Feeding resources and key nutrients of commercial fish in the Ural-Caspian Basin

The article presents information on hydrobiological monitoring in the Ural–Caspian Basin, including phytoplankton, zooplankton, and benthic communities in the Ural–Caspian Basin. The annual dynamics of hydrobiont biomass density have been established, showing seasonal variations with the highest values in summer and the lowest in winter. The qualitative and quantitative composition of aquatic organisms was studied depending on the discharge volume and flow levels of the Ural (Zhaiyk) River. In high-water years, the biomass of phytoplankton and the abundance of zooplankton increased by 1.4 and 2.2 times, respectively. In recent years, an increase in the biomass of gelatinous zooplankton has been observed. Among them, the comb jelly Mnemiopsis is spreading particularly actively, successfully colonizing shallow, low-salinity areas of the sea. This species is a major consumer of zooplankton, which negatively affects the food base of planktivorous fish and the juveniles of all ichthyofauna species in the Caspian Sea. An analysis was also conducted on the feeding preferences of anadromous, marine, semi-anadromous, and freshwater species inhabiting the Caspian Sea. Keywords: forage base, abundance, biomass, ichthyofauna, fish diet.