ECOLOGICAL AND FISHERIES ASPECTS OF THE FORMATION OF THE MODERN HYDROLOGICAL REGIME OF THE WATER BODIES OF THE AZOV-DON REGION. PART 2: FISHERIES REQUIREMENTS TO THE GEOBOTANICAL CONDITIONS OF THE LOWER DON FLOODPLAIN; SALINITY REGIME OF TAGANROG BAY

Background. The second part of the article examines the features of the floodplain landscapes of the Lower Don, evaluates the salinity regime and the formation of desalinated zones in Taganrog Bay in 2006-2023, and considers issues related to the operating efficiency of the Ust-Manych fish passage channels during the spring spawnings of 2017-2024. Relevance. The current understanding of the geobotanical features of the Don River floodplain and the transformation of its landscapes is incomplete, which contributes to the relevance of assessment of the quality of spawning grounds and their compliance with the fisheries requirements. Compilation of historic and current data provided the basis for geobotanical characterization of the floodplain area of the Azov-Don Fishery Region and made it possible to present new data on the habitat conditions of aquatic biological resources in Taganrog Bay, as well as on the efficiency of the Ust-Manych fish passage channels. The aim of this article was to investigate the compliance of the geobotanical conditions in the floodplain areas of the Azov- Don Region with the fisheries requirements, to determine the extent of the formation of desalinated zones in Taganrog Bay, and to evaluate the efficiency of the Ust-Manych fish passage channels in the spring season. Methods. The data collected over the course of the annual expeditionary (Taganrog Bay) and field (Ust-Manych channels) studies, the materials from AzNIIRKH archives, and the information from literary sources on the research topic have been used. When collecting, processing and analyzing samples and relevant data, the methods recommended in [1] have been used, including mathematical and statistical analysis, analytical graph plotting and mapping. Results. The fisheries requirements to the geobotanical conditions in floodplain spawning grounds have been formulated; based on the summarized modern and historical data, the features of the Don River floodplain landscapes have been studied, and an assessment of the compliance of the geobotanical conditions with the fisheries requirements has been given. The analysis of changes in water salinity and the formation of desalinated areas in Taganrog Bay for the period 2006-2023 has been carried out. The operating efficiency of the Ust-Manych fish passage channels has been estimated for the spring seasons of 1917-2024. Conclusion. The transformation of the Lower Don floodplain landscapes led to the non-compliance of its geobotanical characteristics with the fisheries requirements. The increase in salinity of the Taganrog Bay water observed in the period 2006-2023 (from 5.26 ‰ in 2006 to 11.78 ‰ in 2020) and the reduction in the area of desalinated zones in salinity gradations from 4 to 9 ‰ provide evidence of the deterioration of the habitat conditions both for juveniles and for mature individuals of the semi-anadromous commercial fish species of the Azov-Don Region, as well as serve as an indicator of reduction in the volume of the water bodies receiving the released juveniles obtained through the artificial reproduction. In 2017-2024, the Ust-Manych fish passage channels generally ensured the passage of semi-anadromous fish species to their spawning grounds, although during certain periods the flow velocities exceeded 1 m/s, reaching carrying speed.