According to a government long-term monitoring program, two-thirds of Australia's Great Barrier Reef displayed the highest level of coral cover in 36 years, although the reef is still vulnerable to increasingly frequent mass bleaching (Aug 4).

The Australian Institute of Marine Sciences (AIMS) noted in its annual report that the recovery in the middle and northern sections of the UNESCO-designated reef was in contrast to the loss of coral cover in the southern region brought on by outbreaks of the starfish known as the crown-of-thorns.

In a statement, AIMS Chief Executive Officer Paul Hardisty remarked, "This shows how vulnerable the Reef is to the continued acute and severe disturbances that are occurring more often and are longer lasting."

Following a visit by UNESCO specialists in March, the study comes as UNESCO evaluates whether to label the Great Barrier Reef as "in danger." The World Heritage Committee meeting that was scheduled to take place in Russia in June and cover the reef's future was postponed. Based on its extensive surveys of the reef, AIMS classifies hard coral cover of more than 30% as a high-value indicator of reef health.

The average hard coral cover in the northern region increased to 36% in 2022 from a low of 13% in 2017, while the average hard coral cover in the central region increased to 33% from a low of 12% in 2019 - the highest levels recorded for both regions since the institute started monitoring the reef in 1985.

The southern region, which typically has a higher hard coral cover than the other two zones, saw a decline in hard coral cover in 2022, from 38% to 34%.

The recovery follows the first La Nina event and the fourth mass bleaching in seven years, but according to Hardisty, while extensive, the bleaching in 2020 and 2022 was less harmful than in 2016 and 2017.

"These latest results demonstrate the Reef can still recover in periods free of intense disturbances," Hardisty said. The Acropora corals, which according to AIMS are especially susceptible to wave damage, heat stress, and crown-of-thorns starfish, have spurred the growth in cover, which is a drawback.

This means that significant increases in hard coral cover can be quickly reversed by disturbances on reefs where Acropora corals are the dominant species, according to Mike Emslie, leader of the AIMS monitoring program.

These corals are especially susceptible to wave damage, such as that caused by powerful winds and tropical storms, according to Emslie. "The extent of mass bleaching events highlights the critical threat climate change poses to all reefs, particularly while crown-of-thorns starfish outbreaks and tropical cyclones are also occurring. Future disturbance can reverse the observed recovery in a short amount of time."