Bangladesh's coastal waterways are home to the largest known population of the rare of Irrawaddy dolphin, a New York-based Wildlife Conservation Society (WCS) study said on Wednesday.

The WCS researchers estimate that nearly 6,000 Irrawaddy dolphins have been found living in freshwater regions of the country's Sundarbans forests at a time when the global population of this species was believed to be only in the hundreds.

Although these animals were once abundant throughout South-East Asia, this species was red-listed by the World Conservation Union in 2008 for its endangered status, according to the WCS.

Irrawaddy dolphins are related to orcas, also known as killer whales. The dolphins grow up to 8 feet long and frequent large rivers, estuaries, and freshwater lagoons in South and Southeast Asia.

In Myanmar's Ayeyarwady River, these dolphins are known to cooperatively fish with humans, helping to herd schools of fish toward boats and awaiting nets.

"The practice benefits the fishermen-increasing the size of their catches up to threefold-as well as the dolphins, which fill their own bellies with some of the cornered fish and those that fall out of the fishing nets," the study added.

Local fishermen say this is a common occurrence. The scientists also found that the dolphins must cope with declining freshwater supplies, caused by upstream water diversion in India coupled with sea-level rise.

These circumstances also threaten the endangered Ganges River dolphin, which shares part of its range in the Sundarbans with the Irrawaddy dolphin.

The recent likely extinction of the Yangtze River dolphin, or baiji, is a potent reminder of how vulnerable freshwater dolphins are to human impacts on the environment.

The Ministry of Environment and Forests in Bangladesh is cooperating in a conservation project with the WCS for ensuring a protected area network for both Irrawaddy and Ganges River dolphins in the Sundarbans.