Pause in recent coral recovery on much of Great Barrier Reef

In-water monitoring by the Australian Institute of Marine Science (AIMS) reveals that the level of hard coral cover across the Great Barrier Reef remains consistent with last year. However, there has been a slight decrease in the Northern, Central, and Southern regions.

The Annual Summary Report on Coral Reef Condition for 2022/23 from AIMS indicates that while some reefs have shown signs of recovery, the increase in hard coral cover has been offset by coral loss on other reefs. Most reefs have experienced minimal changes in coral cover.

Last year’s report noted the highest levels of coral cover in the Northern and Central regions since AIMS began monitoring 37 years ago.

The interruption in recovery in these regions can be attributed, in part, to the mass coral bleaching event in 2022. Factors such as a low population of coral-eating crown-of-thorns starfish and a cyclone in January 2022 also contributed to coral loss in the Northern region. In the Southern region, continued outbreaks of crown-of-thorns starfish and coral disease have maintained coral cover at similar levels to last year, with bleaching playing a smaller role.

Dr. David Wachenfeld, the AIMS Research Program Director, acknowledges the positive recovery on some reefs but emphasizes that even less severe mass bleaching events have consequences for the Reef.

“The coral bleaching event in 2022 was not as severe as those in 2016 or 2017, but it caused enough mortality to pause the recent regional gains in hard coral cover. The heat stress during the bleaching event likely had sub-lethal effects, including reductions in coral growth and reproduction,” said Dr. Wachenfeld.

“Although the conditions during the 2023 summer were relatively mild, with low levels of coral bleaching and no cyclones crossing the Reef, we are still just one major disturbance away from a rapid reversal of the recent recovery.

“While the Reef remains a remarkable, intricate, and beautiful ecosystem, it is increasingly vulnerable to climate change, which leads to more frequent and severe bleaching events and puts pressure on the ecosystem’s resilience.”

The Report provides the following average hard coral coverage for 2022/23:

• Northern region (north of Cooktown) – 35.7%, down from 36.5% last year;
• Central region (Cooktown to Proserpine) – 30.8%, down from 32.6%;
• Southern region (south of Proserpine) – 33.8%, down from 33.9%.

The report’s data on reef slopes comes from surveys conducted between August 2022 and May 2023 as part of the AIMS Long Term Monitoring Program (LTMP), which has been collecting data for 37 years. The LTMP dataset is the largest, longest, and most comprehensive source of information on the status of the Great Barrier Reef.

Dr. Mike Emslie, the leader of the AIMS Long-Term Monitoring Program, notes that recent recovery has primarily been driven by fast-growing branching and plate corals, known as Acropora. However, these important habitat builders are also susceptible to disturbances such as cyclones, crown-of-thorns starfish, and coral bleaching.

“Acropora corals are abundant and play a significant role in the fluctuations of hard coral cover. They have been experiencing a rapid growth phase in recent years. However, other coral species have also contributed to the recovery,” he said.

Dr. Emslie highlights the increased frequency of mass coral bleaching events on the Great Barrier Reef, with four events occurring since 2016.

“The bleaching event in 2022 was the first ever recorded during a La Niña year, which is typically characterized by cooler temperatures,” he explained.

“If there were no disturbances, we would expect the recent increases in coral cover to continue this year. However, this pause indicates that even a less severe mass bleaching event, with low mortality, is enough to halt coral recovery.”

“This highlights the ongoing risk of decline for the Reef due to more frequent disturbances. AIMS is committed to understanding the impact of climate instability through monitoring and research.”

Dr. Wachenfeld added, “Ensuring the future of the Great Barrier Reef and all coral reefs worldwide requires reducing greenhouse gas emissions to stabilize temperatures, implementing best practice management to mitigate local pressures, and developing interventions to enhance climate tolerance and resilience in coral reefs.”

Background

The LTMP measures long-term trends in the status of coral communities across the Great Barrier Reef.

Hard coral cover is one indicator used by researchers to assess reef condition, along with various other metrics. Trained scientists estimate the percentage of hard coral cover during manta tow surveys, which allows AIMS to provide an overview of the Great Barrier Reef’s status and keep policymakers, managers, and other scientists informed in a timely manner.

Manta tow surveys are an efficient method for assessing large areas of reef. They involve standard measurements of hard coral cover, estimates of crown-of-thorns starfish populations, assessments of coral bleaching levels, and observations of fish and shark populations.

The LTMP also conducts detailed surveys on fixed sites across 73 reefs within the Great Barrier Reef. These surveys provide deeper insights into the corals, fish, and crown-of-thorns starfish at these locations, including information on their abundance, age, and more detailed data on coral disease and bleaching. 3D images are captured to assess reef complexity and changes in structure.

The AIMS monitoring team spent 120 days at sea during the survey period, covering 1016km around the perimeter of the 111 surveyed reefs.

The new survey season for the AIMS LTMP team will begin in late August and continue throughout the summer, concluding around May 2024.

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