U.S. Geological Survey via AP file
This elkhorn coral near St. Croix, U.S. Virgin Islands, was photographed earlier this year and shows signs of stress, having become bleached in appearance.
updated 9/26/2006 12:24:06 PM ET 2006-09-26T16:24:06

Scientists have issued their strongest warning so far this year that unusually warm Caribbean Sea temperatures threaten coral reefs that suffered widespread damage last year in record-setting heat.

Waters have reached 85.5 degrees Fahrenheit around the U.S. Virgin Islands and 85.1 degrees  around Puerto Rico — temperatures at which coral can be damaged if waters do not cool after a few weeks — said Al Strong, a scientist with the U.S. National Oceanic and Atmospheric Administration's Coral Reef Watch.

The warning issued Saturday by NOAA urges scuba-dive operators and underwater researchers in the U.S. Caribbean territories to look for coral damage and use caution around the fragile reefs, which are easily damaged by physical contact.

Coral, which provide a sheltered habitat for fish, lobsters and other animals, die from prolonged bleaching, when the water temperature gets so high that it kills the algae that populate and build the reefs. Reefs are also impacted by sewage and farm runoff into the sea.

The new warning follows two watches issued since July.

Strong said the water was not expected to become as warm as last year, when sea temperatures in the territories hovered near 86 degrees Fahrenheit for months at a time and as much as 40 percent of the coral died around the U.S. Virgin Islands.

He said researchers were monitoring how the heat affects coral recovery from last year.

"There is still so much to learn about the physiology of coral" and which species recover fastest, Strong said.

Scientists have not pinpointed what is behind the warm sea temperatures but some speculate global warming might be the cause.

Calm weather in the Caribbean this year has helped sea temperatures rise.

"As long as the winds stay light and the skies bright, you're going to see the temperature increase," Strong said. "That's a lot of warm water down there."

Millions of people visit the Caribbean each year to dive and snorkel over the region's coral reefs, part of a multibillion-dollar tourism industry.

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