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The characteristics of the quantitative distribution of fish and decapod crustaceans of benthic and demersal communities were first studied in the six main channels of the Tien River in the lower part of the Mekong Delta, and the coastal zone adjacent to estuaries. The material was collected during the low water season from April to June 2018 using standard commercial beam trawl. The region from the estuary zone upstream to a distance of 110 km has been studied. The coordinates of the beginning and the end of the trawling were determined using a GPS navigator. The average weight of the catch was about 30 kg, the maximum was 100 kg and more. The average catch of fish and crustaceans was about 1.5 kg, the maximum was 4 kg; the anthropogenic wastes and remnants of vegetation made up the rest of the catch. The average specific amount and biomass values which were collected at 36 stations based on 102 trawling results were computed for a comparative analysis of the spatial variability of fish and decapods abundance. The presence of wide limits of variations in quantitative indicators was established. Specific number of fish varied from 23.4 to 1978.2 (average 333.3) specimens·ha-1, decapod crustaceans – from 10.6 to 5091.5 (358.1) specimens·ha-1, the biomass of these groups was 219.1–22773.5 (average 1969.9) g·ha-1 and 31.4–9337.1 (average 740.6) g·ha-1, respectively. The main regularities of the spatial distribution of the density of hydrobionts have been identified. The increase in the average specific abundance of fish in the upper part of the investigated watercourses to 656.2 specimens·ha-1 was noted, its value is twice higher than that of the whole areas along the delta, whereas the distribution of the average specific biomass was fairly uniform. These patterns were associated with the presence of juveniles of commercial species and small-sized fish species in the upper reaches of rivers. The trends in the spatial distribution of abundance and biomass of decapods were very close. The highest values were observed at the mouths of rivers and the estuary zone of the delta, as the shrimps of the suborder Dendrobrachiata and the crabs of the suborder Pleocyemata appeared in the communities. Among the fish, the representatives of the marine-originated families which belong to Polynemidae, Ariidae, and Engraulidae were dominant. Their share in catches was 39.4, 20.4 and 13.4 % in abundance and 28.5, 34.9 and 9.2 % in biomass, respectively. Euryhaline estuarine and coastal (shelf) species prevailed. Sea shrimps of the families Penaeidae, Aristeidae and freshwater prawn from the family Palaemonidae prevailed among the decapods. Representatives of the Cyprinidae family (the richest in terms of diversity, abundance and biomass in the Mekong Delta) and a number of other freshwater fish, with the exception of Akysidae, were insignificant in abundance, probably due to their migrations in dry season from upstream sections of the river. The influence of some anthropogenic factors on the quantitative indicators of benthic and demersal communities was analyzed. Large amounts of solid domestic and industrial waste, especially from towns, could be one reason for the relatively low average values of density of fish and decapods. This could have a negative impact on feeding and on the life cycles of these hydrobionts in general. Anomalies in the distribution of water salinity and the specific density of fish and crustaceans in the region of the dam on the river Balay were discovered. Upstream from the dam, the water was fresh, whereas, in downstream part, the salinity of the bottom water layer could reach 17 ‰. The average specific abundance of fish was higher in the lower reaches of the dam, and the average specific biomass was higher in the upper dam. The low average specific abundance and biomass of decapod crustaceans were noted there, possibly due to the cessation of spawning migrations of both marine and freshwater shrimps. The obtained results can be used as the basis for subsequent monitoring studies of the structural changes in the bottom communities of fish and decapod crustaceans of the Mekong Delta.
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