The last survey, conducted between 1997 and 1999, turned up 13 of the mammals, but Turvey said fishing, pollution and boat traffic in the busy river, which was home to about 10 percent of the world population, had likely led to the baiji's demise.

"We twice covered the dolphin's range," Turvey said. "It is difficult to see how we could have missed them."

The dolphins will now be classified as critically endangered and possibly extinct, but Turvey said there is little chance there are any baiji still alive.

Wang Ding, a co-author of the report and deputy director of the Institute of Hydrobiology under the Academy of Sciences, said the dolphins faced "functional extinction".

This means that even if there remains a small population of breeding individuals, it will not be sufficient to sustain the species.

According to the World Conservation Union, an animal can be declared extinct only if it has not been seen for more than 50 years.

"At present, we cannot make the judgment that the dolphins are already extinct because it is only nine years since 13 of them were seen in wild," Wang said.

Researchers have known for years about the dolphin's precarious situation but indecision about how best to save them meant little was actually done, he said.

This underscores the need to act quickly to prevent the extinction of other similar shallow-water aquatic mammals, such as the vaquita found in the Sea of Cortez and the Yangtze finless porpoise, Turvey said.

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