An Assessment of the Fish Assemblage in Dolphin-inhabited Areas of Assam

Several causes have been identified for the steady decline of the Gangetic dolphin (Platanista gangetica gangetica). The Gangetic dolphin is an exclusively piscivorous mammal, and it is well-known that the dolphin prefers the particular riverine habitats where fish assemblage is high, such as stretches where fish congregate. The paper aims to conduct an assessment of the fish assemblage in dolphin-inhabited areas of Assam. As fish are the primary food source of dolphins, an in-depth study was conducted to assess the fish diversity and abundance in selected dolphin-inhabited areas of Assam. Three sampling sites in the Brahmaputra system and five past dolphin-sighting areas were selected for the present study. Water samples were collected from all the sampling sites every month for a consecutive 2 years from December 2021 to November 2023. Ichthyospecies were collected seasonally and identified. Condition factor (K) is used to compare the condition or the well-being of the fish and is based on the hypothesis that fish with a heavier weight for a given length exhibit a better physiological condition. An attempt is made to provide detailed insights into the hydrobiological characteristics, including the major physicochemical parameters, of the Brahmaputra and Barak Rivers in Assam. The various issues about river management and fisheries ecology have been addressed. The K factor was found to be>1.0 in most of the species throughout the year. Among the studied species, Mystus cavasius was in poor condition, and Macrognathus aral, with a K value of 1.807, depicts the excellent condition of the species. Fishing intensity was lowest (0.67±0.20kg) in the monsoon with the cast net, a common fishing net used in the Dibru River and adjoining Maguri Beel. Again, with the cast nets, the catch per unit effort (cpue) was maximum in the post-monsoon season (0.93±0.33 kg), followed by the winter months (0.82±0.45 kg) and the pre-monsoon season (0.61±0.15 kg). The negative impacts of human activities, such as the regulation of water flow, overfishing, and siltation on aquatic habitats and their biota have been pointed out. It also highlights issues such as habitat alteration and calls for urgent conservation efforts to save the aquatic habitats and their associated biota. Therefore, fish conservation strategies must be implemented in conjunction with the strict enforcement of existing fishery laws, particularly in the upstream areas of the Barak River during the fishing ban period, and with the involvement of all stakeholders to maintain habitat quality and the prey base for the Gangetic dolphins.