva-Grigorash (1961) for the roach of the Ucha 

 water-body, those larvae that have a significant reserve of yolk at the stage 

 of mixed feeding possess a clearly expressed diurnal rhythm of feeding, 

 while the larvae that have a small yolk reserve continue feeding on the 

 external food uninterruptedly; this also contributes to the more complete 

 utilization of the food supply. A widening of the range of foods eaten is 

 also achieved, as the smaller larvae are feeding on one kind of food, while 

 larger ones are feeding on another kind. If larvae develop under unfavourable 

 conditions of feeding this difference in size increases, while under favourable 

 conditions it becomes less as the smaller larvae overtake the larger ones in 

 size. The ways in which egg heterogeneity has adaptive importance are 

 different with eggs laid in one batch or a single spawning and those laid in 

 part of a spawning (N. V. Lebedev, 1959; G. V. Nikol'skii, 1953) but most 

 of these ways are connected with a better utilization of the food supply and 

 thus with an increase in population density and biomass. 



CONCLUSIONS 



A knowledge of the forms of adaptive response of populations to changes in 

 the conditions of life, and, particularly, to changes in food supply, is as 

 necessary for a rational organization of a fishery, as is a knowledge of the 

 changes in living conditions of the population. It is impossible to build up a 

 rational fishery industry, based only on studies of the changes in living 

 conditions, without trying to understand the adaptive response of the 

 population. The fishing industry should be planned in such a way that it 

 becomes a component of the environment of the species exploited, i.e. the 

 rate of the industry's influence should not exceed the range of the adaptations 

 of a species. The industry must provide the maximal productivity of the 

 population exploited. 



