WA hit by worst coral bleaching on record across 2000km of coastline

Western Australia’s reefs recently experienced the most widespread coral bleaching event on record, a new report has found, with around 2000 kilometres destroyed as temperatures soared between August 2024 and June 2025.

Heat stress remained persistent for between 80 and 160 days along much of the stretch of coastline, the Status of Coral Reefs of the World 2025 published on Monday afternoon revealed.

But the majority of the report did not capture the most recent bleaching event, and was focused on data collated between 1980 and 2024.

It is “the most detailed assessment of the status of the world’s coral reefs to date”, pulling together more than 21.1 million observations from 87,299 surveys, representing 36,886 sites across 124 countries.

It found in WA, extreme heat stress accumulations of greater than 20 degrees over a span of weeks was expected to result in near-complete, reef-wide coral mortality.

Australian Institute of Marine Science coral reef ecology principal research scientist Dr James Gilmour said that metric was “something like two degrees above normal for a 10-week duration, and that’s very considerable stress”.

“If you find temperatures at any one point in time are more than two or three degrees above normal, then certainly most corals would die from that,” he said.

“In some parts, temperatures were as high as four degrees above normal at one time so essentially, in those areas, many corals are just basically cooked in a one-off rather than over a duration.

“There are patches that survive – usually they’re more the encrusting or circular corals, they’re not the ones that really give a lot of habitat to the reef.”

The report stated that most of the Pilbara reefs experienced extreme bleaching and mortality, at greater than 75 per cent, while coral communities at world-heritage listed Ningaloo Reef – the world’s longest fringing reef – recorded the most severe bleaching on record with between 30 and 70 per cent damaged.

Gilmour said the report did not fully capture the impact of the most recent mass bleaching event in Ningaloo in 2025, which was the worst on record for Australian coral reefs.

“We do see good recovery of our reefs, and that’s a good reflection on our management and the efforts here, but for the first time last year we really saw all of WA’s most significant reefs affected in one event, and that’s fairly new and novel, but that’s not captured in this report,” he said.

“From a severe bleaching, it takes about five to 10 years for them to recover but since 2010, we’re starting to see gaps between bleaching of certainly less than 10 years, and in some places of a few years.

“The Pilbara, in particular, is one area that’s been affected many, many times.”

Gilmour said “pristine reefs” like Ningaloo and Rowley Shoals were also badly affected last year.

“They really haven’t had a severe bleaching at least multiple times before, so this is the concern, particularly with the heat stress within predictions that we’re looking for this coming season,” he said.

Scientists are predicting a “super El Nino” to impact the state’s far north reefs, but the extent of that weather event is difficult to predict, according to Gilmour.

“Last year the worst bleaching occurred due to what was a relatively neutral phase, but we still saw this bleaching,” he said.

“We would expect the worst bleaching to be in the far north for this El Nino, but again, it’s just so hard to predict nowadays because background temperatures are increasing so rapidly.”

The report also found climate change was not the only stressor on WA reefs, with some coastal areas occasionally affected by episodic flood events, cyclones and localised dredging operations.

However, University of Technology Sydney future reefs team leader Emma Camp said not all hope was lost.

“The global decline in coral cover found in the report is highly concerning, but not unexpected given the multitude of threats coral reefs face,” she said.

“Importantly, the trajectory of coral reefs is not yet predetermined.

“The report highlights that more optimistic futures remain possible if we can create conditions that give corals sufficient time and opportunity to recover between successive stress events.”