Fish Assemblage Structure and Environmental Correlates in Nepal's Gandaki River

This study investigates spatial and temporal patterns of fish assemblage composition in Nepal's Gandaki River, which exhibits extremes of altitude and environmental conditions. Samples (n = 156) were made at 81 sites from the river's headwaters at 3100 m altitude north of the Himalaya Mountains to its lowland mainstream 50 m above sea level on the subtropical Gangetic Plain. Fourteen sites were sampled every season for two years. The ichthyofauna consisted mainly of cypriniforms and siluriforms and also included cyprinodontiforms and perciforms. Detrended canonical correspondence analysis indicated that geography, water quality, and stream hydraulics were the principal physiochemical correlates of fish assemblage structure. Monsoon rains and associated parameters had substantial secondary influence, and vegetation composition and abundance were also significant. The fish fauna showed longitudinal replacement and addition of species, as the number of species increased from one in the headwaters to a maximum of 33 in lowland sites.