Analysis of effectiveness of pelagic fish species invasions into the Volga River reservoirs

Two pelagic fish species, smelt and kilka, have spread in the cascade of Volga River reservoirs since the time of its formation. A comparative analysis of the succession and the rate of distribution of these two species in the cascade is performed. Their role in the structure of the fish population in the pelagial is determined. It is found that, after a long period of the range expansion and increase in the abundance of smelt, almost total reduction of its range and abundance in the Volga basin has occurred by the present time. On the contrary, kilka not only has colonized all reservoirs of the cascade but in most cases also preserves its dominating position in the structure of pelagic fishes. The relationship between changes in ranges and abundance of both species and global climatic changes is established. On the basis of analysis of the long-term data on the structure of the fish population in the pelagial of the Rybinsk Reservoir, the effectiveness of the smelt and kilka invasion since the period of the formation of the reservoirs is assessed. According to the parameters of naturalization, kilka is a more effective alien species than smelt. Possible biological and ecological prerequisites of the greater invasive success of kilka as compared to smelt are considered.