But researchers see signs of hope.

Porpoise numbers fell by nearly half from 2006 to 2012 to an estimated 1,040.

But the rate of decline has slowed markedly since then, suggesting that conservation may be making a dent.

A central component of the rescue effort is the introduction of porpoises to several conservation areas off the busy river, where researchers say numbers have actually been increasing.

Encouraging signs

Around 30 to 40 porpoises were brought to the Tianezhou Oxbow Nature Reserve in central China's Hubei province-a curving lake linked to the Yangtze by a stream-at the beginning of the 1990s. There are now around 80.

"We found out that the animals can not only survive, but also reproduce naturally and successfully at Tianezhou," says Wang Ding, 60, a porpoise expert with the Chinese Academy of Sciences.

"That's very encouraging."

Researchers also credit official clampdowns on polluting activities and overfishing, artificial reproduction projects, and growing environmental awareness among China's emerging middle class.

"The voice and supervision of the public have played an important role," says Zhang Xinqiao, the species' project manager at the World Wide Fund for Nature.