Parts of the Great Barrier Reef have experienced the largest annual decline in coral cover in nearly 40 years, with experts warning it faces a future of “increased volatility”.

According to the latest annual survey from the Australian Institute of Marine Science (AIMS), hard coral cover declines in 2025 were largely driven by climate change-induced heat stress, leading to mass bleaching events and exacerbated by cyclones, floods, and outbreaks of crown-of-thorns starfish.

While the coral losses were significant, they came off a high base, with observed coral cover now sitting at “near to long-term average levels”.

“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,” AIMS long-term monitoring program leader Mike Emslie said.

Rising ocean temperatures, floods, and cyclones were all compounding the impacts of recent bleaching events.

“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,” Dr Emslie said.

A man wearing Quiksilver wet suit and red life jacket stands while driving a boat on the ocean

Mike Emslie leads the Australian Institute of Marine Science (AIMS) long-term monitoring program. (Supplied: Australian Institute of Marine Science Long-Term Monitoring Program)

AIMS has been monitoring and surveying the Great Barrier Reef for 39 years. It noted the 2024 mass bleaching event was the fifth to hit the region since 2016, and the largest ever recorded on the Great Barrier Reef.

The stunning, varied colour of the corals in the Great Barrier Reef is the result of a unique, symbiotic relationship between the hard coral and zooxanthellae algae. The coral gives the algae a protective environment, and the algae feeds the coral.

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But when the ocean gets too hot, the coral polyps can become stressed and expel the zooxanthellae, which triggers the bleaching effect. Without food for too long, the coral can starve and die.

In the latest AIMS survey results, the most impacted coral species were the Acropora, which is susceptible to heat stress and a favoured food of the crown-of-thorns starfish.

“These corals are the fastest to grow and are the first to go,” 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.”

A close-up underwater shows a spiky crown-of-thorns starfish

Crown-of-thorns starfish infestations are still at outbreak levels on several reefs in the Southern Great Barrier Reef. (Supplied: Australian Institute of Marine Science Long-Term Monitoring Program)

Southern Great Barrier Reef sees greatest decline

AIMS surveys 124 individual reefs across the Great Barrier Reef (GBR), and analyses results for three regions: the Northern GBR from Cape York to Cooktown, Central GBR from Cooktown to Proserpine, and Southern GBR from Proserpine to Gladstone.

One of the main measures used to monitor the condition of the reef is the percentage of hard coral cover, which describes the proportion of the sea floor that is covered in hard coral.

Compared to figures from 2024, the Southern GBR saw a 30 per cent decline in coral cover — the largest annual decline ever recorded, bringing it below the long-term average.

Coral cover in the Central and Northern GBR remains above long-term averages, but both regions saw significant declines compared to 2024, of 14 per cent and 25 per cent respectively.

Based on historical surveys, AIMS defines reefs with between 30-50 per cent coral cover as “high value” — that is, they’re relatively healthy.

Of the reefs surveyed, AIMS found that two had less than 10 per cent coral cover, 77 between 10 and 30 per cent cover, 33 between 30 and 50 per cent cover, 10 between 50 and 75 per cent cover, and two had greater than 75 per cent coral cover.

While there has been an overall decline, the Great Barrier Reef has “fared relatively better” than other parts of the world because of its size and its varied habitats.

The Caribbean, for example, has suffered high mortality on almost all of its reefs, and very little coral remains.

“The Great Barrier Reef currently retains higher coral cover than many reefs globally; however, mass coral bleaching events are now occurring with increasing frequency, while recovery periods are decreasing,” the report said.

Crown-of-thorns starfish control working, but emissions remain a threat

One of the successes noted in the survey was the ongoing efforts to monitor and control crown-of-thorns starfish.

Crown-of-thorns starfish can grow up to 80 centimetres in diameter, with up to 21 arms covered in hundreds of toxin-tipped thorns, and they can consume vast amounts of coral.

Some thick, spiky red crown-of-thorns starfish are seen crawling around branches of white coral.

Crown-of-thorns starfish are native to the Great Barrier Reef, but their numbers have boomed since the 1960s. (Supplied: Australian Institute of Marine Science)

The starfish are native to the Great Barrier Reef, but since the 1960s their numbers have increased significantly.

Exactly what is behind the booming population is still being studied, but increased nutrient availability from land run-off, which increases phytoplankton, has created more food for COTS larvae, and may be leading to more adults.

Over the past year, the federal government’s crown-of-thorns starfish control program has culled over 50,000 starfish, injecting them with vinegar or ox bile.

“Due to crown-of-thorns starfish control activities, there were no potential, established, or severe outbreaks of crown-of-thorns starfish recorded on Central GBR reefs in 2025,” the AIMS report noted.

As global carbon emissions continue to rise, so too does the temperature of the sea water that covers the Great Barrier Reef.

The latest reef update from the Reef Authority shows that even as water temperatures declined during winter, they were still around 0.8 degrees above the long-term average.

Earlier this year, the GBR experienced its sixth mass bleaching event since 2016. Up until the 1980s, coral bleaching at this scale was rare.

“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,” AIMS CEO Selina Stead 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 the past decade that a reef experienced back-to-back bleaching events in consecutive years. Coral reef systems can take up to 10-15 years to recover.

“The results from this year really underscore the amount of stress the Great Barrier Reef is under,” Dr Emslie said.

“The Great Barrier Reef is such a beautiful, iconic place, it’s really, really worth fighting for. And if we can give it a chance, it’s shown an inherent ability to recover.”An underwater photo shows coral of varying colours, with yellow fish swimming towards the surface

Pompey Reef No.1 in the Southern Great Barrier Reef has a diverse coral assemblage despite impacts from the 2024 bleaching event. (Supplied: Australian Institute of Marine Science Long-Term Monitoring Program)