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Scientists on an expedition to Jarvis Island in the Pacific Remote Island Marine National Monument have found devastating loss of corals due to record warm ocean temperatures from April 2015 to May 2016, the National Oceanic and Atmospheric Administration (NOAA) said Wednesday. 

Jarvis is an unpopulated island about 1,500 miles south of Honolulu, and it is typically a treasure trove of biological diversity. 

According to NOAA, the area has the highest fish biomass, which measures the total amount of fish species in a given area, yet it is studied as part of the agency's Pacific coral reef monitoring program. 

The reefs “looked more like a coral graveyard," Bernardo Vargas-Angel, a scientist with NOAA Fisheries’ Pacific Islands Science Center Coral Reef Ecosystem Program said.

“One would have never believed that just a year before this was a vibrant and colorful coral reef. Coral mortality was widespread across all reef habitats and depths,” Vargas-Angel said in a statement. 

Woods Hole Oceanographic Institution scientist Anne Cohen, who is involved in the research but wasn't on this trip, told the Associated Press that the area around Jarvis typically teems with life.

"It's like the Super Bowl of coral reefs, this place," Cohen said. "The coral cover is astronomical. The amount of life that it supports is just sky high: fish, turtle, dolphins, sharks. You name it, you find it there in large numbers."

Vargas-Angel estimates that “about 95% of the coral colonies died from coral bleaching caused by high and prolonged water temperatures associated with this intense El Niño.”

The El Niño event combined with the effects of human-caused global warming to send ocean temperatures soaring to record levels around the world, with widespread coral reef mortality found this week on parts of Australia's Great Barrier Reef, for example. 

Coral bleaching occurs when coral expels symbiotic algae, known as zooxanthellae, that lives in its tissue, giving it color and nutrients. This action, caused by stresses such as increased water temperature and pollution, leaves the coral skeleton exposed, making it more susceptible to heat stress, disease and pollution.

“One would have never believed that just a year before this was a vibrant and colorful coral reef."

Bleached corals can recover if the ocean waters cool fast enough or pollutants diminish. However, they can die if the stress lasts too long.

In the case of the area around Jarvis Island, ocean temperatures remained in the lethal range for eight months, and were conducive to bleaching for more than a year, NOAA said. At the peak of the event, water temperatures were about 4 degrees Celsius, or 7.2 degrees Fahrenheit, above average. 

Scientists surveyed the area by diving from research vessels shortly before the El Niño began to affect the area, and re-surveyed it a year later with partners from the Woods Hole Oceanographic Institution and Rutgers University earlier this month, just as the water returned to lower temperatures. 

The long duration of unusually high temperatures had decimated the coral reef ecosystems in one of the world's largest marine conservation areas. 

Many of the previously bleached corals observed during a November 2015 research trip were dead, broken and covered with a thick mat of reddish algae -- a telltale sign of coral death.

Some good news, too

While the third-recorded global coral bleaching event is killing reefs around the world, this event also offers an opportunity for scientists to study what makes some species of coral more resilient than others.

For example, some colonies of corals known as Porites survived the bleaching event, scientists found. 

“While many of these died, some looked relatively healthy, providing hope that they, and the exceptional biological productivity and remoteness of Jarvis Island, will be the harbinger to a successful recovery of these unique coral reef ecosystems,” said Rusty Brainard, leader of the Coral Reef Ecosystem Program, in a statement. 

As global warming continues to push air and ocean temperatures higher, marine scientists are working to determine which corals are more resilient, and may therefore need to be better protected from other man-made threats, such as pollution and overfishing.

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