Habitat and Distribution Pattern of Threatened Freshwater Fishes found along the Drangmechhu River and its Tributaries in Pemagatshel District, Eastern Bhutan

Abstract

The distribution of freshwater fishes is primarily determined by physical and chemical water parameters and human activities. However, a complete understanding of how these factors relate to threatened species in the study area is still lacking. This study assessed the habitat association and distribution pattern of three threatened species Tor putitora Hamilton, 1822, Neolissochilus hexagonolepis McClelland, 1839, and Schizothorax richardsonii Gray, 1832, along the Drangmechhu River and its five tributaries in Pemagatshel District,

Bhutan. The field data was collected in March 2025 from 33 sampling plots, with each plot covering a 500 m stretch. Fishes were sampled using electrofishing and cast nets. A significant difference in fish abundance was detected across habitat types (H = 16.64, df = 2, p = .000), with riffles supporting the highest abundance (58.4%), followed by pools (30.1%), and runs (11.5%). The species exhibited distinct habitat association with respect to velocity, depth, and substrate composition. These threatened species preferred varying velocities, depths, and substrates. N. hexagonolepis favored higher velocities (0.5 m/s), shallower depths (0.44 m), and fine substrates, while S. richardsonii preferred lower velocities (0.3 m/s), moderate depths (0.56 m), and coarse substrates. Ecological models revealed highly suitable zones (>0.6) for N. hexagonolepis and S. richardsonii with strong predictive accuracy (AUC = 0.88–0.996), whereas models for T. putitora showed low accuracy (AUC = 0.547–0.569). Conservation implications from the study highlight the importance of habitat-specific management for threatened freshwater fishes along the Drangmechhu River system and its tributaries. Conservation should focus on protecting and restoring riffle habitats.