Many species of shark are facing a serious threat to their existence because of worldwide fishing trends, environmentalists say.

Fishermen "used to cut the lines and let sharks go," said Pete Knights, executive director of WildAid, a San Francisco-based conservation group, at a shark conservation conference Wednesday. In recent years, however, fishermen have harvested the sharks for their lucrative fins.

One-third of the more than 500 shark species are threatened with extinction or are close to being threatened, said Sarah Fowler of the World Conservation Union.

"Fisheries can remove 50 to 90 per cent of an entire shark [species] in only 10 years," said Fowler, according to the Xinhua News Agency.

Fowler told the Associated Press later that the projections were based on a continuing study by the Conservation Union's shark research group.

She and other experts at the conference said governments and non-governmental organizations must work together to educate the public and fishermen about overfishing.

Shark's fin soup threatens species

WildAid, which co-sponsored the conference, persuaded NBA star Yao Ming in August to pledge to give up eating shark's fin soup, a Chinese delicacy, as part of a campaign to promote wildlife protection in his homeland.

WildAid says China is the world's biggest importer of shark fins, which conservationists say are cut from sharks that are thrown back into the ocean to die. WildAid put the worldwide trade in shark fins at 10,000 tonnes a year.

Fowler estimated that 38 to 70 million sharks are killed each year for their fins.

"It is difficult to change people's dining habits, but we can educate and engage the public to achieve sustainable development of the sharks and people's dining culture," said Li Yanliang, deputy general director of the Agriculture Ministry's aquatic wild fauna and flora administrative office, at the conference.

Knights said that as the Chinese economy has grown, shark fin consumption has risen, increasing pressures on shark populations that were already vulnerable to overfishing because they breed so slowly.