The fish fauna of the Atrai River were studied for a period of two years from January 2011 to December 2012. Sampling was carried out by several fishing nets and traps on fortnight basis. A total of 74 fish species belonging to 27 families and 52 genera were recorded. Cyprinidae was the most dominant family contributing 18 species in 9 genera. Two alien species were found- Hypophthalmichthys molitrix and Hypostomus plecostomus. Thirty locally threatened species (Vulnerable 13.51%, Endangered 18.92% and Critically Endangered 8.11%) were recorded. But, no globally threatened species was found. Relative abundance of majority 36.49% species was rare. Global population trend of 24.32% recorded fish species was Declining. The highest number of species (42) was recorded in the November 2011. Whereas, the lowest the number of fish species (12) recorded in June and August 2011. Very high correlation was found between fish species and physical environmental parameters (air temperature, water temperature, and water transparency). This study concluded that Atrai River could be an excellent place for natural conservation of fish species. Nevertheless, efforts to maintain a sustainable fishing pressure, removal of existing alien species and avoiding further introduction of such species are recommended.

1. Worldwide, water regulatory structures have impacts on aquatic ecological connectivity. This study determined the effects of current sluice management on the fish community in the Baral River, a major connection to the largest wetland (Chalan Beel) in Bangladesh. It also examines wider problems for biodiversity conservation (particularly waterbirds) in that wetland, which has shrunk to 30% of its former dry-season size in 50 years. 2. During the flood period, the peak breeding time for native floodplain fishes, sluices were in undershot operation (open by 16-60% of water depth). During this time, fish abundance and species richness were 229% and 155% higher respectively at sites upstream of the sluices, despite similar habitat upstream and downstream. Outside this period, when sluices were fully open, abundance and species richness were similar upstream and downstream. 3. Fish samples were dominated by fry, which are susceptible to damage by sluices. Twenty (41.7%) of 48 fish species captured in this study are classed as threatened in Bangladesh and their abundance was significantly lower downstream of the sluices. Two alien species, Aristichthys nobilis and Hypophthalmichthys molitrix, were recorded, probably escapees from local aquaculture activities.4. Twenty-five species of wetland birds were recorded in the Chalan Beel. From interviews, 64% of these species appear to have decreased in the last 20 years, together with 11 more species that may have become locally extinct over this period. This suggests that widespread ecological disruption is occurring. 5. Improved water management (e.g. gate opening height and duration) or modification (e.g. fish pass) of the Baral sluices is needed, to meet biodiversity and fisheries needs, rather than just for flood control and crop production. Improved hydrological and ecological connectivity and habitat protection are needed, as are a cessation of destructive fishing and seasonal fish ranching practices that currently provide synergistic pressures.

The Padma River is one of the longest rivers and it is believed to be an important spawning and feeding ground for riverine fish species of Bangladesh. This study was conducted from February 2013 to January 2014 and with a view to revealing the diversity of fish fauna in the river. A total of 71 species were recorded belonging to 10 orders, 26 families and 54 genera. The most dominant fish order was Cypriniformes contributing 28 species in 16 genera. Cyprinidae was most dominant family contributing 23 species in 16 genera. Four alien species were found. Twenty eight species have been considered threatened by IUCN Bangladesh. These fishes were belonging to the following categories, Vulnerable (13%), Endangered (18%) and Critically Endangered (8%). Comparing the results with the previous findings, it was revealed that the species diversity have declined in the Padma River over time. Considering all the findings it is concluded that the Padma River could be considered a refuge for conservation of threatened freshwater fishes of Bangladesh. The conservation efforts should ensure minimization of anthropogenic impacts, especially the fishing pressure and introduction of alien invasive species.

Invasive vermiculated sailfin catfish (Pterygoplichthys disjunctivus) has an impact on highly valued native fish species.

Impacts of ineffective wastewater management on the biodiversity of receiving waters in developing countries are poorly documented. Using a before-after-control-impact methodology, we measured the effects of untreated wastewater release on the fish community in the Barnoi River, Bangladesh. In 2006, prior to untreated wastewater discharge, fish abundance, species richness and water quality were similar across sampling sites. In 2016, after 8 years of wastewater release to the downstream reach, fish abundance and species richness were reduced by >47% and >35% respectively at downstream sites compared to unaffected upstream sites and >51% and >41% lower respectively compared to the pre-wastewater discharge period. The wastewater impact was particularly severe during months of low discharge (October–December). Water transparency, dissolved oxygen and pH were lower (P < 0.001) at impacted downstream sites compared to upstream sites. Nineteen species (41.3% of all species we recorded) are threatened in Bangladesh and the abundance of these species, except one, decreased significantly (P < 0.05) at the impacted sites. We recommend improved wastewater management by applying primary treatment facilities and incorporating reedbed filtration as a mean of biological treatment, into the canals carrying wastewaters. The success of such measures should be tested with fish species that were most responsive to wastewater, using the indicator species concept.