KAILUA-KONA, Hawaii - Coral reefs off the Big Island are stabilizing nearly four years after Hawaii's worst bleaching event, an environmental organization said.

Warm ocean water temperatures caused the first statewide bleaching event in 2015, damaging delicate coral reefs, West Hawaii Today reported Wednesday.

Surveys by The Nature Conservancy found that an average of 60 per cent of coral off the island's western side bleached, with some reefs experiencing up to 90 per cent mortality.

Bleaching occurs when coral is stressed by changes in its environment, causing it to release algae living in its tissue. The coral then turns pale or white and becomes more susceptible to disease.

Consecutive years of coral bleaching have led to some of the most widespread mortality of reefs on record. While bleached coral often recovers, multiple years weakens the organisms and increases the risk of death.

Coral reefs provide habitats for a quarter of all marine life, form barriers that protect coastlines from storms and help drive tourism economies around the world.

Scientists with The Nature Conservancy have studied coral reefs for the last three years to identify the most resilient.

"Bleaching events like what occurred in 2015 can overstress a coral reef to the point where it may never recover," said Eric Conklin, director of marine science for the organization's Hawaii program. "We surveyed over 14,000 coral colonies at 20 sites along the West Hawaii coast from Kawaihae to Keauhou and were thrilled to see that many of the area's reefs have stabilized, which is the first step toward recovery."

The scientists found that many of the most resilient reefs are in remote areas with limited shoreline access and human exposure. Those reefs had little or no coral disease, and evidence indicated that new corals were beginning to grow, the scientists said.

The least resilient reefs have multiple stressors, like fishing and pollution.

"Interestingly, the number of stressors affecting an area, not the severity of a single one, was the most important factor," said Kim Hum, the organization's marine program director. "Reefs that are fighting the impacts of several stressors are more susceptible to temperature stress, making them more likely to bleach and less able to recover if they do."

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Information from: West Hawaii Today, http://www.westhawaiitoday.com

News from © The Associated Press, 2019