Western Australia has endured its most extensive coral bleaching on record after an unprecedented marine heatwave over the 2024/25 summer drove prolonged heat stress across reefs from the Kimberley to Ningaloo and the Pilbara, scientists say.
The event, described by researchers as the state’s longest, largest and most intense marine heatwave, delivered bleaching and coral mortality ranging from medium (11–30%) to extreme (>90%) across systems up to 1,500 kilometres apart, according to the WA Coral Bleaching Group convened by the Australian Institute of Marine Science (AIMS). A summary newsletter of the event was released on Monday, as researchers and managers gathered at a Marine Heatwave Symposium at the Indian Ocean Marine Research Centre in Perth, with an open forum to be livestreamed.
Monitoring by the multi-agency group began in September 2024. Heat stress peaked in December 2024/January 2025, eased slightly with a late monsoon, and surged again in March/April before sea temperatures fell in May.
AIMS senior research scientist Dr James Gilmour said the heat took a severe toll on reefs that had previously been spared. “There has been little reprieve this time for any of our northwest reefs. Areas which had given us hope because they’d rarely or not bleached before like the Rowley Shoals, north Kimberley and Ningaloo have been hit hard this time. Finally, climate heating has caught up with these reefs.”
He said duration and intensity set this summer apart. “The length and intensity of the heat stress, and its footprint across multiple regions, is something we’ve never seen before on most of the reefs in WA. For individual coral reefs we are still crunching the numbers around Degree Heating Weeks (DHW) – a measure of heat stress on corals – but early estimates are over 15 DHWs on all reefs and up to 30 DHWs at some Pilbara reefs. Eight DHWs is the level of heat stress we generally consider severe enough to cause coral mortality.”
AIMS coral scientist Nicole Ryan, who coordinates the WA Coral Bleaching Group, said the episode is expected to surpass previous benchmarks. “The heat stress will beat records set during the 2011 La Nina and the 2016 El Nino on all but the southernmost reefs, in the south of Ningaloo, Shark Bay and the Abrolhos Islands.
“It will take several months to understand how much this year’s event will have impacted coral cover on various systems. A bleached coral is not a dead coral – it can recover.”
“For some coral species, it can take time after bleaching for it to recover or die. WA reefs are also spread across thousands of kilometres with many being remote, making them difficult to monitor.”
Field surveys reported:
– Ashmore Reef: medium levels of bleaching and mortality in March, with variable impacts including one lagoon site showing high bleaching.
– Rowley Shoals (Mermaid and Clerke Reefs): very high mortality (61–90%) across habitats, from lagoon to 30 metres depth, recorded in April not long after peak stress.
– Inshore southern Kimberley: high bleaching recorded in February with mortality reported in March.
– Ningaloo: high bleaching and mortality in May, with Tantabiddi and Jurabi particularly affected.
Ocean measurements gathered by AIMS oceanographer Dr Camille Grimaldi indicate conditions were hotter than satellite data suggested. “At Ashmore Reef, for instance, in situ temperatures recorded in the lagoon were up to 1.4 °C higher than satellite estimates. Additionally, temperatures measured at 20 m depth exceeded satellite readings by nearly 0.5 °C, indicating that ocean warming extended not just at the surface, but throughout the water column, spanning the depth range for most corals,” said Dr Grimaldi. “These differences highlight how local oceanographic processes, such as shallow water depth, upwelling or mixing, can amplify or dampen the effects of regional heatwaves, making extreme events either more severe or more tolerable at specific reef locations.”
The Bureau of Meteorology’s Dr Claire Spillman said the backdrop was unprecedented ocean warmth. “2024 was the warmest year on record for global oceans. For Australian waters, sea surface temperatures during the summer of 2024-2025 were the warmest since official records began in 1900.
“The northwest waters of WA also experienced a warm winter in 2024, with persistent warm water at depth, and this background warmth increased the chances of marine heatwave conditions. While the 2010-11 WA marine heatwave was associated with a strong La Nina event, this was not the case this summer. The increased frequency of extreme and record-breaking ocean temperatures is associated with global warming.”
Dr Gilmour said the WA event formed part of a global pattern. “This huge WA bleaching event comes at a concerning time for coral reefs in Australia. It was synchronised with another mass bleaching event on the Great Barrier Reef and is part of the ongoing fourth Global Coral Bleaching Event, which began in 2023 and has circumnavigated our oceans in a wave of coral mortality. Climate change is driving these events, which are becoming more frequent, more intense and more widespread, giving our amazing, valuable coral reefs little time to recover. And they need 10 to 15 years to recover fully.
“The key to helping coral reefs survive under climate change is to reduce greenhouse gas emissions. Good management of local issues like water quality and overfishing and using interventions developed by marine science to help reefs will also assist.”
The WA Coral Bleaching Group, first convened by AIMS in 2015, brings together state and federal agencies, Indigenous Sea Rangers, universities, CSIRO and the Bureau of Meteorology to track heat stress, document bleaching and coordinate monitoring across vast and often remote reef systems. The group shares updates via a regular newsletter and has been central to coordinating science and management responses across Western Australian waters.
