The Australian Broadcasting Corporation reports that Professor Wang Ding, who headed the research team on the six-week Baiji Expedition, says that some of the dolphins may still exist.

"This is only one survey and...you can't have a sample in a survey, so you cannot say the baiji all is gone by the result of only one survey," he said.

"For example, there is some side channels or some tributaries [where] we cannot go because of a restriction of navigation rules, and also we don't survey during the night-time so we may miss some animals in the Yangtze River."

Professor Ding says based on anecdotal evidence, he remains confident the dolphins are still out there.

"I'm pretty much sure there are a few of them left somewhere in the Yangtze River," he said.

"I keep receiving reports from fishermen, they say they saw a couple of baiji somewhere, sometime.