Bangladesh will declare three river areas in its southwest as dolphin sanctuaries, wildlife officials said Monday, in a bid to protect the country's population of endangered freshwater cetaceans.

The forestry department took the decision after studies found three areas in the UNESCO-listed Sundarbans mangrove forest, which straddles Bangladesh and India, were home to large populations of Irrawaddy and Ganges river dolphins.

"We have decided to declare river channels at Dhangmari, Chandpai and Dudhmukhi areas in the eastern Sundarbans as dolphin sanctuaries," Tapan Kumar Dey, senior wildlife conservation official at the forest department, said.

"The channels and adjoining areas are home to hundreds of endangered Irrawady and Ganges river dolphins. Fishermen will be banned from fishing in the areas," he told AFP.

Tens of thousands of fishermen catch fish and shrimp in the channels. Although dolphins are not targeted directly, they often become entangled in the fishing nets and die by the dozen every year.

A series of studies since 2002 by the Bangladesh Cetacean Diversity Project (BCDP) identified the three areas in the Sunderbans' river channels which are key dolphin hotspots.

An earlier BCDP study found the world's largest population of Irrawaddy dolphins -- an estimated 6,000 -- living along Bangladesh's southern coast, including in the Sundarbans.

In other areas where the flat-faced dolphins are known to converge, such as the Mekong delta in South East Asia, populations have been estimated at less than 100.