TAIPEI (Reuters) - Coral reefs off the southeast coast of Taiwan have turned black with disease possibly due to sewage discharge, threatening fragile undersea ecosystems and tourism, a study released Friday said.

The discovery on a problem long suspected but seldom documented shows that coral is suffering widely in waters up to five meters (16.4 feet) deep and 300 meters offshore from two outlying islands, said researcher Chen Chao-lun of Taiwan’s state-funded Academia Sinica.

“This is a large distribution and we had no previous information,” said Chen, whose began doing research with local environmental groups in 2007. “If you snorkel, you’ll see it’s black. If it’s all black, there won’t be too many tourists.”

Coral reefs, delicate undersea structures resembling rocky gardens made by tiny animals called coral polyps, are important nurseries and shelters for fish and other sea life.

They also protect coastlines, provide a critical source of food for millions of people and are potential storehouses of medicines.

Taiwan’s study did not pinpoint a cause for the diseased coral, but untreated sewage may a factor, Chen said.

On Green Island, a tourism hotspot and one the sites surrounded by diseased coral, garbage and excrement are dumped into the surrounding azure waters while reefs are often plundered by coral-robbing tourists, officials and long-time divers say.

The Taiwan researchers have sent their report to the government and plan to check for problems in other offshore areas known to support coral, Chen said.

Coral covers about 400,000 square km (154,000 sq miles) of tropical ocean floor. The biggest is Australia’s Great Barrier Reef, a collection of 2,900 reefs along 2,100 km (1,300 miles) of Australia’s northeast coast in a marine park the size of Germany.