Dhaka - Researchers have identified waters along Bangladesh's coast and deep in the Bay of Bengal as one of the richest areas on Earth for cetacean diversity.

An amazing array of dolphins and whales live in these areas, the US-based Wildlife Conservation Society and Bangladesh's Cetacean Diversity Project said after conducting a joint study over the past six years.

But scientists at the same time warned that the habitats of these aquatic mammals are increasingly in danger because of declining freshwater flows from the Ganges River and global climate change.

"The riverine, coastal and deep-sea waters of the Bay of Bengal are ideal places for cetacean diversity," Rubaiyat Mansur Mowgli, the principle researcher of the project, said after releasing the study ahead of a four-day photo exhibition and film show on dolphins and whales in Bangladesh.

The exhibition, aimed at building awareness about marine diversity and environmental conservation, kicked off Thursday in Dhaka.

The prime cetacean habitat extends across the world's largest contiguous mangrove forest in Bangladesh's Sundarbans Reserve Forest, a UNESCO World Heritage Site at the mouth of the Ganges and the habitat of rare species like the Bengal tiger, as well as offshore at a 900-plus-metre-deep undersea canyon known as the Swatch of No Ground.

The dolphin species ranged from Ganges River dolphins and Irrawaddy dolphins in the rivers, finless porpoises and Indo-Pacific humpbacked dolphins in the coastal waters and the Indo-Pacific bottlenose, spinner, and pantropical spotted dolphin along with Bryde's whales at the Swatch of No Ground.

The long-term prospects of these mammals remaining in these waters are endangered by increasing natural and manmade threats, said Brian D Smith, the project's director in Bangladesh.

Among them, he said, were declining freshwater flows from rivers and sea-level rises caused by global climate change.

Other major threats include incidental killing of cetaceans in gillnet fisheries, depletion of their prey, loss of spawning habitat and toxic contamination from large upstream human population centres.

Both local and international experts urged Bangladesh's government to take initiatives to conserve cetaceans. Doing so, they said, could transform the mammals into a foreign-exchange resource.

In having the most diverse habitat for dolphins and whales, Bangladesh could become an important tourist destination, especially for dolphin lovers, they said.

"The diversity of cetaceans and the abundance we have recorded is remarkable and indicates that a large population of these species remain in our waters," said Benazir Ahmed, who began supervising the project beginning in July 2006.

He said the Irrawaddy dolphin population of about 6,000 in Bangladesh was probably the world's largest.