The inter-seasonal oscillation of the deoxygenated layer off the Kerala-Karnataka Coast and its encroachment to shallower regions of the shelf due to upwelling and its effects on both demersal and pelagic fisheries of the region were studied in detail. The results in broad, indicated that the oxygen depletion worked out differently on pelagic and demersal groups of fishes stressing the need for studies of tolerance limits of different species of fishes for deoxygenation of waters and the toxic effects of bacteria on fishes, as upwelling areas are known for heavy bacterial loads