[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-13] Larval Fish Assemblages in Coastal Waters of Bangladesh: Spatial and Seasonal Dynamics

*SK AHMAD AL NAHID¹, Saifuddin Rana¹, Nargis Sultana¹, Jannatul Mawa¹, Sazeed Mehrab Souhardya², Ilias Ebne Kabir², Shahida Arfine Shimul¹, Md Masum Billah³, Md Khurshid Alam Bhuiyan⁴, Afsana Kabir Dipty⁵, Sk Istiaque Ahmed^1,6, Md Jalilur Rahman², Md Mehedi Iqbal⁶ (1.Chattogram Veterinary and Animal Sciences University, Khulshi, Chattogram 4225,, 2.Enhanced Coastal Fisheries in Bangladesh (ECOFISH II Activity), WorldFish Bangladesh & South Asia Office, Dhaka 1213, Bangladesh, 3.Inter-Departmental Research Centre for Environmental Science-CIRSA, University of Bologna, Ravenna Campus, Via S. Alberto 163, 48123 Ravenna, Italy, 4.Institute of Marine Research (INMAR), Department of Biology, Faculty of Marine and Environmental Science, University of Cádiz, Spain, 5.Department of Fisheries and Marine Science, Noakhali Science and Technology University, Bangladesh, 6.Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8564, Chiba, Japan)

Keywords:Bay of Bengal, Estuary, Fish larvae, Biodiversity, spatiotemporal distribution

Understanding the abundance and distribution of fish larvae in estuaries is significant for comprehending the functioning of estuarine ecosystems and their role as nursery grounds for estuarine and marine fish. Bangladesh is enriched with an enormous maritime area and marine fish diversity. However, there are limited studies and data on the larval fish assemblages in the coastal waters of Bangladesh. Proper management of marine fisheries and biodiversity in Bangladesh requires scientific investigations on the spatiotemporal status of fish larval abundance and the relationship of fish larval abundance with environmental variables. To address the information gap on larval fish in Bangladesh, this study examined the spatial and seasonal dynamics of larval fish in the coastal waters of Bangladesh using a two-year sampling effort (January 2020 and December 2021). Monthly sampling of fish larvae was conducted using a bongo net with horizontal towing at a depth of 0.5–1.5 meters at four sites (Bakkhali River Estuary, Moheshkhalipara, Naf River Estuary, and Rezu Khal Estuary) in the southeastern coast of Bangladesh. Water flow and water quality variables (salinity, DO, pH, TDS, alkalinity, and water transparency) were recorded during each sampling. The samples of fish larvae were identified morphologically up to the family level and the diversity, abundance, richness, and evenness were determined on spatial and seasonal scales. Pearson’s correlation analysis was employed to investigate the relationships between fish larvae abundance and physicochemical variables. A total of 10,387 fish larvae representing 25 families were identified morphologically. 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. Among the study sites, the Moheshkhalipara coast exhibited the highest number of families (21) followed by the Rezu Khal Estuary (19), and Bakkhali River Estuary (15), whereas Naf River Estuary exhibited the lowest number of families (12). The highest abundance of fish larvae was recorded in the Rezukhal Estuary (3974 larvae/1000 m³), whereas the lowest abundance was found in the Bakkhali River Estuary (1462 larvae/1000 m³). The abundance of fish larvae didn’t significantly vary among the sampling sites (p > 0.05) but significantly varied among seasons (p < 0.05). The highest abundance of fish larvae was observed during the monsoon whereas the lowest was observed during winter. Among the four sites, the highest diversity and evenness of fish larvae was observed at the Moheskhalipara in March 2020, and the highest richness in the Rezu Khal Estuary in September 2020. In contrast, the Bakkhali River Estuary exhibited the lowest diversity and richness values throughout the study period and the Naf River Estuary exhibited the lowest evenness in May 2020. The salinity, DO, pH, TDS, alkalinity, and water transparency levels ranged from 8 to 36 ppt, 3.8 and 9 mg/L, 6.1 to 9.1, 320 to 655 ppm, 52 to 309 ppm, and 26 to 126 cm respectively. Significant seasonal variation (p < 0.05) was observed in the water temperature, salinity, DO, pH, TDS, and transparency. The correlation between environmental variables and larval abundance indicated that salinity negatively influenced the abundance of fish larvae. The results of this study can be helpful in determining the areas and times at which various marine fish nurse during their early stages of development. The knowledge base of fish larvae research will be improved by the findings of this study for Bangladeshi researchers, policymakers, and managerial organizations involved in fisheries management and conservation initiatives.