Coral reef restoration costly, premature, and doomed to fail says report

Global restoration efforts are failing to protect coral reefs from the effects of climate change, pollution and overfishing, says an international team of researchers

The report comes as Australia’s coral reefs have been bleached yet again, for the sixth time in nine years.

Coral reef bleaching is a global phenomenon, its effects felt not only by the millions of organisms which rely on these ecosystems for food and shelter, but by the more than 1 billion people for whom reefs represent livelihoods, as well as protection from storms and coastal erosion.

University of Western Australia doctoral candidate, Clelia Mulà estimates that around 11,700 km² of global coral reef were degraded between 2009 and 2018 through combined bleaching, overfishing and pollution.

Global coral reef coverage may have already declined by as much as 50%, says Dr Giovanni Strona at the European Union Joint Research Centre. He expects that “bleaching events and coral deaths will only become more common, with projected losses of total coral cover of more than 90% by the end of the century.”

Restoration efforts are dwarfed by the size of this problem, and are failing to deliver protection to reefs, says Mulà.

A biology refresher: coral bleaching is the breaking up of a symbiotic relationship between coral and algae.

Dr Kerry Cameron of Reef Recruits told Cosmos: “Coral polyps host dinoflagellate algae, ‘Symbiodinium’, that provide enough photosynthetic energy, like a solar panel for polyps”, to build the familiar limestone of the reef.  “We wouldn’t have a reef without algae”, she adds.

This is a delicate relationship, sensitive to marine heatwaves. Under higher-than-normal temperatures, Symbiodinium produce toxins that irritate the coral polyps, degrading their tissue. Polyps react by ejecting the algae, which leaves them colourless ‘bleached’ and prone to starvation and disease.     

Corals can take the algae back if the temperature goes down again, says Cameron, but “it’s like getting hit by a bus” she says.  “With bleaching there’s degrees of severity and it’s mostly about how long it lasts. Bleached coral isn’t necessarily dead coral, it’s a very unhappy coral. It depends how hot, but typically if they bleach for a month, they’re unlikely to recover. And that’s when you get that mortality.”

Overfishing and pollution make matters worse, further stressing the coral and making mortality more likely

Countries around the globe have been involved in restoration and rehabilitation efforts in the wake of such bleaching events. These can take many forms, including coral gardening — selective breeding and out-planting coral colonies in affected areas, removing seaweed overgrowth or coral-eating fish, introducing other fish species to help ecosystem function or construction of artificial reefs.

But many of these efforts fail, says Mulà. She and her collaborators Strona and Corey Bradshaw from Flinders University examined why such efforts were not protecting reefs.

Analysis was done in collaboration with researchers from the University of Milan-Bicocca, the Marine Research and Higher Education Centre (Maldives) and the University of Helsinki.

Small scale, high costs per hectare and a focus on restoring already-compromised reefs were found to be barriers to successful restoration, says Mulà.

The size of the problem is hard to comprehend, and potential restoration costs are staggering. Successful rehabilitation of just 10% of the 11,700 km² of global coral reef degraded between 2009 and 2018, would cost more than $US1 billion, nearly four times the investment in coral restoration over the last decade, Mulà says.

And that is at the lower boundary, she adds; at the upper end of the scale, costs could get into the trillions.  Estimates vary between $US6000 and $US261 million per hectare, she says, depending on method, with coral gardening, the more ‘economical’ end, at $US3.3 billion in total. 

“Most restoration projects only operate over several hundred or a few thousand square metres. Compared to the 14% loss and degradation of coral reefs between 2009 and 2018, equating to nearly 12,000 square kilometres, we come nowhere close to the scale of restoration that is needed to offset the losses from climate change,” says coauthor, Bradshaw

“This combination of adverse factors, coupled with the fact conservation efforts can target sites that don’t have the highest change of success, means over a third of projects fail.”

Mulà says, poor planning, use of unproven technologies, insufficient monitoring and subsequent heatwaves are often causes.

Restoration projects suffer from a lack of agreed format for collecting and reporting data, says Strona, making it hard to work out what happened, why they worked (or didn’t) and slows down the pace of improvement.

“We found that no measures of human impacts, pre- and post-restoration bleaching risk, coral diversity, remoteness, or type of restoration [that] could explain why a project was successful or not.”

“Although coral restoration has the potential to be a valuable tool in certain circumstances, our research makes it clear it is not yet and might never be feasible to scale up sufficiently to have meaningful, long-term, and positive effects on coral reef ecosystems,” says Bradshaw.

Professor David Bellwood of James Cook University expressed a similar view in a 2024 Nature Climate Change paper, saying large-scale coral restoration was “costly, premature, and doomed to fail” unless the root cause of climate change was addressed by lowering carbon emissions.

In 2019 the Turnbull Government granted $535.8m to the Great Barrier Reef Foundation (GBRF) for restoration work. It plans to deploy 10m heat tolerant corals annually across 100 “priority reefs.” It’s been estimated there are 2900 individual reefs off the Queensland coast.

The Albanese Government says it is spending $1.2bn before 2030: “This funding will accelerate actions to improve water quality, strengthen partnerships and stewardship, reduce fishing-related risks to protected and threatened species and strengthen management of the Reef. “It will support projects to help the Reef adapt to a changing climate”

The “”Protecting the Great Barrier Reef” report” says more than 1.2m coral eating starfish have been culled across 437 reefs since 2012; and the Australian Institute of Marine Science has begun to design “coral factories” – modular containerised, aquaculture systems to propagate 100,000 corals at a time for reef restoration. The program planned to deploy 10,000 devices carrying the baby corals – but the project team says its subsequent aim is to deliver up to three million devices in the next iteration of deployments over a three-to-five-year period, bringing millions of baby corals to the Reef. “But the scale of this deployment was some years off yet,” it says. 

“We need a fundamental rethink,” says Bellwood. “Too much is at stake. At the moment, coral restoration is, at best, psychological relief and cosmetic conservation, and at worst, a dangerous distraction from climate action.”

Bellwood did acknowledge the role of coral gardening in a small-scale context.

“This reality check should stimulate constructive debate about when and where restoration is most feasible and important. But the truth is that without stemming the pace and magnitude of climate change, we have little power to save coral reefs from massive losses over the coming century and beyond,” says Bradshaw

The paper was published in Nature Ecology and Evolution

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