Chinese scientists on Sunday began a survey of the dwindling population of an endangered porpoise in the country's longest river, as the animal edges towards extinction from man-made threats.

Researchers will spend more than a month tracking the finless porpoise -- known as the "river pig" in Chinese -- in the Yangtze River, China's longest waterway at more than 6,000 kilometres (3,700 miles), organisers said.

It marks the most comprehensive survey of the species, found only in China, since 2006, when a similar expedition found only 1,800. The current number could be far lower, they said.

"Our expectation is maybe only 1,000 of them are left, but we have to see how it turns out from the survey," said Wang Ding, a research professor at the Institute of Hydrobiology under the Chinese Academy of Sciences.

"People have started to think of the finless porpoise as the symbol of the Yangtze River which also indicates the current status of the river," Wang, who is leading the expedition, told AFP.

The earlier 2006 expedition declared another species, a freshwater dolphin called the "Baiji", to be extinct.

The finless porpoise, which has only a small dorsal ridge rather than a fin, has been hurt by human intrusion and environmental degradation, said global conservation organisation WWF, which is supporting the expedition.

Deaths of the creature have been caused by boat strikes and fishing gear accidents as well as degradation of rivers -- and dolphin food sources -- due to pollution and severe droughts blamed on climate change.

"If we are going to save the Yangtze finless porpoise... we must take immediate action to keep the Yangtze River and its lakes healthy," Lei Gang, director of WWF China's freshwater programme, said in a statement.

"This means better laws and enforcement -- we need to see harmful fishing practices stop, sand-dredging better controlled and new reserves developed."

The WWF has said the porpoise could become extinct in 15 years if no action is taken.

Waterways in China have become heavily contaminated with toxic waste from factories and farms -- pollution blamed on more than three decades of rapid economic growth and lax enforcement of environmental protection laws.

Environmental activists have also blamed China's huge Three Gorges Dam and other hydropower projects on the Yangtze for upsetting the delicate ecological balance and harming aquatic life in the river.

The government-backed expedition set sail Sunday from the central Chinese city of Wuhan aboard two boats equipped with sonar and acoustic surveying equipment, organisers said.

They will travel upstream to Yichang, gateway to the Three Gorges Dam, before heading back downstream all the way to China's commercial hub of Shanghai at the mouth of the Yangtze, they said.