Organization of Fish Assemblage in the Mekong Delta as a Potential Indicator of Saltwater Intrusion

Abstract—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.