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. Read the full report here.
Source: by Anbarasan Ethirajan, BBC News, Dhaka.
