Australian Reef Survives 22 °C-Weeks of Heat Stress in 2025 Marine Heatwave While Nearby Reefs Suffered Major Coral Death

Coral reefs on the Houtman Abrolhos Islands off Western Australia were almost untouched by a prolonged heatwave in early 2025 that killed vast swathes of coral globally. Researchers dived at 11 sites across the Houtman Abrolhos archipelago in July 2025 and found that apart from a few tiny patches, there were no signs of stress such as fluorescing coral.

Up to 60 per cent of corals died at Ningaloo Reef during the same 2025 heatwave.

Marine heatwaves that year proved catastrophic elsewhere, yet the full array of coral species at Houtman Abrolhos appeared immune to heat stress that was disastrous at other locations. Prolonged heat stress generally leads to coral bleaching when corals expel symbiotic algae. Heat stress is measured in degree heating weeks, abbreviated as DHW.

Over 4 °C-weeks, significant bleaching is expected, while above 8 °C-weeks the situation becomes dire. ” Waters around the Houtman Abrolhos Islands hit 4 °C-weeks in early February 2025 and reached 8 °C-weeks by early March 2025. By mid-April 2025, corals at Houtman Abrolhos had experienced 22 °C-weeks of heat stress.

“We expected to see mass bleaching with lots of white colonies, and likely mortality of reefs, given we did surveys after many months of marine heatwave. We did not see this,” Kate Quigley stated. Quigley, who is at the University of Western Australia in Perth, and her colleagues were most surprised that every coral species at the site showed the same resilience.

To test the limits of this tolerance, colonies from several coral species were brought to the lab and subjected to prolonged high temperatures. At 8 °C-weeks, survival rates at Houtman Abrolhos were twice as high as currently accepted thresholds and bleaching resistance was nearly four times higher. There was still nearly 100 per cent survival at around 16 °C-weeks.