The northern waterways of the Sundarbans mangrove forest encompass the farthest downstream range of the endangered Ganges River dolphin or shushuk (Platanista gangetica). In a generally narrow geographic band, occurring within the same habitat is the farthest upstream distribution of a seasonally mobile population of the Irrawaddy dolphin (Orcaella brevirostris). Farther offshore but still occurring in habitat influenced by freshwater inputs is the Indo-Pacific humpback dolphin (Sousa chinensis) and finless porpoise (Neophocaena phocaenoides). Then, a relatively short distance from the fluvial habitat of shushuks is the Swatch-of-No-Ground where a burst of biological productivity created by upwelling currents supports large groups of Indo-Pacific bottlenose dolphins (Tursiops aduncus), pantropical spotted dolphins (Stenella attenuata) and spinner dolphins (Stenella. longirostris), as well as a possible resident population of Brydes whales (Balaenoptera edeni).
The diversity of cetaceans occupying this relatively small area is remarkable, and rigorous abundance estimates of shushuks, Irrawaddy dolphins and finless porpoises indicate that large populations of these species remain. In fact, the Irrawaddy dolphin population in Bangladesh is probably the worlds largest, possibly by an order of magnitude.
However, optimism about the long-term survivability of cetaceans in these waters is tempered by increasing threats from incidental killing in gillnet fisheries, depletion of prey due to a loss of fish and crustacean spawning habitat and to massive non-selective catch of fish fingerlings and crustacean larvae in small mesh mosquito nets, and toxic contamination from large, upstream human population centers. An additional threat is declining freshwater flows from upstream abstraction in the Ganges-Brahmaputra-Meghna river system as well as sea-level rise caused by global climate change.