BEIJINg - An international expedition to search for a rare Chinese river dolphin has ended without a single sighting, and researchers said Wednesday that the aquatic mammal is facing imminent extinction.

Related readings: Hopes dim for baiji

A few of the white Yangtze River dolphins, known as baiji, may still exist in the massive waterway that cuts through eastern China, but their numbers are insufficient to stave off extinction, said August Pfluger, the Swiss co-leader of the expedition.

"We have to accept the fact that the Baiji is functionally extinct. We lost the race," Pfluger said in a statement released by the expedition. "It is a tragedy, a loss not only for China, but for the entire world. We are all incredibly sad."

The baiji, shy and nearly blind, is one of the world's oldest dolphin species, dating back some 20 million years. Scientists believe their disappearance would be the first instance of a large aquatic mammal being driven to extinction since hunting killed off the Caribbean monk seal circa 1952.

Overfishing and shipping traffic, whose engines interfere with the sonar the baiji uses to navigate and feed, are likely the main reasons for the mammal's declining numbers, Pfluger said. Though the Yangtze is polluted, water samples taken by the expedition every 50 kilometers (30 miles) did not show high concentrations of toxic substances, the statement said.

For nearly six weeks, Pfluger's team of 30 scientists scoured a heavily trafficked 1,700-kilometer (1,000-mile) stretch of the Yangtze, where the baiji once thrived. The expedition's two boats, equipped with high-tech binoculars and underwater microphones, trailed each other an hour apart without radio contact so that a sighting by one vessel would not prejudice the other.

Around 400 baiji were believed to be living in the Yangtze in the 1980s. The last full-fledged search, in 1997, yielded 13 confirmed sightings, and a fisherman claimed to have seen a baiji in 2004, Pfluger said in an earlier interview.

At least 20 to 25 baiji would now be needed to give the species a chance to survive, the group's statement said citing Wang Ding, a hydrobiologist and China's foremost campaigner for the baiji.