6,000 Rare, Large River Dolphins Found in Bangladesh

A previously unknown population of Irrawaddy dolphins discovered in Bangladesh has given scientists "great hope" for the survival of the rare species, conservationists said Wednesday. 

A research team estimated that 6,000 Irrawaddy dolphins thrive in the country's Sundarbans mangrove forests and nearby waters of the Bay of Bengal.
The group is the largest ever found—previously, scattered groups of only about a hundred Irrawaddy dolphins each had been found throughout the dolphin's Southeast Asian habitat, which stretches from the mouths of rivers feeding the Bay of Bengal across open waters to Indonesia (map of the region). 

The species' total worldwide population is unknown. 

(Related: "Irrawaddy River Dolphin Closer to Extinction Despite Reports, Experts Say.") 

"Thats why this is so exciting … ," said Howard Rosenbaum, head of the ocean giants research program at the Wildlife Conservation Society (WCS), the conservation group that made the discovery. 

"Here you have this area where we found nearly 6,000 animals—it gives us hope for protecting the entire species and this really important habitat." 

Few marine-mammal biologists had previously explored the diverse water ecosystem where the new dolphin group was found, which ranges from freshwater mangroves to brackish water to deep ocean canyons in just a small area. 

Because the 6.5- to 8-foot-long (2- to 2.5-meter-long) mammals surface only occasionally, researchers used a transect method to gather data about the population. 

The team steered a boat along a straight line, noting any dolphin sightings along each run. 

A wider population estimate was then made from that data, presented Wednesday at the First International Conference on Marine Mammal Protected Areas in Maui, Hawaii.
Six thousand is a "tremendous amount" of individuals for the species—listed as vulnerable on the International Union for Conservation of Nature's Red List. But "it doesn't mean we're out of the woods yet," Rosenbaum said. 

For one thing, the dolphins, relatives of the killer whale, easily get entangled in fishing nets.
And declining flows of fresh water from dams upstream in India, along with sea-level rise from global warming, further threaten the sensitive mammals, Rosenbaum said. 

Populations of the Irrawaddy's cousin, the endangered Ganges River dolphin, are also plummeting due to the same threats. Likewise, the Yangtze River dolphin, which is thought to be nearly extinct, is a "potent reminder" of how humans can impact dolphins. 

Dekila Chungyalpa, director of the Mekong River program for WWF-US, said the decline of what she calls the "cutest" of the dolphins has been a huge concern for her conservation group. 

"To know that there's a very large population elsewhere is quite a relief," Chungyalpa said. 

But, she added, "just because we're finding these wonderful numbers doesn't mean the urgency is any less strong." 

To that end, the discovery has motivated WCS and its partners to speed up the creation of a marine protected area in the Sundarbans mangroves, WCS's Rosenbaum said. 

The group is working with the Bangladeshi Ministry of Environment and Forests to set aside a sanctuary for both the Irrawaddy and Ganges dolphins.

Christine Dell'Amore
National Geographic News
March 31, 2009