Countering their expectations, biologists working in Bangladesh have found a thriving population of 6,000 Irrawaddy dolphins, a species restricted to brackish bays and rivers from southern Asia to northern Australia that marine mammal experts had worried was vulnerable to extinction. The population, many times larger than any other known regional groups of the dolphins, was revealed in 2004 in the first systematic survey for marine mammals along Bangladesh " s coast of waterways, bays and mangrove-fringed islands. The full results were described Wednesday in Hawaii at the first international conference on protected areas for marine mammals and in a paper in the winter issue of the Journal of Cetacean Research and Management. American and Bangladeshi biologists conducted the dolphin survey by boat. The researchers said that the six to eight-foot dolphins, while apparently thriving, needed to be protected and monitored in view of rising threats like entanglement in fishing nets, a decline in freshwater flows because of dam construction and inland diversions of water along the rivers that sustain the coastal ecosystems. The scientists also signaled a long-term threat to the dolphins from global warming, which climate studies project will raise sea levels and change the river flows as Himalayan glaciers erode. This would shrink the species" range, which is restricted to water with low salinity. The Wildlife Conservation Society, which led the study, is working with the Bangladesh Ministry of Environment and Forests to create protected areas for the dolphin and for another species, the Ganges River dolphin, and seeking money for the effort, said Howard Rosenbaum, a biologist who directs the "ocean giants" program of the nonprofit group. "This mother-lode population in Bangladesh really gives us hope for the survivability of the species in the long term," Dr. Rosenbaum said. 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, experts concluded that the baiji, a river dolphin that thrived in the Yangtze River for 20 million years in what is today China, had been driven extinct by a variety of activities by the nearly half billion people now living in that watershed. The vaquita, a porpoise living in 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 river"s flow from dam construction.

单选题 Marine biologists used to assume that Irrawaddy dolphins ______.

单选题 Biologists called on urgent protection of dolphins on account of ______.

单选题 The long term threat Irrawaddy dolphins are faced with is______,

单选题 By saying "the mother-lode population in Bangladesh" in paragraph 5. the author refers lo ______.

单选题 The example of "the baiji" is cited to show that ______.