A 2006 survey by Chinese and foreign scientists failed to find any white-flag dolphins on China's longest river.
In 2007 white-flag dolphins were declared "functionally extinct," meaning the population is too small for the species to effectively reproduce, China's state-run Xinhua news agency reported.
"There is still the possibility of finding one or two white-flag dolphins on the Yangtze," Wang Kexiong, deputy head of the survey team, said. "We are looking forward to a miracle."
The white-flag dolphin, among the 12 most endangered species in the world, is under Chinese state protection.
In 2004, the body of a white-flag dolphin was found on the bank of the Yangtze, the last known proof of the species' existence, Xinhua said.