Hill streams in mountainous regions are fast-flowing, narrow watercourses that support diverse freshwater ecosystems. A study was conducted from 2022 to 2024 in the Bichom River, Arunachal Pradesh, to assess fish diversity and the reproductive biology of Schizothorax richardsonii (snow trout). Sampling was conducted at six sites across pre-monsoon, monsoon, and post-monsoon seasons. The riverbed, composed of boulders, cobbles, and gravel, supported typical hill stream features such as riffles, pools, and dense bank vegetation. S. richardsonii was the most dominant species, followed by Neolissochilus hexagonolepis and S. progastus. Both S. richardsonii and S. progastus undertake upstream migration during the breeding season and return downstream after breeding. Fish abundance and diversity were highest during the pre-monsoon and post monsoon periods, declining during the monsoon due to habitat disturbances. Spatial differences in fish assemblages reflected transitions between lotic and lentic habitats. The family Cyprinidae dominated the fish community, comprising approximately 85% of the total population, while Danionidae (2%), Balitoridae (1%), and Psilorhynchidae (1%) were least represented (Fig. 1).
S. richardsonii, locally known as ‘Asla’, is listed as Vulnerable (IUCN, 2025-1) and is threatened by habitat fragmentation, overfishing, and invasive species. A total of 120 specimens were examined, and six maturity stages were identified. Ova were large and distinct during pre- and post-monsoon seasons and smaller during the monsoon. Ova diameter ranged from 0.4 to 2.0 mm, with a mean absolute fecundity of 4,304 ova and relative fecundity of 12 ova/kg body weight. The gonadosomatic index (GSI) ranged from 0.11 to 3.83. Length at first maturity was 16.5 cm for males and 17.5 cm for females, with males outnumbering females across all seasons. The biological data generated through this study provide critical insights for the conservation and management of this indigenous species in the Eastern Himalayan region.