Thailand on Thursday indefinitely banned diving at seven marine parks popular with tourists to try to protect deteriorating coral reefs.

The ban applies to some 22 places in the parks where "bleaching" covers 80 percent or more of the coral reefs. The parks are on the Andaman Sea on Thailand's west coast.

The recovery of the coral would be monitored before the ban is lifted, said Sunan Arunnopparat, chief of the National Parks Department.

The ban is likely to hurt Thailand's lucrative tourist industry. Nearly 16 million international visitors were expected last year, and Thailand's beaches are a major draw.

An educational campaign would be launched to help explain the situation to tour operators, tourists and the public, Sunan said. Authorities will also step up action against illegal fishing.

"Bleaching" occurs when coral becomes stressed and expels the algae that live inside and give it color. The weakened coral can be damaged by people, boat anchors and buoys, and long-term stress can kill the coral, depriving fish of food and habitat.

The problem is often caused by water changes, including warmer temperatures, sedimentation and acidification. Coral may recover if the algae returns, but they're still significantly weaker and more vulnerable to disease.

Warm waters harmed reefs in Indonesia, Malaysia, Thailand and Vietnam last year.

Marine biologists working for the Wildlife Conservation Society observed coral bleaching off Indonesia's Aceh province as surface waters in the Andaman Sea peaked at 93 degrees — 7 degrees higher than long-term averages.

Subsequent surveys found 80 percent of the bleached corals had died.