The baiji's demise is attributed to overfishing, dam-building, environmental degradation, and ship collisions.
The large-ship traffic on the Yangtze, one of the world's busiest waterways, confounds the sonar that the nearly blind dolphin uses to find food, Pfluger said.
"When you're on the river and you see so many ships, you feel that an animal like a dolphin does not have any chance of survival," he said.
"That's a personal feeling, not a scientific statement."
Although the Yangtze suffers from heavy pollution, it is less polluted than other rivers in China, such as the Yellow River.
(See a photo of the Yellow River running red due to pollution.)
Water samples taken by the scientists did not show toxic pollutants in concentrations high enough to have killed the baiji.
Beginning of "Wave of Extinctions"?
Zeb Hogan, who studies large river fish in Asia, says unprecedented use of freshwater rivers has led to the decline of populations of many aquatic species.
"Perhaps nowhere is this pattern more apparent than in populations of species such as river dolphins and large-bodied fish," said Hogan, a researcher with the University of Nevada at Reno and a National Geographic Emerging Explorer based in Phnom Penh, Cambodia.
"Globally, a pattern has emerged; these large aquatic animals are disappearing," Hogan said.
"The world's river dolphins and large freshwater fish face the biggest threats, including overfishing, dams, navigation projects, pollution, and habitat destruction."
The extinction of the baiji dolphin should serve as a wake-up call that more needs to be done to protect river life, Hogan added.
"Unless concrete steps are taken soon to better protect these vulnerable species, this is the beginning of a wave of extinctions that is likely to occur over the next 20 to 30 years."
There are now five species of freshwater dolphins left in the world, four of them living in major freshwater systems in Asia. All are critically endangered.
During their search for the baiji, the scientists also surveyed the population of the endemic Yangtze finless porpoise. They found that there may be fewer than 400 animals left there.
"We have to consider these animals in a better way than we did the baiji," said Pfluger, the expedition organizer.
"We know that if the baiji was doing badly, the finless porpoise is doing badly too. This animal needs our action now. There is no time to waste."
Yesterday, Pfluger said, he watched video footage that he had shot of Qi Qi, a male baiji who was rescued in 1980 and died in captivity in 2002 (see photo).
"I consider myself a strong man," he said. "But when I saw that footage I cried for several minutes. It's just so terribly sad."
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