The composition of the fish population of the Mekong River is one of the richest on the planet, however, despite the high diversity ( 1200 species) and productivity, the basis of fishing in the Mekong Delta is made up of several dozen species, the most valuable of which are members of the families Pangasiidae, Polynemidae and Sciaenidae. Information on the distribution of these taxa is fragmentary, and the role of various parts of the delta in their reproduction has not been studied. Distribution of families Pangasiidae, Polynemidae and Sciaenidae in the Mekong Delta was assessed in the dry season by midwater trawl catches in 2019 (April, December), 2021 (January, April) and 2022 (March–May). A total of 237 trawls were carried out, in the catch of which 45.9 thousand fish belonging to 36 families were found. Representatives of each of the families Pangasiidae, Polynemidae, and Sciaenidae were noted in 42–57

The problem of saltwater intrusion into the Mekong Delta is a research topic involving various branches of science. Among the causes of this phenomenon are the global rise in sea level and the regulation of the natural river discharge due to hydropower dam construction. The scale of the delta and its water dynamics complicate instrument-based observations. Meanwhile, communities of living organisms can serve as indicators of environmental heterogeneity. This study is aimed at analyzing the spatial variability of fish assemblage organization in the Mekong Delta for use as an indicator of saltwater intrusion. The composition of fish assemblages in different parts of the delta are determined by midwater trawl catches in January and April 2021. A total of ~15 000 individual fish from 74 trawl hauls were captured and analyzed. The null hypothesis was that the fish assemblages in the Mekong Delta are represented by three taxonomic complexes that are localized in the upper course (freshwater), the lower course (brackish), and the middle course (zone of the contact of the fresh and brackish waters) of the delta. The taxonomic composition of the freshwater complex was the poorest and was represented by nine families. Meanwhile, marginal (the contact zone) and brackish complexes included 26 and 23 families, respectively. Specimens of families Cobitidae, Eleotridae, Plotosidae, and Siluridae, which are only found in the zone of contact of fresh and brackish waters, could be considered indicator species of its position. The actual boundaries of the brackish and freshwater complexes were determined and the distance from them to the marine delta edge was equal to 34 and 78 km, respectively. Assessing the salinity values that correspond to the complex boundaries is a task for future investigations. The position of the boundaries of freshwater and brackish fish taxonomic complexes has a biological basis and could be used as an indicator of salt water intrusion into the Mekong Delta.

The problem of salt water intrusion into the Mekong Delta is a research topic involving various branches of science. Among the causes of this phenomenon are global sea level rise and regulation of the natural river discharge due to hydropower dam construction. The scale of the delta and its water dynamics complicate instrument-based observations. Meanwhile, communities of living organisms can serve as indicators of environmental heterogeneity. This study aimed to analyze the spatial variability of fish assemblage organisation in the Mekong Delta for use as an indicator of salt water intrusion. The composition of fish assemblages in different parts of the delta were determined by midwater trawl catches during January and April 2021. A total of ~15 thousand individual fish from 74 trawl hauls were captured and analyzed. The null hypothesis was that the fish assemblages in the Mekong Delta are represented by three taxonomic complexes that are localized in the upper course (freshwater), the lower course (brackish) and the middle course (zone of the contact of the fresh and brackish waters) of the delta. The taxonomic composition of the freshwater complex is the most poor and is represented by 9 families. Meanwhile, marginal (the contact zone) and brackish complexes include 26 and 23 families, respectively. Specimens of families Cobitidae, Eleotridae, Plotosidae and Siluridae that are only found in the zone of contact of fresh and brackish waters could be considered as indicator species of its position. The actual boundaries of the brackish and freshwater complexes were determined and the distance from them to the marine delta edge equals 34 and 78 km, respectively. Assessment of the salinity values that correspond to the complex boundaries is a task for future investigation. The position of the boundaries of freshwater and brackish fish taxonomic complexes has a biological basis and could be used as an indicator of salt water intrusion into the Mekong Delta.

It is found that a large species of the order Cladocera, Daphnia (Ctenodaphnia) magna Straus, has disappeared from the zooplankton composition in the alpine Lake Sevan, Armenia, with an increase in the density of whitefish, Coregonus lavaretus L. This led to an increase in the quantitative characteristics of heterotrophic nanoflagellates, resulting in a decrease in the number and biomass of bacterioplankton. At the same time, a number of seasonal features of plankton transformation were observed. In particular, the number and biomass of planktonic invertebrates, including representatives of Cladocera, D. longispina O.F. Müller, and Diaphanosoma lacustris Kořínek increased in July. The main reason for this was the change in the distribution of whitefish caused by an increase in their abundance, depletion of food supply due to the disappearance of D. magna, and a high water temperature. This contributed to the formation of the maximum fish density at greater depths than usual, where the trophic and temperature conditions were optimally combined. The reduction of a significant effect of fish on zooplankton can be associated with an increase in the trophic status of the lake under which grazing is compensated by a higher fecundity of invertebrates. An increase in the trophic status of the lake is indicated by an increase in the phytoplankton biomass, which is determined by the increasing amount of phosphorus regenerated by Cladocera against the background of a high water temperature. This not only compensated for grazing of algae and cyanobacteria, but also contributed to an increase in their biomass. In October, the pressure of fish on invertebrates in the water column increased due to an increase in their density and the patterns of their vertical distribution due to oxygen deficiency in the near-bottom layers. Under these conditions, the number of species and the proportion of Rotifera in the total abundance and biomass of zooplankton increased, while the total abundance and biomass of zooplankton decreased due to the filter-feeding crustaceans D. lacustris and D. longispina. However, the decrease in their abundance did not lead to changes in the biomass and structure of phytoplankton, which is associated with the decrease in the input of phosphorus excreted by Cladocera. Thus, the response of invertebrates and the groups of planktonic organisms they control to an increased density of the fish population depends on the vertical distribution of planktophages determined by the water temperature, oxygen concentration, and the quantitative characteristics of food objects.