Twelve finless porpoises, an endangered mammal, were recently found dead in Dongting lake, a major freshwater lake that feeds into the Yangtze River in central China's Hunan province, the Xiaoxiang Morning Post reports.

A finless porpoise was found dead near a wharf at the mouth of the Poyang Lake in Jiujiang, eastern China's Jiangxi province on March 8, 2012. The 20-kilo adult mammal died about one month ago, apparently from head injuries inflicted by the propellers of boats. [CFP]

Nine of the dead mammals were found in one week, with 12 overall from March 3 to April 15. Autopsies showed that the mammals (including several from Poyang Lake) all share one symptom: no food residue in the digestive system.

Xie Yongjun, a veterinarian who conducted three dissections, believed the mammals could have died from infectious diseases, poisoning or starvation.

Dr. Dong Lijun with the Institute of Hydrobiology of the Chinese Academy of Sciences said that finless porpoises live in families and that the death of one mammal usually means at least three others are in grave danger.

Fang Liang, another researcher with the Institute, said that considering the deterioration of the environment in the Yangtze River, the only feasible option now is to relocate the porpoises for better protection.

Wei Baoyu, program director of the World Wide Fund for Nature (WWF), the international wildlife NGO, in Changsha said that the dying-out of finless porpoises could herald a biological disaster for the middle and lower reaches of the Yangtze River, as the mammal is at the top of the river's food chain.

Finless porpoises are threatened by food shortages, water pollution and damage to the river bed (on which the mammals breed) from digging in Dongting lake, Wei said. They can also be easily injured by the propellers of the many boats in the lake.

Scientists estimated there were between 1,200 to 1,400 finless porpoises in the Yangtze in 2006, only half of the number in 1991. A sonar and sighting survey in January this year put the number at 85 in the Dongting lake, with the number between 300 to 400 in Poyang lake, the report says.

As well as the finless porpoise, the Yangtze River also used to be home to another freshwater mammal: the baiji, or Chinese river dolphin. The animal was declared extinct in 2006, China Daily reports.

Hunan province is yet to designate a nature reserve area for the finless porpoise in the Dongting lake, following other reserves established in Hubei, Jiangxi, Anhui and Jiangsu provinces along the Yangtze.