"Too many tourists will bring it to an unsustainable level," oceanographer Vassen Kauppaymuthoo told Reuters.

Mauritius is proud of its sandy beaches, tropical waters and turquoise lagoons protected by coral.

But agriculture, fishing, and other human activity have been stressing the reefs for hundreds of years.

Now marine scientists fear the combination of warmer seas and mushrooming human activity will place even more strain on the island's few remaining pockets of quality coral.

Anchor damage, snorkellers, divers, water-skiers, pollution, and hotel waste water all play their part, Kauppaymuthoo said.

"Human development and tourist development around the island are now becoming more of a threat to the coral," Kauppaymuthoo added. "It is because of the quantity of hotels and the amount and volume of effluents discharged."

He told Reuters he had participated in an official study, which concluded in 1998 that, even with proper management, Mauritius could not welcome more than 750,000 tourists per year without destroying the marine environment.

Tourism officials were not available for comment.

At least some coral damage is likely to be beyond the immediate control of humans.

Global warming causes periodic spikes in the sea temperatures that can bleach the coral, making it much more vulnerable to pollution and human contact.

Sugar fields, hotels, and an international airport encircle Blue Bay marine park, which arguably has the island's best coral.

Snorkellers there see some enormous corals, but in parts it has broken and died.

Small fish nibble at the coral, much of it colourless, and the big fish are conspicuous by their absence.

"When you look at your ecosystems here, they are clearly stressed," Ian Watt, a marine ecologist with Reef Conservation Mauritius, a non-profit organisation, told Reuters.