Bangladesh is declaring three areas in the southern Sundarbans mangrove forest as dolphin sanctuaries to protect freshwater dolphins, officials say. Conservationists say the mangrove forest is the only place in the world where the Ganges river dolphins and Irrawaddy dolphins are found. These dolphins are among the world\'s most endangered mammals. Fishermen normally do not target them, but the animals get entangled in fishing nets and drown. They are also threatened by rising salinity levels and pollution. \"We have decided to declare Dhangmari, Chandpai and Dudhmukhi areas of eastern Sundarbans as dolphin sanctuaries so that these mammals can survive in a safe environment,\" Tapan Kumar Dey, a senior wildlife conservation official, told the BBC. He said the three water segments were identified as dolphin hotspots by the Wildlife Conservation Society\'s Bangladesh Cetacean Diversity Project (BCDP), which has been doing research in Bangladesh. \'Clearly demarcated\' Mr Dey said an official notification on establishing the sanctuaries will be issued by the ministry of environment soon. \"The waterways in these areas will be clearly demarcated and there will be signpostings so that local fishermen will not venture into this region for fishing,\" Mr Dey said. Environmentalists say the diverse aquatic ecosystem of the Sundarbans support an impressive variety of cetaceans - whales, dolphins and porpoises. While Ganges river dolphins find safe haven in the upper regions of Sundarbans, Irrawaddy dolphins thrive in the southern parts, which are closer to the Bay of Bengal. The decision by the forest department coincided with a new survey by the BCDP which, apart from freshwater dolphins, also reported sightings of the finless porpoises and an Indo-Pacific humpback dolphin in western Sundarbans. These two cetacean species, which are normally found along the coast, migrate upriver in Sundarbans mostly during winter, when the salinity level is high. They go back after fresh water starts flowing into the rivers. The nine-day survey was conducted in the western part of Sundarbans mangrove forests earlier this month. \"This year we encountered many of them during the recent survey, soon after the rains when the salinity level is low. Their presence in this region at this time may be an indication of the rising salinity level,\" Rubaiyat Mansur Mowgli, principal researcher of the BCDP said. \"Our preliminary results indicate that there is a high density of dolphin population in western Sundarbans as well and some areas there might also be identified as dolphin hotspots,\" Mr Mowgli said, However, he warned that the identification of new dolphin hotspots does not mean the animals are thriving in Sundarbans. \"Declining freshwater supplies and rising sea levels due to global climate change are affecting the dolphin population,\" Mr Mowgli said. Two years ago, researchers found that there were nearly 6,000 Irrawaddy dolphins - which are related to orcas or killer whales - in the waterways of Sundarbans mangrove forests and the nearby coastal waters of the Bay of Bengal.