Hilsa shad (Tenualosa ilisha), is a biologically, nutritionally, economically, socially and culturally important species in the Bay of Bengal and Persian Gulf regions, but Bangladesh enjoys the major share wherein it contributes about 517,000 tons/year. However, this important fishery declined in the late 1990’s that led the Government to formulate the Hilsa Fishery Management Action Plan (HFMAP) and started its implementation from 2005. Since then, hilsa production increased @5%/year till 2015. To improve the annual incremental production further, Department of Fisheries (DoF) and WorldFish have jointly been implementing “Enhanced Coastal Fisheries in Bangladesh (ECOFISH-Bangladesh)”, a USAID supported project (2014-2019). The project supports the DoF and local communities to establish a science-based “adaptive co-management” that focuses on the brood hilsa protection, juvenile conservation, illegal gears control, and overall ecosystem resilience involving all stakeholders. As a synergistic impact of all the initiatives taken and the proactive supports of the law enforcing agencies, the declined hilsa fishery from both inland and marine habitats revived @ 11% annual incremental total hilsa catch. The increased hilsa production and average size (from 510 g to 915 g) resulted in increased household income of fishers by 52%. The interventions additionally improved catfish abundance and other fish biodiversity.

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.

The anadromous Hilsa shad (Tenualosa ilisha) live in the Bay of Bengal and migrate to the estuaries and freshwater rivers for spawning and nursing of the juveniles. This has led to two pertinent questions: (i) do all Hilsa shad that migrate from marine to freshwater rivers come from the same population? and (ii) is there any relationship between adults and juveniles of a particular habitat? To address these questions, NextRAD sequencing was applied to genotype 31,276 single nucleotide polymorphism (SNP) loci for 180 individuals collected from six strategic locations of riverine, estuarine and marine habitats. F_ST OutFLANK approach identified 14,815 SNP loci as putatively neutral and 79 SNP loci as putatively adaptive. We observed that divergent local adaptations in differing environmental habitats have divided Hilsa shad into three genetically structured ecotypes: turbid freshwater (Western Riverine), clear freshwater (Eastern Riverine) and brackish-saline (Southern Estuarine-Marine). Our results also revealed that genes involved in neuronal activity may have facilitated the juveniles’ Hilsa shad in returning to their respective natal rivers for spawning. This study emphasized the application of fundamental population genomics information in strategizing conservation and management of anadromous fish such as Hilsa shad that intersect diverse ecotypes during their life-history stages.

The migration of anadromous fish in heterogenic environments unceasingly imposes a selective pressure that results in genetic variation for local adaptation. However, discrimination of anadromous fish populations by fine-scale local adaptation is challenging because of their high rate of gene flow, highly connected divergent population, and large population size. Recent advances in next-generation sequencing (NGS) have expanded the prospects of defining the weakly structured population of anadromous fish. Therefore, we used NGS-based restriction site-associated DNA (NextRAD) techniques on 300 individuals of an anadromous Hilsa shad (Tenualosa ilisha) species, collected from nine strategic habitats, across their diverse migratory habitats, which include sea, estuary, and different freshwater rivers. The NextRAD technique successfully identified 15,453 single nucleotide polymorphism (SNP) loci. Outlier tests using the F_ST OutFLANK and pcadapt approaches identified 74 and 449 SNPs (49 SNPs being common), respectively, as putative adaptive loci under a divergent selection process. Our results, based on the different cluster analyses of these putatively adaptive loci, suggested that local adaptation has divided the Hilsa shad population into two genetically structured clusters, in which marine and estuarine collection sites were dominated by individuals of one genetic cluster and different riverine collection sites were dominated by individuals of another genetic cluster. The phylogenetic analysis revealed that all the riverine populations of Hilsa shad were further subdivided into the north-western riverine (turbid freshwater) and the north-eastern riverine (clear freshwater) ecotypes. Among all of the putatively adaptive loci, only 36 loci were observed to be in the coding region, and the encoded genes might be associated with important biological functions related to the local adaptation of Hilsa shad. In summary, our study provides both neutral and adaptive contexts for the observed genetic divergence of Hilsa shad and, consequently, resolves the previous inconclusive findings on their population genetic structure across their diverse migratory habitats. Moreover, the study has clearly demonstrated that NextRAD sequencing is an innovative approach to explore how dispersal and local adaptation can shape genetic divergence of non-model anadromous fish that intersect diverse migratory habitats during their life-history stages.

Almost a half million fishers in Bangladesh are predominantly reliant on the hilsa shad (Tenualosa ilisha) fishery in the Meghna River and estuarine ecosystem. This paper adopts a broadened concept of social-ecological traps to frame the complex dynamics that emerge from social and ecological interactions in this highly natural resource-dependent social-ecological system (SES). We analyze how endogenous self-reinforcing processes in the system and poor initial conditions, particularly debt and lack of livelihood options outside fisheries, keep fishing households in poverty. We identify a policy decision in favor of incentive-based fisheries management as a critical juncture that influenced a trajectory of recovery in hilsa shad stocks in this complex adaptive system. Normative assessment of stakeholder perceptions indicates that fishers perceive a nominal improvement in well-being as a result of this policy. Compensation in return for compliance with a seasonal fishing ban in sanctuary areas does not, however, disrupt trap dynamics perpetuating the cycle of poverty, social exclusion, and political disempowerment in which fishing households are entrenched. Poverty and lack of alternative livelihood opportunities remain significant reasons for noncompliance with the ban as long as fishers do not have any meaningful representation in resource management and decision making. A secured tenure system through adaptive comanagement involving fishers in monitoring and enforcement of compliance with fishing bans, supported by sustainable finance for livelihood improvements outside of natural resource exploitation and predicated on responsive and accountable institutions for and by people who depend on the fishery, can form the foundation for local stewardship in a unique demonstration of contemporary large-scale open-water fisheries governance in this complex SES.