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.
Interpol investigating stolen passports on missing flight
Scarlett Johansson steps out with fiance after pregnancy reveal