A survey of endangered porpoises in China's longest river has yielded fewer sightings as intense ship traffic threatens their existence, scientists said Monday.

Chinese researchers spent 44 days tracking the finless porpoise -- or "river pig" in Chinese -- along a little over half of the 6,000-kilometre (3,700-mile) Yangtze River.

The finless porpoise, which has only a small dorsal ridge rather than a fin, has been hurt by human intrusion and environmental degradation, global conservation group WWF, which supported the survey, said last month as it began.

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

The survey, which began on November 11, marked the most comprehensive study of the species, found only in China, since 2006. A similar expedition then discovered just 1,800 of the porpoises.

Based on sonar tracking results, they said a total of 91 finless porpoises were detected while travelling the 3,400 kilometres from the cities of Yichang to Shanghai, down sharply from 177 in 2006.

The scientists said that was an initial estimate and warned it was too early to announce an actual number. Further data and analysis was needed and they expected to have a result within two months.

"There are fewer and fewer finless porpoises in the mainstream of the Yangtze River, while the animals were concentrating in the harbour areas in groups of three to five," said a release issued by the scientists from the Yangtze River city of Wuhan.

"Our initial analysis concluded that this may be because there are comparatively more food resources in the harbour areas," said Dr Wang Kexiong of the Chinese Academy of Sciences.

"However, in the mainstream of the Yangtze River, especially in the harbour areas, traffic is very intense, which is a huge threat to the survival of finless porpoises," added Wang, who is deputy director general of the survey.

"Such intense ship traffic is a potentially deadly threat to finless porpoises, who totally depend on their sonar system to survive," said Dr Zhang Xinqiao, a survey team member and WWF official.

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

Finless porpoise deaths 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.

China's waterways 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.