Chinese officials are keen to avoid a repeat of the baiji, or Yangtze dolphin-the river's only other aquatic mammal, which since 2007 has been considered "functionally extinct"-which was a huge conservation setback for China.

Losing the "smiling angel" would be a further tragedy, conservationists say.

One of the world's few freshwater porpoise subspecies, it is considered a natural barometer of the overall health of China's most important river.

'River pig'

The finless porpoise is mentioned in ancient Chinese poems and has been considered a harbinger of rain.

Some locals call it the "river pig" for its plump body and rounded head.

Adults can reach two meters long.

Since China reopened to the world four decades ago, living standards have soared, but so have air and water pollution.

But in January 2016, President Xi Jinping called for a river protection push. Steps have included curbs on development, stricter fishing rules and other protection projects.

Later that year, a formal porpoise action plan was launched, including increased relocations away from the river, more reserve sites and research on artificial breeding.

The Tianezhou reserve, established in 1992, claims to be the world's first and only example of cetaceans-which include dolphins and porpoises-surviving and reproducing after relocation.

Local fishermen near the lake were encouraged to change professions and Wang Hesong, 46, became a patrolman at the reserve.

"Look over there, a mother and a baby," Wang says, as his pilot cut their patrol boat's engine at the sight of two arched backs breaking the lake's silvery surface.