HANGZHOU -- Overfishing and increasing pollution are destroying one of the world's great fisheries in the East China Sea, new studies show, confirming the fears of fishermen and environmentalists.

Eighty-one percent of the sea area has been rated category four for pollution, the second worst of five pollution grades, in a survey by the Zhejiang Provincial Environmental Bureau. The polluted area has expanded from 53 percent rated category four in 2000.

Known in China as the Zhoushan Fishery, the East China Sea area was listed among the world's largest in the last century with its 20,800 square kilometers providing a tenth of China's total catch in 2002.

The Zhoushan Fishery Bureau said on Tuesday that the annual catch dropped from over 1.3 million tons in 2001 to 980,000 tons last year, and the quality of fish species netted was degraded.

Meanwhile, the number of people employed in the Zhoushan fishing industry has fallen from a high of 250,000 to an estimated 210,000.

The warning has been backed up by evidence from former fishermen such as Yu Zhaozhang who decided to abandon his 30-year fishing career in 2003.

"There were fewer and fewer cash fish and more juvenile fish in each haul. I realized that the lack of fish would soon put a lot of fishermen out of business," said Yu, who now owns a sea-food restaurant.

The government of Zhoushan, the island city from which the fishery get its name, has appropriated funding and provided training to help fishermen retrain and set up new businesses, such as aquaculture, sea-food processing and marine tourism, but the dwindling fishery is still trawled by thousands of vessels.

The ocean environmental survey, carried out by east China's Zhejiang Province, which administers the fishery, has also shown the actual fishing area has been nearly halved due to restrictions on fishing around the burgeoning number of undersea pipelines and cables.

Chinese law forbids fishing within two kilometers of fiber-optic lines, oil pipelines and electricity lines in the Zhoushan Fishery, putting 8,000 square kilometers of the area technically out of bounds.

Marine environmental monitoring has shown that half of China's "red tides" caused by pollution now appear in the Zhoushan Fishery. Pollutant samples show petrochemical waste and heavy metal sediments are the main contaminants.

Ma Chaode, a water expert with the environmental group WWF China, said pollution was making the Zhoushan Fishery unsustainable and destroying fish stocks in one of the world's major sea fisheries.