Identifying imperilled fish species and potential causes of decline in the Himalaya biodiversity hotspot

Despite the global nature of the aquatic biodiversity crisis, drivers of extirpation vary among regions, making local-level data collection crucial for adaptive management in understudied regions like the Himalaya Biodiversity Hotspot (HBH). As human population growth, economic development and environment change accelerate across the HBH it is important to identify imperilled species and determine any ecological traits that predispose them to extirpation.In the present study, Monte Carlo analysis was used to determine fish species with significant range reductions ('significantly declining species') from the 1980s to the 1990s, and from the 1990s to the 2010s, at 38 sites on Nepal's Kaligandaki-Narayani River (KNR). Relative to a null model under which all species were equally likely to lose individuals between time periods, distributions of 16 of 83 species declined significantly across at least one of the two time periods, and two species, Hamilton's baril (Opsarius bendelisis) and the trout barb (Raiamas bola), declined across both.Mid-water fishes native to both lowlands and hills were most likely to experience range reduction. International Union for the Conservation of Nature Red List status was not representative of actual population trends, highlighting the critical role of local, long-term data collection and management strategies for effective conservation.Opsarius bendelisis and R. bola urgently need protection in the KNR to stem current population declines. Conservation efforts here should focus on speciose lower-elevation sites, where many of the 14 other significantly declining species also occur. Overfishing is likely to have contributed to species declines, suggesting that the network of terrestrial protected areas already in place across the HBH can aid aquatic biodiversity conservation efforts owing to their status as fish sanctuaries. Rivers such as the KNR represent a unique opportunity to conserve an intact, albeit threatened, fish species, before human pressures lead to widespread extirpations of the type documented in many areas of the world.