Divers can still find plenty of attractive hard coral on Australia’s Great Barrier Reef compared with many parts of the world, but it’s the underlying trends that are concerning scientists.
Hard-coral cover across the GBR has declined substantially from the high levels of recent years and returned to near long-term average levels – emphasising how volatile the situation has become, according to the latest annual survey report produced by the Australian Institute of Marine Science (AIMS).
The GBR experienced the biggest annual decline in coral cover in two of its three regions (northern, central and southern) since AIMS began monitoring 39 years ago.
This was driven predominantly by climate change-induced heat-stress leading to coral mortality from the 2024 mass-bleaching event, but also by the impact of cyclones and crown-of-thorns starfish outbreaks.
In the northern region coral cover dropped over the year by almost 25% to 30%; the central region saw a smaller 13.9% drop to 28.6% and southern region cover fell by 31% to 26.9%.
Off a high base
“This year’s record losses in hard-coral cover came off a high base, thanks to the record high of recent years,” says AIMS’ Long-Term Monitoring Program leader Dr Mike Emslie. “We are now 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. We have seen coral cover oscillate 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.”
Of the 124 coral reefs surveyed, 77 recorded 10-30% hard-coral cover, 33 reefs 30-50% and only two had more than 75% cover. Two reefs had less than 10% cover.
Coral reefs dominated by the Acropora species were among those hardest-hit by mass coral-bleaching and the two cyclones, says Emslie.
“We’ve said in the past that these corals are the fastest to grow and are the first to go, as they are susceptible to heat-stress, cyclones and are a favourite food of crown-of-thorns starfish, and this year’s results illustrate that.
“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.”
Biggest footprint yet
Last year’s mass bleaching, part of a global event that began in the Northern Hemisphere in 2023, was the fifth on the GBR since 2016 and had the biggest footprint yet, with high to extreme bleaching across all three regions.
“This year Western Australian reefs also experienced the worst heat-stress on record,” commented AIMS CEO Prof Selina Stead. “It’s the first time we’ve seen a single bleaching event affect almost all the coral reefs in Australia.”
Such events were becoming more intense and frequent, she said, as evidenced by those of 2024 and 2025, the second time in a decade that the reef had experienced mass bleaching in two consecutive years.
“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.”
AIMS’ 2025 Long-Term Monitoring Program quantifies trends in the status of GBR coral communities, and the latest annual summary reports the results of reef surveys from August 2024 to May 2025, and assesses the impact of the 2024 mass bleaching event.
