Nearly four years ago, coral reefs on the west side of the Big Island experienced the worst bleaching event in the state’s history.

The Nature Conservacy has some good news: Those reefs appear to be recovering.

“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 TNC’s Hawaii program, in a news release. “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.”

In 2015 higher than normal ocean temperatures associated with that year’s El Nino caused the first statewide coral bleaching incident. When water is too warm, coral expel the algae living in their tissue, causing them to turn white, or bleach, according to the National Oceanic and Aviation Administration website. Coral may survive bleaching, but are under more stress and may be more likely to die.

Scientists from the nonprofit conducted surveys of the reefs over the course three years to identify the most resilient coral species.

Overall, they found that an average of 60 percent of coral in West Hawaii were bleached, with some reefs experiencing up to 90 percent mortality.

Of the 25 coral species identified in the survey, scientists found that lobe coral (Porites lobata), one of the area’s most dominant species, proved to be the most resilient, with a 50 percent rate of bleaching. Scientists found that cauliflower coral (Pocillopora meandrina) was the hardest hit, with a 98 percent rate of bleaching.

Recent surveys, however, show that cauliflower coral was beginning to recover.

Many of the most resilient reefs, scientists found, lie in remote areas that have limited shoreline access and exposure to human impacts. The least resilient sites suffered from multiple “stressors,” including fishing activity, land-based pollutants and runoff.

“Interestingly, the number of stressors affecting an area, not the severity of a single one, was the most important factor,” said Kim Hum, Nature Conservancy’s marine program director, in the release. “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.”

More management of Hawaii’s coral reefs is needed to ensure their survival, according to The Nature Conservancy, which hopes the survey results will help determine how to ensure the state’s most resilient reefs are protected.

During the World Conservation Congress in 2016, state officials made a legacy commitment, “Hawaii 30 by 30 Oceans,” aiming to increase management of nearshore waters to 30 percent by 2030. Currently, only 6 percent of state waters out to 3 nautical miles, and 12 percent of nearshore waters to a depth of about 164 feet, have some form of management, according to TNC.

“With more frequent and severe bleaching anticipated in the years ahead, there is a lot we need to do in West Hawaii and across the state to minimize the impacts of a warming climate on our reefs,” Hum said. “We can make sure remote areas with few stressors stay that way, and we can reduce pressures from over-fishing, land-based pollutants and runoff in more populated areas.”