Aquatic Mammal mammals that have secondarily taken to water. These include (a) the pinniped carnivores, represented by seals and walruses, (b) the sirenians, represented by dugongs and manatees, and (c) the cetaceans as a whole, represented by whales, dolphins and porpoises. Of these, the representatives of the first group, namely seals and walruses, are cold sea forms, and are unrepresented in Bangladesh waters. The second and third groups, namely sirenians and cetaceans, are both represented in warmer waters, and some of them occur in Bangladesh also. Aquatic mammals of Bangladesh are represented mainly by the cetaceans.
There are stray reports about sightings of solitary specimens of unidentified whales in the bay of bengal. There are, however, occasional cases of whales being cast up on the coasts of Bangladesh. The earliest recorded case of the stranding of a whale, at the time identified as Balaenoptera indica, took place on 15 September 1842 near Chittagong. During the half century since 1943 there have been several recorded cases of the stranding of whales on the southern coast of offshore islands of Bangladesh, mostly sandwip. These isolated cases mostly strayed in the exclusive economic zone (EEZ) of Bangladesh, and got washed up in heavy seas whipped up by foul weather. Most of these stranded specimens were tentatively identified as humpback whales (Megaptera novaeangliae).
The small cetaceans are represented in Bangladesh water by a number of species, of which one is freshwater and the rest are all estuarine and/or marine. While the riverine form is the Ganges river dolphin (Platanista gangetica), the others have been identified as Irrawaddy dolphin (Orcaella brevirostris), finless porpoise (Neophocaena phocaenoides), short-beaked common dolphin (Delphinus delphis), Indo-pacific humpbacked dolphin (Sousa chinensis), and bottlenose dolphin (Tursiops truncatus). The Irrawaddy dolphin, the finless porpoise and Indo-pacific humpbacked dolphin, though essentially coastal, occasionally venture some distance up the rivers.
There are reports on the occurrence of the dugong, Dugong dugon, in Bangladesh. The Chittagong District Gazetteer (1908) states: "... The dugong appears to have been captured on one occasion, more than 20 years ago, off Maheshkhali Island, and has been seen of late years at the mouth of the matamuhuri river". The death around January-February 1976 of a strange aquatic animal in the Moiscal channel near cox's bazar, and the circumstantial evidence later collected, strongly suggest the occurrence of the dugong there.
No account of aquatic mammals of Bangladesh will be complete without a reference to otters representing the family Mustelidae under the order Carnivora. Otters are semi-aquatic carnivores, which obtain most, or all, of their food in the water and spend much of their time in water swimming with strong limbs and a powerful cylindrical tail. Otters are represented in Bangladesh by 3 species, viz, Asian Small-clawed Otter (Aonyx cinerea), Eurasian Otter (Lutra lutra), and Smooth-coated Otter (Lutra perspicillata).
River dolphin may appear to be a competitor with man for fish. But the services they render to fisheries resources far outweigh any harm they are thought to cause to the same. For example, by their rocking and side-swimming habits they mix up nutrients and dissolve gas in the water and 'plough' and 'rake' the riverbed thus increasing productivity of the water and of the riverbed, and also releasing toxic gases bound in the bottom-mud. Further, they usually catch the physically weaker individuals among the prey-animals thus effecting what agricultural and livestock farmers would respectively call 'weeding' and 'culling'.
This way river dolphins do a great service in the management of fisheries resources. Due to the close physiological resemblance of river dolphins to humans, they are important sources of knowledge about respiratory and cardiac problems and of the effects of pollutants on mammalian systems. Dolphins may be maintained in oceanaria or in marine parks for research, education and recreation. Otter fishing is an attractive sport that can also help develop the country's tourism industry. [AKM Aminul Haque]