During an expedition on the Chinese Yangtze River, 30 Chinese and foreign scientists spent nearly a month searching for the rare baiji or white-flag dolphin, but not a single one was spotted. The scientist say the result is a sign that the mammal's Yangtze River habitat is increasingly being despoiled by pollution and human activity.

Members of the Swiss-based Baiji.org Foundation kept a diary during the 1,000 mile quest that confirmed reports from the Xinhua News Agency that the expedition was a failure. An expedition launched in 1997 found 13 baiji; a long, narrow dolphin that the foundation calls the rarest mammal in the world.

"We can't say the white-flag dolphin is extinct," Xinhua quoted Wang Ding, an expedition leader and a hydrobiologist with the Chinese Academy of Sciences. "However, the population has dropped dramatically."

Wang's estimate put the current baiji population at "no more than 50."