The marine heatwave that hit reefs from the World Heritage-listed Ningaloo to the remote Ashmore Reef left many scientists shocked.<\/p>\n
Sign up: AU Breaking News email<\/a><\/p>\n A group of scientists from state and federal government departments, agencies and universities, who monitor coral bleaching in Western Australia<\/a>, will issue a detailed update on Tuesday.<\/p>\n The heatwave, which started to build off the north-western WA coast in August 2024, caused heat stress in corals across the state\u2019s northern reefs that lasted until May this year in \u201cthe most severe coral bleaching on record for Western Australian coral reefs,\u201d the update said.<\/p>\n Quick GuideWhat is coral bleaching?Show Coral bleaching describes a process whereby the coral animal expels the algae that live in its tissues and give it its colour and much of its nutrients.<\/p>\n Without its algae, a coral\u2019s white skeleton can be seen through its translucent flesh, giving off a bleached appearance.<\/p>\n Mass coral bleaching over large areas, first noticed in the 1980s around the Caribbean, is caused by rising ocean temperatures.<\/p>\n Some corals also display fluorescent colours under stress when they release a pigment that filters light. Sunlight also plays a role in triggering bleaching.<\/p>\n Corals can survive bleaching if temperatures are not too extreme or prolonged. But extreme marine heatwaves can kill corals outright.<\/p>\n Coral bleaching can also have sub-lethal effects, including increased susceptibility to disease and reduced rates of growth and reproduction.<\/p>\n Scientists say the gaps between bleaching events are becoming too short to allow reefs to recover.<\/p>\n Coral reefs are considered one of the planet\u2019s ecosystems most at risk from global heating. Reefs support fisheries that feed hundreds of millions of people, as well as supporting major tourism industries.<\/p>\n The world\u2019s biggest coral reef system \u2013 Australia\u2019s Great Barrier Reef \u2013 has suffered seven mass bleaching events since 1998, of which five were in the past decade.\u00a0<\/p>\n Thank you for your feedback.<\/p>\n Dr James Gilmour, a senior research scientist at the Australian Institute of Marine Science that coordinates the monitoring group, said on some coral reefs that were inspected, more than 90% of corals had either bleached or died.<\/p>\n \u201cWhen you have more than about 75% of corals that have bleached or died, it can be very hard to find any healthy corals,\u201d he said.<\/p>\n The update from the WA Coral<\/a> Bleaching Group said the amount of bleaching and coral death on reefs ranged from medium (11%- 30%) to extreme (greater than 90%) across systems up to 1,500km apart.<\/p>\n The worst-hit area was the Rowley Shoals, Gilmour said \u2013 a group of three remote reefs that had previously escaped the effects of global heating and had been a \u201chope spot<\/a>\u201d for many scientists.<\/p>\n A map shows the location of coral reefs across WA that experienced severe levels of coral bleaching and death.<\/a>The location of coral reefs across WA that experienced severe levels of coral bleaching and death during the 2024\/25 summer marine heatwave.<\/p>\n \u201cThe first time reefs bleach badly, the mortality tends to be quite high,\u201d he said.<\/p>\n Throughout the heatwave, scientists were monitoring a metric over reefs known as Degree Heating Weeks \u2013 a measure of accumulated heat stress.<\/p>\n Gilmour said as a general rule, 8DHWs is high enough to cause some corals to die. During the marine heatwave, most WA reefs from Ningaloo north reached at least 15 DHWs, with some reefs off the Pilbara coast hitting 30DHWs.<\/p>\n \u201cAt pretty much all of these reefs, it was the highest heat stress ever recorded,\u201d he said.<\/p>\n The marine heatwave in WA coincided with the most widespread global bleaching event on record that started in January 2023 and is ongoing. More than 80% of the planet\u2019s reefs<\/a> have seen heat stress high enough to cause corals to bleach.<\/p>\n skip past newsletter promotion<\/a><\/p>\n Sign up to Breaking News Australia<\/p>\n Get the most important news as it breaks<\/p>\n Privacy Notice: <\/strong>Newsletters may contain info about charities, online ads, and content funded by outside parties. For more information see our Privacy Policy<\/a>. We use Google reCaptcha to protect our website and the Google Privacy Policy<\/a> and Terms of Service<\/a> apply.<\/p>\n after newsletter promotion<\/p>\n Previous marine heatwaves in WA have been associated with La Ni\u00f1a patterns of ocean heat in the Pacific that see warmer waters pulled south by a current that sweeps down the coast.<\/p>\n This extreme marine heatwave in WA happened in the absence of a La Ni\u00f1a.<\/p>\n \u201cWe still got this dramatic warming, so we are only left with climate change [as a cause] after that,\u201d said Dr Alistair Hobday, a marine heatwave expert at the CSIRO.<\/p>\n Dr Claire Spillman, a member of the bleaching group from the Bureau of Meteorology, said the summer of late 2024 and early 2025 recorded the warmest ocean temperatures for the Australian region on a record going back to 1900.<\/p>\n \u201cThe increased frequency of extreme and record-breaking ocean temperatures is associated with global warming,\u201d she said.<\/p>\n![\"\"](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/08\/1754948592_780_2828.jpg\")
<\/p>\n![\"WA](\"https:\/\/www.europesays.com\/us\/wp-content\/uploads\/2025\/08\/5760.jpg\")
WA reefs experiencing ‘worst coral bleaching event’ ever to hit state \u2013 video<\/p>\n