Number and distribution of fingerlings of semi-anadromous fish in the western part of the Northern Caspian Sea in conditions of low-water flow of the Volga river

The results of surveys on the number of fingerlings of roach (Rutilus rutilus caspicus), bream (Abramis brama) and zander (Sander lucioperca) in the western part of the Northern Caspian Sea in 2021-2024 are presented. According to the hydrological parameters of high water, these years are characterized as low-water years, when the flow volume from March to June does not exceed 100 km3, that largely determined the low number of generations of roach, bream and zander born in 2021-2024. From 2021 to 2024, the number of roach fingerlings decreased 2.9 times, reaching the lowest value for the entire observation period (since the 1930s) of 1.4 billion specimens in 2024, which, in addition to unfavorable breeding conditions, is also due to the deep depression of the species. The bream population, which is in a relatively prosperous state, also reacts to the deterioration of reproductive conditions by reducing the number of juveniles (2.1 times from 2021 to 2024), but not as critically as the roach. The zander, whose population is gradually recovering, is characterized by significant fluctuations in yield due to unstable breeding conditions. The number of fingerlings of this species varies quite widely even with the low flow of the Volga River, however, remaining at the level of low-yielding generations. The generations of all three species of semi-anadromous fish born in 2021-2024 are estimated as low-yielding. The roach fingerlings most widely explored the sea area. The feeding area of bream fingerlings due to reduced salt tolerance and especially of zander due to low abundance is smaller than one of the roach. Despite the fact that fingerlings of roach, bream and zander were found at depths up to 16 m with a salinity of up to 13‰, the roach stuck to deeper water areas and for most of its generation foraged in waters of increased (over 8‰) salinity. Fingerlings of bream and zander mainly foraged in shallow waters up to a 5-meter isobath at a lower salinity than the roach.