A dead zone on the north-western Bay of Bengal's continental margin and its alarming impact on the distribution of demersal fishes

Hypoxia in the mesopelagic and demersal realms of marine ecosystems directly or indirectly affects fish population structure, community composition and ecosystem functioning. This may result in excess mortality or the emigration of organisms from the area, resulting in dead zones. The present study reports on a dead zone on the north-western Bay of Bengal's continental margin with no discernible fish catch. The phenomenon is associated with stratification, small residence time of suspended matter in the water column and weak vertical/horizontal mixing due to a retention area (and increased biological flux) observed in the region as a result of cooccurring warm and cold core eddies, resulting intense decompose activities in the regional bottom. As with similar ocean zones globally, dominance of a single species (Parascombrops pellucidus) around the zone's periphery (edge effect) is also reported which is attributed to their feeding patterns, locomotory characteristics, prey preferences and availability. The dead zone's seasonal occurrence indicates the influence of natural or anthropogenically induced climatic variability can even affect demersal/bottom-dwelling organisms. The present work discusses the drivers and implications of dead zones using in situ oceanographic and fishery data, remote sensing observations and model outputs.