The last such expedition, in 1997, located 13 of the world's rarest dolphins, which lives only in China's longest river, the Yangtze, Xinhua news agency said.

The white-flag dolphin is considered to be more endangered than the giant panda, which lives in remote forest areas, because the dolphin faces additional threats from pollution and environmental degradation, it said.

"We can't say the white-flag dolphin is extinct. However, the population has dropped dramatically over the past decade," Wang Ding, vice director of the hydrobiology institute of the Chinese Academy of Sciences, was quoted as saying.

Wang and his colleagues estimate that no more than 50 are left.

During a 1,700-kilometer (1,100-mile) trip down the river that ended in Shanghai, the scientists found that the population of black finless porpoises had also fallen.

Wang said pollution, overfishing, shipping and a large number of water conservancy projects had affected the habitats of the animals, Xinhua said.

"If the situation cannot be improved, the white-flag dolphin may be extinct within ten years, and the black finless porpoise will also be endangered," Wang said.

There are an estimated 1,000 black finless porpoises in the river, about half the number 12 years ago, Wang said.

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