If the white-flag dolphin is extinct, it will be the first whale species to vanish as a result of human activity, said Wang Ding, vice director of the hydrobiology institute of the Chinese Academy of Sciences.

Scientific data indicates that more than 90 percent of white-flag dolphin deaths are caused by human activity.

Wang insisted that it is still too early to say whether the white-flag dolphin is extinct, even though scientists found none during their 3,400-km expedition along the main section of the Yangtze River.

Even if there are a few specimens of the rare fish left, the species' chances of survival are virtually nil, according to foreign conservationists.

The white-flag dolphin, or baiji, was traditionally thought by the Chinese to be a river god.

The research team made up of conservationists from China, America, Britain, Japan, Germany and Switzerland used sonar monitoring equipment to search for the white-flag dolphins. But their efforts were in vain. Instead scientists picked up sonar signals from 700 to 900 cowfish, a smaller freshwater mammal.

Experts estimate that there are about 1,200 to 1,400 cowfish in the Yangtze River. Their number, however, is also well down on previous surveys, as the ecological condition of the river deteriorates, said the experts.

The mammals share the river with ships, towboats and fishing vessels, as the Yangtze has developed into China's busiest waterway. The research team's monitoring results show that there are 12 vessels per km on the river.