A comprehensive study on the fish assemblages of the River Cauvery was conducted during 2017–2020 by structured fish sampling surveys, encompassing various sampling sites situated along the upper, middle, lower, and estuarine stretches. The present study recorded a total of 146 fish species belonging to 52 families from the River Cauvery. The highest recorded fish diversity was at Hogenakkal (76 species) as it is situated in the transition zone between Deccan Plateau and the plains of Tamil Nadu. Lowest diversity was recorded at Bhagamandala (23 species), as this site represented a narrow hillstream habitat. As per SIMPER analysis, the silver razor-belly minnow (Salmostoma acinaces) was the most representative species in Upper Cauvery, with percentage similarity contribution (% SC) of 36.55%. The Middle Cauvery situated in the plateau zone was mainly represented by Labeo calbasu (14.01% SC), the exotic Nile tilapia (Oreochromis niloticus) with 8.45%, and the bagrid catfish Mystus cavasius (5.1%). The exotic species contributed to maximum fish catch and gradually replaced the native fish fauna in Lower Cauvery. The fish fauna of Cauvery Estuary is distinct from other zones with a characteristic fish assemblage represented by the glassy perchlet, Ambassis miops (11.97% SC), Crenimugil buchanani (11.14 %), and Mugil cephalus (6.23 %). Among the fish species recorded during the present study, 29 species were endemic to Western Ghats, of which 10 species were categorized as threatened by the IUCN, including 2 critically endangered (Barbodes bovanicus, and Hemibagrus punctatus), 5 endangered (Dawkinsia arulius, Hypselobarbus curmuca, H. mussullah, H. micropogon, and Nemacheilus pulchellus) and 3 vulnerable species (Hypselobarbus kolus, Hyporhamphus xanthopterus, and Wallago attu). A holistic study on the fish communities of Cauvery along the spatio-temporal scale is imperative to ensure sustainable management for conservation of endemic fish fauna.

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