Houtman Abrolhos Corals Show High Resilience to 2025 Heatwave, Unlike Global Losses

Coral reefs on the Houtman Abrolhos Islands off Western Australia remained almost untouched by a prolonged heatwave in early 2025 that killed vast swathes of coral globally, according to New Scientist. Researchers, diving at 11 sites across the archipelago in July 2025, observed no signs of stress such as fluorescing coral, apart from a few tiny patches.

This resilience contrasted sharply with nearby Ningaloo Reef, where up to 60 percent of corals died during the same event.

Kate Quigley at the University of Western Australia in Perth and her colleagues conducted the surveys after months of marine 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,' Quigley said.

Prolonged heat stress typically causes coral bleaching, where corals expel symbiotic algae that provide most of their food. Scientists measure such stress in degree heating weeks (DHW), factoring in duration and temperature intensity. Over 4 °C-weeks, significant bleaching is expected, and above 8 °C-weeks, conditions become dire for corals.

'Values of around 8 °C-weeks are generally considered catastrophic and are often associated with widespread bleaching and mortality,' Quigley stated. Waters around the Houtman Abrolhos Islands reached 4 °C-weeks in early February 2025, escalating to 8 °C-weeks by early March. By mid-April 2025, the corals had endured 22 °C-weeks of heat stress.

Yet the full array of coral species at the reef appeared immune, showing no mass bleaching or mortality. To assess this tolerance, Quigley and her colleagues collected colonies from several species and exposed them to prolonged high temperatures in the lab. At 8 °C-weeks, survival rates were twice as high as currently accepted thresholds, and bleaching resistance was nearly four times higher.

Nearly 100 percent survival persisted at around 16 °C-weeks. 'Clearly substantial and higher than what has been documented at other reef locations studied so far around the world,' Quigley said of the results. Resistance extended across many species at Houtman Abrolhos.

'I think this location has a particular set of environmental factors that has driven the evolution of heat tolerance generally for the species that live there,' she added. Petra Lundgren at the Great Barrier Reef Foundation highlighted the site's potential.

'They may also hold the key to advancing selective breeding and other interventions aimed at enhancing thermal resilience in conservation aquaculture and coral restoration,' Lundgren said.

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