Environmental drivers shape fish diversity and community composition in the Ruo and Lichenya rivers, Southern Malawi

Riverine fish diversity and community composition are strongly influenced by environmental conditions, yet such information from southern Malawi remains limited despite its usefulness in determining the health status of the river. This study assessed fish abundance, diversity and community composition in the Ruo River and Lichenya River and examined key environmental drivers influencing fish assemblages. Fish were sampled using a 1.5-mm mesh seine net. Physicochemical parameters, including temperature, dissolved oxygen (DO) and pH, were measured in situ, while total dissolved solids (TDS), total suspended solids (TSS), nitrate, nitrite, ammonia, total phosphorus and chlorophyll-a (Chl-a) were analyzed in the laboratory. Fish diversity, abundance and community composition between the two rivers were compared using an independent t-test, while multivariate analyses were used to examine differences in fish assemblages and identify environmental drivers. Results showed significantly higher fish diversity and abundance in the Ruo River compared to Lichenya (p < 0.05), and fish assemblages differed significantly between the two rivers (R = 0.62, p < 0.001). Environmental variables, including TDS and phosphorus, were identified as key predictors of fish community structure (p < 0.05). These findings demonstrate that physicochemical conditions play an important role in shaping fish diversity and assemblage patterns in riverine ecosystems. Continued monitoring of environmental drivers is therefore essential for informing sustainable management and conservation of riverine fisheries.