Andrew Revkin reports.
Dolphin and porpoise species that have adapted to rivers and deltas around the world have long been considered some of the most vulnerable of marine mammals because of their restricted habitats. In 2007, the baiji, a river dolphin that thrived in the Yangtze River for 20 million years in today’s China, was said by experts to have been driven to extinction by a mix of impacts from the nearly half billion people now living in that watershed. The vaquita, a porpoise living in brackish waters where the Colorado River empties into the Gulf of California, is critically endangered, biologists say, depleted by fishing nets and the disruptions in the great river's flow in the 20th century from dam construction.
But in the great weaving of mangrove-fringed islets and channels that make up the sprawling coast of Bangladesh, biologists have found a thriving population of another species that marine mammal experts had also thought depleted—the Irrawaddy dolphin. After methodical surveys, biologists from the Wildlife Conservation Society and Chittagong University in Bangladesh estimate that the region is home to 6,000 of the dolphins, by far the largest known population.