A freshwater dolphin found only in China is "effectively extinct", an expedition has declared following a fruitless six-week search.
The baiji was considered the world's most endangered mammal (Image: Stephen Leatherwood/ZSL)
The Yangtze River dolphin, or baiji, was listed as "critically endangered" on the Red List of Threatened Species.
It has been dying out due to habitat degradation, overfishing, pollution and ship traffic - which confounds the sonar the animal uses to find food.
Zoologists announced a plan to save the mammal earlier this year.
"We have to accept the fact that the baiji is extinct. We lost the race," said August Pfluger, co-head of the expedition and director of baiji.org, an environmental group dedicated to saving the animal.
"It is a tragedy, a loss not only for China, but for the entire world," he added.
Mr Pfluger admitted it was possible that animals had been missed despite the use of optical and acoustic equipment and trained observers.
The baiji lives along the lower reaches of China's environmentally-degraded Yangtze River and is thought to have been in existence for about 20 million years.
If confirmed, it would be the first large aquatic mammal driven to extinction since hunting and overfishing killed off the Caribbean monk seal in the 1950s.
The damage to the baiji's habitat is also affecting the Yangtze finless porpoise, whose numbers have fallen to below 400, the expedition found.