Hard coral cover across the Great Barrier Reef has fallen sharply from recent record highs back to near long‑term averages, with two of the three regions recording their largest annual declines since monitoring began 39 years ago, the Australian Institute of Marine Science (AIMS) has reported.
AIMS’ annual Long-Term Monitoring Program summary, which assessed 124 reefs between August 2024 and May 2025, attributes the downturn primarily to climate change‑driven heat stress and coral mortality from the 2024 mass bleaching event, compounded by cyclone damage and crown‑of‑thorns starfish outbreaks.
Over the past year, hard coral cover dropped in the northern region (Cape York to Cooktown) by a quarter, from 39.8% to 30%; in the central region (Cooktown to Proserpine) by 13.9%, from 33.2% to 28.6%; and in the southern region (Proserpine to Gladstone) by almost one third, from 38.9% to 26.9%.
AIMS LTMP leader Dr Mike Emslie said the scale of losses was tempered by the strong growth recorded in recent years. “This year’s record losses in hard coral cover came off a high base, thanks to the record high of recent years,” he said.
“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.”
Species in the Acropora genus, which help build the Reef’s three‑dimensional structure, bore the brunt of the heat and storm impacts. “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,” Dr Emslie said. “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.”
AIMS said the 2024 event formed part of a global mass bleaching that began in the Northern Hemisphere in 2023. It was the fifth mass bleaching on the Great Barrier Reef since 2016 and had the largest spatial footprint recorded, with high to extreme bleaching prevalence across all three regions.
AIMS CEO Professor Selina Stead said other Australian coral systems were similarly affected. “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 said.
“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.
“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.”
Of the 124 reefs surveyed by AIMS’ manta tow method, most (77) recorded hard coral cover between 10% and 30%, 33 had between 30% and 50%, while two reefs had more than 75% and two had less than 10%. The LTMP uses percentage hard coral cover as a key indicator of reef condition, providing an overview for managers and policymakers. AIMS also undertook targeted, in‑water bleaching surveys during and after the 2024 event to verify the manta tow results, with findings to be submitted for peer review.
Beyond broad coral cover assessments, AIMS’ fixed‑site surveys on 71 reefs track coral types, fish communities, and causes of mortality, including crown‑of‑thorns starfish, disease and bleaching, to build a longer‑term picture of how the Reef is responding to mounting environmental pressures.
