The Old Brahmaputra is a distributary of the Brahmaputra system that becomes separated from the main flow near Jamalpur and extends through several districts, including Jamalpur and Mymensingh. Its waters provide habitat for fish that use rivers, floodplains and connected wetlands for feeding, spawning, refuge and movement. Bangladesh contains an extensive network of rivers, canals, lakes, estuaries and floodplain habitats, and the country is home to hundreds of native freshwater fish species. Yet these ecosystems face mounting pressure from overfishing, fishing during breeding seasons, pollution, habitat degradation, altered flow and climate-related changes. Previous surveys of the Old Brahmaputra have reported different numbers of species, reflecting differences in geography, timing, sampling effort and fishing gear. The new assessment focused on the Jamalpur Sadar stretch, where the river remains ecologically and economically important but where seasonal fish diversity and environmental variation had not previously been examined together in a recent, site-specific study.

Researchers sampled three sites—Nandina, Nurundi and Pyarpur—located about nine kilometres apart. Fish were collected monthly from local boats using several commonly employed gears: cast nets, seine nets, gill nets, push nets and traps. In total, 4,340 specimens were recorded. The researchers also consulted 30 local fishers through open-ended surveys and focus-group discussions, using local knowledge to confirm species availability and traditional names. Specimens that could not be identified in the field were preserved in buffered formalin and examined in a laboratory using morphometric and meristic characteristics, such as measurements of body proportions and counts of anatomical structures. Sampling was organized around three locally recognized periods: pre-monsoon from March to May, monsoon from June to August and post-monsoon from September to November. Because fishing gears differ in the sizes and species they capture, the results represent a composite view of the exploitable fish community rather than a complete census of every fish inhabiting the river.

The 44 recorded species belonged to 33 genera, 20 families and seven orders. Cypriniformes, the order that includes many carps and minnows, was dominant with 18 species. Siluriformes contributed 11 species, while Anabantiformes contributed eight; Synbranchiformes and Clupeiformes accounted for three and two species, respectively, and Ovalentaria and Cyprinodontiformes were represented by one species each. The most abundant species in the overall catch was Amblypharyngodon mola, a small indigenous fish, while the least abundant was the critically endangered river catfish Bagarius bagarius. That species made up only 0.03 percent of the catch and was recorded only during the post-monsoon period. The conservation profile of the catch was mixed: 59.09 percent of the species were classified as least concern in Bangladesh, but about 29.55 percent fell into vulnerable, endangered or critically endangered categories. Species of conservation concern included Botia dario, Rita rita, Clupisoma garua, Mastacembelus armatus, Ompok pabda, Bagarius bagarius and Labeo boga.

Seasonal comparisons revealed a pattern shaped by the river’s flood pulse. The post-monsoon period produced the highest documented abundance and species richness, followed by pre-monsoon and then monsoon. At first glance, the result may seem surprising because the monsoon brings more water and expands aquatic habitat. Flooding can indeed create spawning and feeding opportunities by connecting the river with floodplains and transporting nutrients into the system. However, fish become dispersed across a much larger area during high water, making them harder to capture in the main channel. As floodwaters recede, fish and other aquatic organisms can become concentrated in remaining channels and pools, increasing both their apparent abundance and the number of species recorded by fisheries sampling. The Bray-Curtis cluster analysis supported this interpretation by showing that pre-monsoon and post-monsoon fish assemblages were more similar to one another than either was to the monsoon assemblage. The reported similarity between the first two groups was 0.86, compared with 0.76 between the monsoon group and the others.

Three diversity measures reinforced the seasonal signal. The Shannon-Wiener index, which combines species richness with the relative balance of abundances, ranged from 2.91 during the monsoon to 3.41 after the monsoon. Pielou’s evenness index increased from 0.84 in the monsoon to 0.91 in the post-monsoon period, indicating that the community was more evenly distributed among species after the flood season. Margalef’s richness index ranged from 4.32 during the monsoon to 5.67 during the pre-monsoon. These indices must be interpreted alongside the sampling method: lower monsoon values do not necessarily mean that fish vanished from the river. Instead, fish may have occupied newly inundated habitats beyond the sampled channel, while high water and stronger flow reduced the efficiency of nets. The researchers also observed species-specific responses. Amblypharyngodon mola was more abundant during the monsoon, when elevated water levels, flooding and nutrient-rich runoff may have created suitable spawning and feeding conditions. Other species, including Macrognathus aculeatus, Channa orientalis and Nandus nandus, were more associated with the pre-monsoon period, whereas Labeo boga, Labeo bata, Mastacembelus pancalus, Xenentodon cancila and Ailia coila were more prominent after the monsoon.

Water-quality measurements help explain why the fish assemblages separated so clearly by season. Researchers measured temperature, pH, dissolved oxygen, total dissolved solids, alkalinity, electrical conductivity and transparency at each site, collecting water samples in triplicate and taking in-situ readings with a calibrated multiprobe. The pre-monsoon period had the highest average water temperature, 30.37 degrees Celsius, and the lowest dissolved oxygen concentration, 5.32 milligrams per litre. It also had the lowest pH, 7.43, and the lowest total dissolved solids, 171.04 milligrams per litre. The monsoon had the highest average pH, 8.68, and the lowest average alkalinity, 91.32 milligrams per litre. Post-monsoon water had the lowest reported temperature, 28.18 degrees Celsius, the highest alkalinity, 97.22 milligrams per litre, and the highest total dissolved solids, reported as 175 milligrams per litre. Electrical conductivity reached 351.76 milligrams per litre after the monsoon, compared with 347.39 before it. Some reported units for conductivity and temperature-related measurements are reproduced as presented in the study.

Statistical analysis identified dissolved oxygen and pH as central variables in the seasonal water-quality pattern. Principal Component Analysis showed that its first axis explained 84.01 percent of the environmental variance, while the second explained 14.57 percent, with dissolved oxygen and pH making major contributions to the separation of seasons. The study also found a strong positive correlation between dissolved oxygen and total fish abundance, with r = 0.949 and statistical significance reported at p < 0.01 in the abstract. Water temperature showed a strong negative correlation with dissolved oxygen, r = -0.874, consistent with a basic property of aquatic systems: warmer water generally holds less oxygen than cooler water. Canonical Correspondence Analysis, a method that relates species distributions to environmental gradients, found that the first two axes explained 71.78 and 28.22 percent of the constrained variation, respectively. The model was statistically significant, and the first axis linked seasonal fish composition with temperature, alkalinity, total dissolved solids, electrical conductivity and pH. These results do not prove that any single variable caused a particular change, but they show that seasonal environmental gradients coincide with predictable shifts in the fish community.

The findings carry practical implications for conservation and fisheries management in the Old Brahmaputra. Protecting habitats used during spawning, nursery development and seasonal migration would help maintain the connectivity on which riverine fish depend. The researchers point to the potential value of fish sanctuaries, protection of migratory habitats, stronger safeguards for threatened species and restrictions on fishing during sensitive periods. They also call for improved knowledge of fish life histories and consideration of measures such as channel excavation or the release of fish fry, although such interventions would require careful ecological assessment. The study’s evidence is a baseline rather than a final diagnosis: it covers one annual cycle, three locations and fishery-dependent catches, so rare, cryptic or gear-avoiding species may have been missed. Nutrient concentrations, flow velocity and habitat structure were not included, and multivariate correlations cannot establish causation. Longer-term monitoring across more sites and years would show whether the seasonal pattern remains stable as climate, river flow and human pressure change. Even with those limitations, the study demonstrates that oxygen, water chemistry and flood-driven connectivity are inseparable from the river’s biological rhythm—and that conserving fish diversity will require managing the river as a changing ecosystem rather than as a static channel.

Subject of Research: Seasonal relationships between fish diversity and water-quality variables in Bangladesh’s Old Brahmaputra River

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

Article References: Begum, N., Jaman, A., Harun-Al-Rashid, A., Hasan, M. S., Samanta Chandan, C. S., Kunda, M., & Pandit, D. (2026). Seasonal variation in fish diversity and hydrological variables of the Old Brahmaputra River, Bangladesh. Discover Conservation, 3(1), Article 37. https://doi.org/10.1007/s44353-026-00106-x

Image Credits: AI Generated

DOI: 10.1007/s44353-026-00106-x

Keywords: Old Brahmaputra River, Bangladesh fisheries, fish diversity, seasonal ecology, dissolved oxygen, water quality, river conservation, freshwater fish, monsoon hydrology, Seasonal, variation, fish