The Baiji, the rare freshwater dolphin of China's Yangtze River, has been declared extinct after a six week expedition failed to find any remaining signs of the species.

The dolphin had existed for 20 million years but was pushed to extinction in recent decades through habitat degradation, increased shipping traffic, pollution, over-fishing and ecosystem impacts of the Three Gorges dam project further up river.

"The baiji is functionally extinct. We might have missed one or two animals but it won't survive in the wild," said August Pfluger, a Swiss naturalist involved in the expedition. "We are all incredibly sad."

The loss represents the first time in decades that the world has lost a first large aquatic mammal to extinction. The last was over half a century ago when the Caribbean monk seal was killed off by hunting and over-fishing.