Record hard coral cover on the Great Barrier Reef helped protect it amid a worsening bleaching crisis, the Australian Institute of Marine Science (AIMS) says.
In its annual survey of the Reef, it found the largest annual decline in coral cover in two of three regions that they have monitored for 39 years.
Coral cover falls in 2025
- northern region (Cape York–Cooktown) fell by a quarter (39.8% to 30%).
- central region (Cooktown–Proserpine) fell 13.9% (33.2% to 28.6%).
- southern region (Proserpine–Gladstone) fell almost a third (38.9% to 26.9%).
The institute says the cause is mainly, but not solely, due to heat stress; other causes given by them include the impact from cyclones and crown-of-thorns starfish outbreaks.
The leader of the Institute’s Long-Term Monitoring Program (LTMP), Dr Mike Emslie says the impact of bleaching has been cushioned by record high levels of protective hard coral.
“We are seeing increased volatility in the levels of hard coral cover. This is a phenomenon that emerged over the last 15 years and points to an ecosystem under stress,” he says.
“We have seen coral cover oscillate (fluctuate) between record lows and record highs in a relatively short amount of time where previously such fluctuations were moderate.
“Coral cover now sits near the long-term average in each region.
“While the Great Barrier Reef is in comparatively better condition than many other coral reefs in the world following the global mass coral bleaching event, the impacts were serious,” Dr Emslie says.
AIMS says that researchers look at hard coral cover as one sign of a reef’s health; the percentage of cover is calculated by scientists to help assess the overall health of the Reef which is passed onto politicians, managers and other scientists.
Dr Emslie says those reefs dominated by the Acropora species of coral were among the hardest hit by bleaching and two cyclones.
“We’ve said in the past that these corals are the fastest to grow and are the first to go; they are susceptible to heat stress, cyclones and are a favourite food of crown-of-thorns starfish. This year’s results illustrate that,” he says.
“This is also the first time we’ve seen substantial bleaching impacts in the southern region, leading to the largest annual decline since monitoring began.”
LTMP REEF SUMMARY
The LTMP summary of reef surveys (August 2024 to May 2025) covered 124 coral reefs. It found that:
- 77 reefs had a hard cover of 10–30%.
- 33 reefs had a hard cover of 30–50%.
- 2 reefs had a hard cover of more than 75%.
- 2 reefs had a hard cover of less than 10%.
BLEACHING ‘SPREADING SOUTH’
AIMS chief executive officer Professor Selina Stead says bleaching has not been confined to the Reef this year.
“This year Western Australian reefs also experienced the worst heat stress on record. It’s the first time we’ve seen a single bleaching event affect almost all the coral reefs in Australia,” she says.
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“Mass bleaching events are becoming more intense and are occurring with more frequency as evidenced by the mass bleaching events of 2024 and 2025.
“This was the second time in a decade that the Reef experienced mass bleaching in two consecutive years,” Prof. Stead says.
“These results provide strong evidence that ocean warming, caused by climate change, continues to drive substantial and rapid impacts to Reef coral communities.
“The future of the world’s coral reefs relies on strong greenhouse gas emissions reduction, management of local and regional pressures, and development of approaches to help reefs adapt to and recover from the impacts of climate change and other pressures.”
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