Extensive coral bleaching detected on southern Great Barrier Reef

Extensive coral bleaching has swept across the southern Great Barrier Reef, with authorities revealing the scale of the damage across multiple surveyed zones in what could mark the start of a seventh mass bleaching event.

Aerial surveys conducted over the weekend by the Great Barrier Reef Marine Park Authority and Australian Institute of Marine Science documented bleaching across 48 surveyed reefs — 27 inshore sites around the Keppel Islands and Gladstone area, and 21 offshore reefs in the Capricorn Bunkers. The bleaching was "extensive and fairly uniform," with most coral showing visible signs of stress.

"Most coral displayed some level of bleaching with white and fluorescent colonies observed in shallow reef areas," said Mark Read, the authority's director for reef health.

The damage became apparent during the tail end of an unusually warm summer. Scientists at James Cook University conducting routine surveys of the Keppel Islands had begun documenting signs of coral stress weeks earlier, with water temperatures reaching 29 degrees Celsius — two degrees above the regional summer average.

"I have been working on these reefs for nearly 20 years and I have never felt the water as warm as this," said Maya Srinivasan, a researcher at James Cook University's centre for tropical water and aquatic ecosystem research. "If the water temperature is higher than normal for an extended period, then the coral will die."

Coral bleaching is a stress response. When water temperatures rise too high, corals expel the symbiotic algae that live within their tissue and provide their primary food source. If conditions cool rapidly, some corals can recover. But prolonged heat means starvation — the organism's white skeleton becomes visible through the transparent tissue, a visible marker of ecological collapse.

Reef managers are bracing for broader impacts. While the southern section showed the most severe bleaching, the authority has received reports of bleaching from across other regions of the marine park. A CNN team documenting the reef a fortnight earlier had already observed bleaching at Lady Elliot Island, the reef's southernmost coral cay, and at multiple outer reefs near Cairns to the north.

"With the temperature profile that we see at the moment, it's not surprising that we're seeing these signs of stress," said Roger Beeden, a chief scientist with the Reef Authority. But the full extent of the damage leaves little room for optimism. "Given the extensive nature of it and the heat stress, some will die off," he cautioned.

The Great Barrier Reef spans nearly 345,000 square kilometres and harbours more than 1,500 fish species across 411 species of hard coral. The ecosystem contributes billions annually to Australia's economy and stands as one of the world's most recognisable natural wonders. Mass bleaching events pose an existential threat not only to the reef's biology but to its economic and cultural significance.

Authorities plan to conduct in-water surveys across the full reef system in the coming weeks, seeking to measure the true severity and spatial extent of the bleaching. The investigation will determine whether the reef is entering its seventh mass bleaching episode — a prospect that underscores the reef's vulnerability to rapid ocean warming and the urgency of climate action.