Last River Porpoises Dying in Polluted Yangtze
Kevin Holden Platt in Beijing, China
for National Geographic News
|April 23, 2008|
The planet's last river-dwelling finless porpoises are dying in part due to exposure to insecticides and mercury in China, a new study says.
The mammals had already been declining as their natural habitat in and around the Yangtze River deteriorated.
In the new research, scientists also found high concentrations of polychlorinated biphenyls (PCBs) and other pollutants in the organs of porpoises found in central China's Dongting Hu Lake, which is connected to the Yangtze. (See China map.)
However the researchers haven't yet established medically the toxicity level that will kill a porpoise.
"In recent decades the [Yangtze finless porpoise] population decreased sharply each year by approximately 7.3 percent because of human activities on the river, including fishing, pollution, transportation, and dam construction," said study co-author Wang Ding of China's Institute of Hydrobiology.
A recent census turned up just 1,800 porpoises, and Wang warned that "the Yangtze finless porpoise will become extinct within 24 to 94 years if no protective measures are taken."
The baiji, a Chinese freshwater dolphin that also lived in the Yangtze, was declared extinct in December 2007.
Wang and colleagues from the Chinese Academy of Sciences started examining porpoise tissue and organ samples after the animal was declared an endangered species by the World Conservation Union in 1996.
Some contaminants found in tissue samples, such as PCBs, likely originated from industrial wastewater and agricultural pesticides and herbicides, according to Beat Mueller, a geochemist at the Swiss Federal Institute of Aquatic Science and Technology who has tested the waterway.
Wang said that some of the toxic substances were also found in water samples taken in "heavily polluted areas near the sewerage outfalls of a medicine factory and a paper mill around the lake."
Local agriculture and industry, including paper mills and oil refineries, have increased dramatically in the region in the past few decades, Wang said.
"It is estimated that approximately 800 million tons of wastewater are discharged into the lake each year."
(See photos of life along China's dangerously toxic Yellow River.)
During the same period, "declines of aquatic animal populations and of species diversity in [the] lake have been observed," he added.
The study was published in a recent edition of the journal Environmental Toxicology and Chemistry.
In a companion study published in Fresenius Environmental Bulletin, Wang and colleagues discovered hazardous amounts of mercury, a highly toxic and persistent pollutant, in porpoise organ samples.
Porpoise calves were discovered to have the highest levels of mercury poisoning, which could account for the dwindling porpoise population, he said.
These discoveries provide additional evidence that contaminants need to be reduced throughout the porpoises' habitat to avert its extinction, said Swiss geochemist Mueller.
Li Lifeng, director of WWF China's freshwater program, agreed.
"Urgent measures need to be undertaken to save this porpoise.
"Pollution control is one of the most important, but this will take time and huge efforts by the government and companies," he said.
WWF China has helped maintain a natural preserve for the Yangtze porpoise along an oxbow of the river at Tian-e-Zhou.
But the survival of the species will depend on reversing contamination of the water and limiting harm caused by shipping and "large infrastructure projects including dams, sluices, roads, bridges, and harbors," Li said.
The Institute of Hydrobiology, meanwhile, has scored small successes with its captive-breeding program, with a third calf expected to be born at its dolphinarium this summer.
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