Sydney: Australia's Great Barrier Reef's hard coral cover has sharply declined, returning to near long-term average levels after recent record highs, a report revealed Wednesday.

The findings underscore a new volatility in coral health, according to the Australian Institute of Marine Science's (AIMS) annual survey report of the world's largest coral reef system off the coast of Australia's Queensland.

The Great Barrier Reef has experienced the largest annual decline in coral cover in two of its three regions since AIMS began monitoring 39 years ago, the report said.

This decline was mainly caused by climate change-driven heat stress from the 2024 mass bleaching event, alongside damage from cyclones and crown-of-thorns starfish outbreaks, it said.

According to AIMS, coral cover fell by about 25 per cent in the northern section, nearly 14 per cent in the central region, and around one-third in the southern region, which experienced such severe bleaching for the first time, Xinhua news agency reported.

AIMS Long-Term Monitoring Program (LTMP) leader Mike Emslie said that over the past 15 years, hard coral cover has become more volatile, fluctuating rapidly between record lows and highs, pointing to an ecosystem under stress.

Coral reefs dominated by the Acropora species, known for their fast growth but high vulnerability, were among the most impacted, "as they are susceptible to heat stress, cyclones and are a favorite food of crown-of-thorns starfish," Emslie said.

AIMS' 2025 LTMP surveyed 124 reefs between August 2024 and May 2025, finding that most reefs had hard coral cover of 10-30 per cent, 33 reefs had 30-50 per cent, two reefs exceeded 75 per cent, and two were below 10 per cent.

AIMS CEO Selina Stead said the findings strongly demonstrate how ocean warming due to climate change is causing significant and rapid harm to the Great Barrier Reef's coral communities. (IANS)