Bay of Bengal Biodiversity
The near shore waters of Bangladesh support a taxonomically diverse and relatively abundant cetacean fauna, which can probably be explained by the wide variety of environmental gradients (river-sea and shallow-deep) available within a relatively small area and the enormous biological production driven by extreme fluvial and oceanographic processes (Smith et al. 2008).
The Ganges River dolphin (Platanista gangetica gangetica) is "endangered" and ranges far upstream in the river system of Nepal, India and the Padma-Jamuna–Meghna river system and in the comparatively much smaller Karnaphuli–Sangu river system of southern Bangladesh. The Irrawaddy dolphin (Orcaella brevirostris) is "vulnerable" and occurs in some of Asia's largest rivers (Mekong, Ayeyarwady and Mahakam) and in coastal waters of the Indo-Pacific with freshwater inputs. Both Ganges River and Irrawaddy dolphins are threatened by incidental killing in gill nets, destruction of fish-spawning habitat, pollution from large human population centers, the enormous by-catch of fish and crustacean fingerlings in mosquito nets used for collecting shrimp and prawn fry.
Waterways of the Sundarbans support about 225 Ganges River dolphins, which represent a significant portion of the species population (Smith et al. 2006). The density of Ganges River dolphins is particularly high in the low-salinity eastern portion of the mangrove forest with even greater concentrations found at channel confluences. Incidental mortality in fishing gears especially gillnets and long-lines are considered among the most severe threats to the endangered Ganges River dolphin (Fahrni et al. 2008). However, almost no information is available about actual interactions of the species with fisheries. These events indicate the importance of monitoring mortality rates and establishing a protected area network in channel segments where the species occurs in relatively high numbers.
The world's second largest documented population of Indo-Pacific bottlenose dolphins (Tursiops aduncus) lives at the northern tip of the Swatch-of-No-Ground (SoNG) in Bangladesh. A total of 1,144 Indo-Pacific bottlenose dolphins (T. aduncus) individually identified through Photo-ID around 100 m depths of mid part of SoNG.
The Sundarbans and adjacent estuarine waters provide habitat for the world's largest population of Irrawaddy dolphins, estimated at about 6,000 individuals, with about 450 occurring in the mangrove forest (Figs. 2 and 5). The density of Irrawaddy dolphins is particularly high in the high-salinity western portion of the mangrove forest but also with important habitat where the range of both species overlaps in the eastern side. The Eastern Sundarbans Reserved Forest is the only location in the world where Asia's two last remaining species of freshwater dolphins, the Ganges River dolphin or Shushuk and Irrawaddy dolphin, are known to co-occur.