Seasonal changes in feeding habits of hilsa (Tenualosa ilisha) were studied monthly in 2019 in the upper Meghna estuary, Bangladesh, through gut content analyses. Tychoplanktonic diatoms followed by copepods were abundant in all months and size groups of hilsa. The inverse relationship between niche breadth and relative length of the gut revealed hilsa consume a variety of food at their early stages while their feeding habit changes towards diatoms during the adult stage with the development of gill rakers. Young hilsas prefer mostly tychoplanktonic diatoms (Aulacoseira sp., Triceratium sp., Nitzschia sp., Synedra sp., and Coscinodiscus sp.) and copepods (Pseudodiaptomus sp.). However, adult hilsas rejected Aulacoseira sp. during their spawning season. Ingested sand grains correlated with tychoplankton, revealing their food value from epipelic microalgae and bacteria clinging to them and epibenthic foraging by young hilsas. Thus, this study revealed that young hilsas primarily feed on tychoplanktonic diatoms and copepods from water and near bottom of the upper Meghna estuary during January to June while later stages with the development of gill rakers allow hilsas to feed on planktonic diatoms. Epibenthic feeding needs to be considered more fully in evaluating the biology of hilsa and, perhaps, other clupeids, and in evaluating possible human impacts on this foraging habitat.

The distribution of fish larvae in estuaries is crucial for comprehending the functioning of these ecosystems and their role as nursery grounds for marine fish species. Data regarding larval fish assemblages in the coastal waters of Bangladesh are currently scarce. This study examines the spatial and seasonal dynamics of larval fish communities in these waters using a two-year sampling effort with a bongo net. This study investigates the diversity, abundance, and distribution of larval fishes at four sites (Bakkhali River Estuary, Moheshkhalipara, Naf River Estuary, and Rezu Khal Estuary) on the southeastern coast of Bangladesh. A total of 10,387 fish larvae representing 25 families were identified. Clupeidae (42.9%), Engraulidae (35.5%), Ambassidae (10.1%), Myctophidae (2.1%), and Gobiidae (1.9%) were the five most dominant families based on the larval catch composition. Marked seasonal differences in the larval fish abundance were observed. However, these differences were not observed when comparing the four sampled sites. Additionally, the correlation between environmental variables and the larval abundance indicated that salinity negatively influences (p < 0.001) the larval fish abundance. This is the first comprehensive investigation of larval fish assemblages in the coastal waters of Bangladesh. The findings of this study enhance our understanding of the nursery habitat requirements for the early development stages of both migratory and resident species in subtropical coastal waters, providing valuable information for potential applications in management and conservation efforts.

A thorough examination of fish diversity indices, gear efficiency, catch composition, and decline causes of fish diversity was carried out in the Feni River (Bangladesh). Monthly data collection took place from July to December 2023 at three stations of the river. A total of 8 fish species under 7 orders and 8 families were recorded. The order Perciformes was the most prevalent, representing 30% of the total. Approximately half of the identified species are classified as least concern, while the remaining 4 species are classified as Not Threatened (12.5%), Vulnerable (12.5%), Endangered (12.5%), and Critically Endangered (12.5%). The mean values of the Margalef’s richness (d), Pielou’s evenness (J), Simpson’s index (1-D), Shannon-Weaver diversity (H), and 0.973±0.002 were calculated as follows: 1.720±0.139, 0.699±0.112, and 0.248±0.034, respectively. Based on the Bray-Curtis similarity matrix, two groups were observed at a similarity of 58% within six months across the three sites. The chandi net and ghera net recorded the highest CPUE (kg gear-1day-1) at 5.93±0.966 and 0.53±0.041, respectively. The highest fish catch was recorded in July (2249 ± 668.71 kg), while the lowest was in October (1564 ± 465.05 kg). There was no significant difference (p>0.05) in the monthly fish catch. Fish biodiversity in the Feni River is declining day by day as a result of pollution, overfishing, the use of harmful gear, and a lack of optimal water and pollution. To improve and conserve fish species in the Feni River, it is strongly advised that appropriate fishery management techniques should be applied, that overfishing be closely monitored, and that fishermen be made more aware of their rights.