Tens of researchers aboard two vessels set sail from Wuhan in Central China's Hubei province Friday to start a 40-day survey on the quantity and distribution of endangered finless porpoises along the Yangtze River.

The Finless porpoise, a rare freshwater cetacean, is a species peculiar to China. The animal is found only in the Yangtze River, China's longest, as well as Dongting and Poyang lakes, two freshwater bodies that link to the waterway.

This year's survey is the third, after surveys were conducted in 2006 and 2012. It will cover a 3,400-kilometer stretch of the Yangtze River from Yichang in Hubei to Shanghai, as well as major river branches and the two lakes.

"The priority of the survey is to investigate the population and distribution of finless porpoise in the last five years," said Hao Yujiang, director of the survey and researcher at the Institute of Hydrobiology at the Chinese Academy of Sciences.

He said researchers will also investigate the environmental quality of the animal's habitat, to provide statistics for making new protection plans.

During the survey, scientists will track porpoises through sonar systems and visual observation. In a variation on previous surveys, drones will be employed to monitor the habitats of the animal.

Known for their "smiling" faces and friendliness with humans, the finless porpoises are also dubbed "water elves". However, this lovely animal is on the verge of extinction.

In the 1990s, the Institute of Hydrobiology at Chinese Academy of Sciences estimated there were 2,700 finless porpoises. In 2012 a survey found the population in the wild had dropped to 1,040.

Apart from finless porpoises, the survey will also try to find Baiji, a functionally extinct species of freshwater dolphin formerly found only in the Yangtze River.

"If any of our ships trace the trail of Baiji, we will stop our survey and get the image information of Baiji first," Hao said.