Originally published 2019-02-17.
Did Hawaii's coral reefs really come back from the dead? Not quite, but the real story is still good news. After the record marine heatwave of 2015, The Nature Conservancy (TNC) spent three years surveying reefs along the west coast of Hawaii Island. Its scientists reported in January 2019 that many of those reefs had stopped declining and stabilised, which is the first step towards recovery. The healthiest reefs were the ones with the fewest human pressures, a finding that points to practical things people can do while ocean temperatures keep rising.
What the West Hawaii surveys found
TNC divers examined more than 14,000 coral colonies at 20 sites between Kawaihae and Keauhou, a stretch of the Kona and Kohala coast that includes busy harbours, resort beaches and remote lava shorelines. The aim was to work out which reefs had the best chance of surviving future heat stress, so that protection could be focused where it would do the most good.
The headline result was stability. Many reefs that had been hit hard in 2015 were no longer losing coral, and some were showing new growth. Eric Conklin, who led TNC's Hawaii marine science work, told West Hawaii Today that heat stress on this scale can push a reef past the point of recovery, which is why the stabilisation was encouraging rather than a sign that the danger had passed.
The most resilient sites shared three features: plenty of living coral, little or no coral disease, and young colonies settling and growing. They were mostly in remote places where shoreline access is difficult and few people fish, swim or build. The weakest sites, by contrast, were all dealing with several problems at once, including fishing pressure, land-based pollution and polluted runoff.
How bad the 2014 and 2015 bleaching was
Hawaii had rarely seen mass bleaching before 2014. That year, and again in 2015, unusually warm water sat around the islands for weeks. A large Pacific heatwave nicknamed "The Blob" and a strong El Niño both played a part. The 2015 event was the first bleaching to affect the whole state, and it was the worst in Hawaii's recorded history.
In West Hawaii, an average of about 60% of corals bleached in 2015, and on some reefs up to 90% of the coral died. Other surveys painted a similar picture across the islands: at Hanauma Bay on Oahu, nearly half of the corals bleached, and on West Maui and around the Big Island large shares of reefs were affected.
Species did not suffer equally. TNC identified 25 coral species in West Hawaii. Lobe coral (Porites lobata), one of the dominant reef builders, held up best, with roughly half of its colonies bleaching. Cauliflower coral (Pocillopora meandrina), a branching species that gives fish and invertebrates shelter, was hit hardest, with about 98% bleaching. Later surveys found that even cauliflower coral had started to come back.
Bleached is not the same as dead
Much of the confusion about "dead" and "recovered" reefs comes from what bleaching actually is. Reef-building corals live in partnership with tiny algae called zooxanthellae, which sit inside the coral's tissue. The algae photosynthesise and can supply up to 90% of the coral's energy. They also give corals much of their colour.
When water stays too warm for too long, the partnership breaks down and the coral expels its algae. The white skeleton shows through the clear tissue, and the coral looks bleached. At that point it is starving but still alive. If temperatures drop soon enough, algae can move back in and the colony can recover. If the stress lasts, the coral dies and is usually covered in turf algae within weeks.
So a reef can bleach heavily and still survive, or it can lose most of its living coral. Recovery also takes time: a damaged reef may need 10 to 15 years to rebuild, and young corals need years before they are mature enough to reproduce. When the West Hawaii reefs "stabilised", that meant the losses had stopped and new growth was starting, not that the reefs were back to their old condition.
Why the number of stressors mattered most
The most useful finding was about what makes some reefs tougher than others. TNC's marine programme director Kim Hum explained that the number of stressors affecting an area, rather than how severe any single one was, made the biggest difference. In her words, reefs facing several stressors are "more susceptible to temperature stress, making them more likely to bleach and less able to recover if they do."
That makes sense biologically. Fishing removes plant-eating fish such as surgeonfish and parrotfish, which keep seaweed from smothering young corals. Sediment and nutrients from land cloud the water and feed algae. Sewage and chemicals add disease and stress. A coral already coping with these problems has fewer reserves when a heatwave arrives.
No local action can stop ocean warming, which is driven by global greenhouse gas emissions. But local action can decide how many other burdens a reef carries when the next heatwave arrives, and that can be the difference between a reef that bleaches and bounces back and one that does not.
What Hawaii planned to do next
Hum's recommendations were straightforward: keep remote reefs with few stressors in that state, and reduce overfishing, land-based pollution and runoff near populated areas. She also warned that more frequent and more severe bleaching should be expected in the years ahead.
At the time of the survey, only about 6% of state waters and 12% of nearshore waters had some form of management. The State of Hawaii had committed to effectively managing 30% of its nearshore waters by 2030, and TNC said its survey results were meant to guide where that effort and money should go. Reefs matter to the islands' economy as well as their ecology: TNC values reef-related tourism at more than $1.2 billion a year and the flood protection reefs provide at more than $836 million a year.
Hawaii has also taken other steps, such as limiting sunscreen ingredients that may harm coral. Those measures address smaller pieces of the problem, but they follow the same logic of reducing the load on stressed reefs.
The bigger picture since 2019
The West Hawaii recovery took place against a difficult global backdrop. The 2014 to 2017 global bleaching event was the longest and most damaging on record at the time, affecting more than 70% of the world's reefs. A fourth global event began in 2023 and was confirmed by the US National Oceanic and Atmospheric Administration in April 2024. By 2025, around 84% of reefs had been exposed to bleaching-level heat.
Against that background, the Hawaii surveys are a reminder that reefs are not simply doomed or saved. Some reefs recover when given the chance, others shift to seaweed-dominated systems, and local care can tilt the odds. Readers who want to help can choose reef-safe sunscreen, avoid touching or standing on coral, support local marine protected areas, and back efforts to cut emissions.
Common myths about coral recovery
"A white reef is a dead reef." Not necessarily. Bleached corals are alive but stressed, and many survive if temperatures fall quickly.
"If a reef recovered once, it is safe." Recovery takes years, and repeated heatwaves can arrive before a reef has rebuilt. Each event leaves less time to heal.
"Local protection is pointless while the ocean warms." The West Hawaii data suggest the opposite: reefs with fewer local stressors coped better, so local measures buy time.
Sources: West Hawaii Today: Coral bouncing back in West Hawaii (23 January 2019); The Nature Conservancy: Restoring Hawaii's reefs; Wikipedia: Coral bleaching.
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