China's finless porpoises face a fight for survival in and around the Yangtze River but researchers see signs of hope, Kelly Wang reports in Wuhan. 

In an oxbow lake along the middle reaches of the Yangtze River, a breathy sigh pierces the surface stillness as one of China's most endangered animals comes up for a gulp of hazy air.

A slick black back with no dorsal fin arches briefly above the water line before plunging back down.

Such glimpses of the shy Yangtze finless porpoise, the only aquatic mammal left in China's longest river and known in Chinese as the "smiling angel" for its perma-grin, are increasingly rare.

Pollution, overfishing and shipping traffic have rendered them critically endangered, worse off even than China's best-known symbol of animal conservation, the panda.

China's government estimates there were 1,012 wild Yangtze finless porpoises in 2017, compared to more than 1,800 giant pandas that are no longer endangered.