Seasonal variation in fish diversity and hydrological variables of the Old Brahmaputra River, Bangladesh

The Old Brahmaputra River is a transboundary river and serves as an important habitat for many freshwater fish species of Bangladesh. To determine its fish diversity in relation to environmental variables, this study was carried out from March to November 2021 covering three seasons: pre-monsoon, monsoon, and post-monsoon in this river at Jamalpur district of Bangladesh. A total of 44 native fish species were recorded, spanning 33 genera, 20 families and 7 orders. The order Cypriniformes was found to be the most abundant with 18 native species. Fish abundance and species richness were significantly higher ( p < 0.05) at post-monsoon followed by pre-monsoon and monsoon. Amblypharyngodon mola was the most abundant species and Bagarius bagarius was the least abundant. Shannon-Wiener diversity, Margalef’s richness, and Pielou’s evenness indices were tested and all the scores changed with the seasons. Multivariate analyses showed that dissolved oxygen (DO) and pH were negatively linked to water temperature, transparency, total dissolved solids, alkalinity, and electric conductivity. DO and pH were identified as the primary factors responsible for seasonal differences in water quality parameters. Principal Component Analysis and Canonical Correspondence Analysis confirmed significant seasonal differentiation in fish assemblages, with DO exhibiting a strong positive correlation with fish abundance ( r = 0.949, p < 0.01). To protect the environment and fish species in riverine systems, this study gives basic data and information for long-term management and conservation of native fish species.