Spatio-temporal variation in fish assemblages and factors affecting them in the Madi River, Nepal

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ABSTRACT Understanding spatial and temporal variations in fish assemblages within river systems is vital for evaluating their ecological significance and predicting the impacts of anthropogenic activities. Despite the economic and ecological importance of freshwater fisheries in Nepal, limited information is available on mid-hill rivers. We investigated the spatio-temporal dynamics and environmental factors influencing fish abundance and species richness in the Madi River, Nepal, from 2021 to 2023. We conducted seasonal sampling across different elevations using electrofishing. We recorded 26 fish species belonging to four orders, seven families, and 17 genera. Cypriniformes was the dominant order, while Cyprinidae represented the most abundant family. The Canonical Correspondence Analysis revealed that environmental variables, temperature and dissolved oxygen influenced fish assemblages. Spatio-temporal clustering showed higher similarity among sites during summer and autumn, whereas greater variation was observed in winter. Fish assemblage composition was varied among sampling sites (R=0.506; p<0.001), but no seasonal variation was detected (R=-0.029; p=0.848). Fish abundance and species richness were significantly associated with water temperature, free CO 2, and total hardness. Fish abundance increased with higher water temperature (β=0.149; SE=0.016; p=0.001) and dissolved oxygen concentration (β=0.071; SE=0.023; p=0.002), while both abundance and richness declined with higher free CO₂ levels. This study provides baseline information for assessing anthropogenic impacts and supporting the sustainable management of fish in the Madi River. Our findings underscore that conservation strategies should prioritize addressing the spatial variation in habitat conditions over seasonal interventions to maintain fish assemblages in this river. We therefore recommend prioritizing site-specific conservation measures to enhance and maintain fish diversity in Madi River networks.

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