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.
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.
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