Beijing, Dec. 3 (PTI): A team of Chinese and foreign scientists have failed to trace the rare white-flag dolphin during a 26-day expedition in the Yangtze river.

"We can't say the white-flag dolphin is extinct," Wang Ding, Vice-director of the Hydrobiology Institute of the Chinese Academy of Sciences, said.

"However, the population has dropped dramatically over the past decade," he said.

Wang and his colleagues estimate the current number to be around 50, Xinhua news agency reported.

The white-flag dolphins, facing threat from pollution and environmental degradation, are more endangered than the giant panda, he said.

The last expedition in 1997 had spotted 13 white-flag dolphins, the world's rarest ones, in the Yangtze river.

If the white-flag dolphin disappears, it will be the first whale species extinct due to human activities. More than 90 per cent deaths of the white-flag dolphins were caused by human activities, scientific data showed.

During the 1,700-kilometre expedition from Yichang to Shanghai, the group of 30 scientists also observed the dwindling number of black finless porpoises.

In addition to deck observers equipped with high-performance telescope to spot the dolphins, the team members used hydrophone machines to detect their acoustic signals which will be analysed in detail.

"It's the last hope of the scientists," Wang said.