The survival of finless porpoises, a highly-endangered freshwater dolphin in China's Yangtze River, is under threat from lingering drought as the river's water level keeps dropping, experts warned Tuesday.

Workers at a national natural reserve in Shishou, Hubei Privince attempts to help freshwater dolphins Friday whose lives were threatened by dangerously low water levels due to a severe drought. [Xinhua]

The drought in central China, lasting for about 200 days, has lowered the water level to 27.38 meters in the Swan Island National Nature Reserve in Shishou City, Hubei Province, said Wang Ding, a dolphin expert at the Hydrobiology Institute under the Chinese Academy of Sciences.

"The level is three meters lower than last year, the lowest level over the past decades. Finless porpoises cannot survive if the level continues to drop," Wang said.

The river section for some 30 dolphins to live has been reduced to ten kilometers long from 21 km in normal days, Wang said.

"If the activity area is reduced, they might be stranded on the bank and will die if they can not swim back," he said.

The lowering of the water also endangered aquatic plants, which might decrease the number of fish to cause food shortage for the dolphins, he said.

Finless porpoise, one of the six porpoise species and a protected mammal in China, is known locally as the "jiang zhu" (river pig). The gray, smoothly shaped dolphin only lives in the middle and lower reaches of the Yangtze, as well as in Dongting Lake and Poyang Lake.

The dolphin population is only 1,000, even fewer than that of giant panda, and is decreasing at a rate of 6.4 percent annually, Wang said.

Five finless porpoises were found in the Swan Island National Nature Reserve in the 1990s, and now the population has exceeded 30.

Extreme weather and human activities are the main threats to the species, Wang said.

"Some farmers pump water from the reserve to relieve the drought these days," he said. Local government was alerted and has banned the pumping.

Wang added that the situation of finless porpoises in Poyang Lake was not good, either, as the drought had shrunk the area of the lake to one-tenth its usual size, which brought unprecedented threats to the species.

"The river should be at least three meters deep for finless porpoises to swim. But once the protected areas suffer low water level, they have to move to the main course of the river where ship propellers are a top killer," he said.

Once the dolphin is injured by the propeller, polluted river water will cause a high mortality, he said.

In the past decade, at least five rare animal species have become extinct and another five species, including the white-flag dolphin, have not been seen for years, said Cao Wenxuan, fish biologist and academician at the Chinese Academy of Sciences.

Fishing was banned in the lower reaches of the Yangtze River in the spring of 2002 to protect the environment and the ban was expanded to the entire river in 2003, which largely improved the environment in the river.

However, Wang Ding feared that the drought and the fishing will threaten finless porpoises even more after the three-month fishing ban expires at the end of June.

"The next ten years will be critical to the protection of finless porpoises. If we fail, the species will be the next white-flag dolphin," he said.