In the Caspian Sea, as in any natural body of water, the distribution and dynamics of the number of fish, including mullet, is due to the combined influence of environmental factors. In the present period of regression of the sea and changes in the level of runoff of the Volga River, salinity plays an important role in the life cycle of all representatives of the ichthyofauna. The article is devoted to the topical issue of the impact of the salt regime on the spatial and quantitative characteristics of the mullet population. The distribution and its relative abundance in 2016 and 2019 observations are presented. The distribution areas are mapped with a detailed description of the features of the formation of concentrations during two contrasting periods: desalination and salinization of the sea. Analyzing the data of trawl catches, it was found that in 2016, when the powerful Volga runoff led to the expansion of desalination zones, the average catch of mullet was 7 copies/an hour of trawling. It is noteworthy that this indicator has grown to 11 copies/an hour of trawling, during the salinization of the Northern Caspian in low-water 2019 Graphical data confirming the presence of a direct correlation between the relative abundance of mullet and increasing salinity gradients are presented. It is shown that for two years the value of the correlation coefficient was 0.74–0.76. Mullet was caught in areas with salinity from 2 to 17%. It was found that the majority of concentrations (68–74%) were formed in marine waters from 8%. In conclusion, it is concluded that the increase in salinity indicators of the northern part of the Caspian Sea has a positive effect on the expansion of the range of mullet.

The article describes in detail all the most significant components of the hydrological regime in the lower reaches of the Volga. As a result of the regulation of the Volga runoff by the Volgograd hydroelectric dam in 1958, the ecological conditions of natural reproduction of fish changed significantly: the areas of spawning grounds decreased, the hydrological regime was violated, which led to deformation of the spring flood regime, intra-annual redistribution of runoff. Their size is of fundamental importance for the fisheries of the Volga-Caspian basin. Attention is drawn to the deep deformation of all flood indicators that occurred after the regulation of the Volga runoff on the Lower Volga, while in the worse direction for the reproduction of aquatic biological resources. The process of degradation of aquatic biological resources of the Volga-Caspian reflects the lack of the minimum hydroecological conditions necessary to ensure the formation of resilient juvenile fish in the process of their natural reproduction in the overwhelming number of years. The modern operation of the Volga-Kama cascade of reservoirs does not meet the requirements of the Volga-Caspian fisheries. There is a tendency to increase intraannual redistribution of runoff, due to high winter releases of water into the lower reaches of the Volgograd hydroelectric complex and low spring releases. The system of management and intersectoral distribution of water resources during the flood does not currently meet the necessary hydroecological conditions under which it is possible to form resilient fish offspring and, as a consequence, preserve and increase stocks of the most valuable commercial fish species. The main reasons for the decrease in water reserves are determined by the regulation of the Volga runoff and as a result of this reduction in the volume of water supply to the lower reaches of the Volgograd hydroelectric complex. Especially this negative manifestation of a sharp deterioration in the flooding of spawning grounds occurs in low-water years, when the priority is to fill the reservoirs of the cascade to a normal retaining level in order to ensure the generation of hydroelectric power during the inter-war period (summer, autumn, winter), the operation of water transport, the implementation of water intake for irrigation, utility purposes, etc.