Freshwater input to estuaries may be greatly altered by the river barrages required to meet human needs for drinking water and irrigation and prevent salt water intrusion. Prior studies have examined the salt plugs associated with evaporation and salt outwelling from tidal salt flats in single-channel estuaries. In this work, we discovered a new type of salt plug formation in the multi-channel Pasur River Estuary (PRE) caused by decreasing river discharges resulting from an upstream barrage. The formation of a salt plug in response to changes in river discharge was investigated using a conductivity-temperature-depth (CTD) recorder during spring and neap tides in the dry and wet seasons in 2014. An exportation of saline water from the Shibsa River Estuary (SRE) to the PRE through the Chunkhuri Channel occurred during the dry season, and a salt plug was created and persisted from December to June near Chalna in the PRE. A discharge-induced, relatively high water level in the PRE during the wet season exerted hydrostatic pressure towards the SRE from the PRE and thereby prevented the intrusion of salt water from the SRE to the PRE.

The Pasur River estuary (PRE) provides vital fishery resources and supports millions of livelihoods in the southwestern coastal region of Bangladesh. Our research focused on phytoplankton community assemblages, alpha diversity indices, and the seasonal succession of major phytoplankton species in relation to physicochemical parameters in the tidal mangrove creeks of the Pasur River estuary. Spatial and temporal variations were assessed by water sampling at 17 stations in the study area from January to December 2019. The mean salinity level in the tidal mangrove creeks of the PRE was significantly (p < 0.05) higher during the dry season than during the wet season. Spatially, no significant variation (p > 0.05) was observed in the dissolved inorganic nitrogen and dissolved inorganic phosphorus between PRE and mangrove creeks, but temporally, the variables varied significantly (p < 0.05). Spatially, no significant variation (p > 0.05) was observed in the alpha diversity of the phytoplankton community but significantly (p < 0.05) varied temporally. Blue-green algae became dominant in the oligohaline conditions during the wet season, while diatoms were dominant during the dry season which severely depleted dissolved silica. In terms of phytoplankton species diversity, our study classifies the study areas as highly diversified zones. Phytoplankton succession from diatoms (dry season) to blue-green algae (wet season) is attributed to the changes in the physicochemical and nutrient parameters depending on seasonal environmental parameter fluctuations. This study illustrated that phytoplankton diversity and density varied with the degrees of habitat and seasonal changes, implying the potential impacts of anthropogenic activities and natural causes on their community structure in tropical estuaries and mangrove creeks.

Variation in salinity is one of the major environmental factors influencing the species diversity of fish in an estuary. Therefore, evaluating the relationship between salinity and species diversity is important. In this study, fish diversity was assessed by fish sampling and visiting local fish markets from February to November 2021. Mean salinity was 10.59 psu (Practical Salinity Unit) and 0.46 psu during the dry and wet seasons, respectively. Harpadon nehereus, Odontamblyopus rubicundus and Pseudapocryptes elongatus species were found as polyhaline (0.06~18.1 psu) species. Anguilla japonica and Arius gagora were abundant in brackish water conditions (0.35~14.2 psu). However, Acanthopagrus latus and Setipinna phasa were found in freshwater conditions (0.06~0.11 psu). The suitability index indicates that commercially important fish species such as Liza parsia, Macrobrachium rosenbergii, Mugil cephalus, Penaeus monodon and Scylla serrata can be used for mariculture during the dry season, and Acanthopagrus latus, Pethia canius and Setipinna phasa during the wet season. Overall, these findings suggest that salinity, water temperature, and chlorophyll-a had a significant (p < 0.05) effect on the fish distribution and assemblage composition in the study area. This finding will be helpful in developing policies for the conservation and management of the aquatic resources in the coastal zone to enrich the blue economy.

Fisheries sector plays a significant role in national food security, empowerment and foreign exchange earnings in the national economy of Bangladesh. At the same time, Bangladesh is blessed with enormous water resources of inland culture (8,21,923 ha), inland capture (38,90,282 ha) and marine (1,18,81,300 ha), having great potentials of fisheries production. Inland culture, inland capture and marine fisheries provides 56.76%, 28.19% and 15.05% fisheries production, respectively. The aim of this review paper is to provide a comprehensive overview on fisheries resources of Bangladesh, those are underutilized and/or need emphasis to enhance more fish production. Recently, inland culture fisheries production has increased rapidly due to adoption of improved farming practices by the farmers. However, inland capture and marine fisheries production has not yet been attained to its optimum level due to over exploitation, fish habitat degradation, siltation of water bodies and water pollution. Therefore, to increase inland capture fisheries production and conserve biodiversity, various approaches such as community-based fisheries management, beel nurseries, stocking fingerlings of endangered species, establishment of fish sanctuaries, cage and pen culture systems can be adopted. Additionally, existing banning rules and regulations of catching fish during breeding season from inland capture water bodies like rivers, haors, baors, beels etc. should be implemented properly for conserving biodiversity and increasing fish production, as banning of 22-days in rivers and 65-days in marine water have significantly increased hilsha fish production. In addition, blue economy has recently gained impressive attention in Bangladesh due to getting huge marine water resources from Myanmer in 2012 and India in 2014. Therefore, marine aquaculture systems for example farming of seaweed, sea bass, mullets, promfrets, mud crab etc. should be adopted for achieving the United Nations agenda on sustainable development goal 14 on conservation, sustainability and use of oceans, seas and marine resources for increasing economic benefits.

The tropical estuarine ecosystem is fascinating for studying the dynamics of water quality and phytoplankton diversity due to its frequently changing hydrological conditions. Most importantly, phytoplankton is the main supplier of ω3 polyunsaturated fatty acids (PUFA) in the coastal food web for fish as they could not synthesize PUFA. This study evaluated seasonal variations of water quality parameters in the Meghna River estuary (MRE), explored how phytoplankton diversity changes according to hydro-chemical parameters, and identified the major phytoplankton groups as the main source of PUFA for hilsa fish. Ten water quality indicators including temperature, dissolved oxygen, pH, salinity, dissolved inorganic nitrogen (DIN = nitrate, nitrite, ammonia) and phosphorus, dissolved silica and chlorophyll-a were evaluated. In addition, phytoplankton diversity was assessed in the water and hilsa fish gut. Principal component analysis (PCA) was used to analyze the spatio-temporal changes in the water quality conditions, and the driving factors in the MRE. Four main components were extracted and explained 75.4% variability of water quality parameters. The most relevant driving factors were dissolved oxygen, salinity, temperature, and DIN (nitrate, nitrite and ammonia). These variabilities in physicochemical parameters and dissolved inorganic nutrients caused seasonal variations in two major groups of phytoplankton. Peak abundance of Chlorophyta (green algae) occurred in water in nutrient-rich environments (nitrogen and phosphorus) during the wet (36%) season, while Bacillariophyta (diatoms) were dominant during the dry (32%) season that depleted dissolved silica. Thus, the decrease of green algae and the increase of diatoms in the dry season indicated the potential link to seasonal changes of hydro-chemical parameters. The green algae (53.7%) were the dominant phytoplankton group in the hilsa gut content followed by diatoms (22.6%) and both are contributing as the major source of PUFAs for hilsa fish according to the electivity index as they contain the highest amounts of PUFAs (60 and 28% respectively).

The objective of this study was to investigate the changes in physicochemical, microbiological, and sensory properties of sun-dried Mystus vittatus during storage at ambient temperature. The fish was dried under sun exposure on bamboo mats for 5–6 days until the moisture content reduced to approximately 10–15%. The dried fish was then packed into airtight polyethylene bags and stored at ambient temperature (24 to 29 °C) for 90 days. The physical and sensory properties revealed that the dried fish was acceptable for human consumption for up to 60 days. The pH value slightly decreased from 6.42 to 5.95 during the storage period. The moisture content increased significantly (p < 0.05), while no significant difference was observed for the protein, lipid, and ash contents (on a dry matter basis) with the increase in the storage time. The peroxide value, acid value, and conjugated dienes of the lipids increased significantly during the storage period. The microbial load also increased with the increase in the storage period. The results of this study suggest that the product was slightly oxidized during the storage period and could be stored for up to two months.