Abstract
Abstract The dynamics of the number of introduced commercial fish (asp) Leuciscus aspius (Linnaeus, 1758), bream Abramis brama (Linnaeus, 1758), carp Cyprinus carpio Linnaeus, 1758, Caspian roach Rutilus caspicus (Yakovlev, 1870), wels catfish Silurus glanis Linnaeus, 1758) in the Kapchagay reservoir (Balkhash–Ili endorheic basin, Republic of Kazakhstan) from the moment of its filling to the present day, is considered. The dependence of the relative abundance of the species under consideration on the hydrological regime, hydrochemical factors, food availability, competition, and predator pressure, is analyzed. A connection was noted between commercial stocks of carp and bream and the number and biomass of zooplankton and zoobenthos. It was shown that the introduction of caspian roach into the reservoir intensified competition for food resources for carp. Correlations were found between the volume of industrial catch of predatory fish species (wels catfish and asp) and the relative abundance of potential prey (bream and carp), recorded several years ago (3 and 10–11 years, respectively). It is assumed that the high number of bream in the reservoir is due to insignificant pressure from predators (primarily wels catfish). The low population size of carp can be explained by a change in the hydrological regime of the reservoir, competition from other fish species (caspian roach) for food resources, and a high number of predators (asp).