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This story was originally published by Grist and appears here as part of the Climate Desk collaboration.

Out among a scattering of islands spilled like beads into the Indonesian shallows, an extended experiment in coral restoration has revealed something marvellous: With a tender touch and a community to care for it, a reef can fully recover from the devastation of blast fishing in just four years.

The Spermonde Archipelago, which lies a dozen miles off the coast of South Sulawesi, Indonesia, was long home to some of the most dynamic reefs in the world, where schools of fish rainbowed over coral blanketing the seafloor. But dynamite fishing turned swaths of those wonders into wastes. That was until 2018, when academics, government agencies, non-profits and local communities came together to restore them with a novel approach developed over years of testing and refinement. Now, a team of marine biologists and reef ecologists has released the first results in a suite of studies investigating the program’s achievements. The study, published in Current Biology, shows that the method can help reefs rebuild in just a few years.

“We do always refer to corals, in particular in reefs, as these slow-growing ecosystems that take a long time to recover, which they are,” said Rebecca Albright, a coral biologist at the California Academy of Sciences who was not involved in the study. “So showing that they can regain rapid growth within four years is very encouraging.”

Promoting this recovery in Sulawesi is particularly important because the island sits at the centre of the Indonesian archipelago and in one corner of the Coral Triangle. This region, and Indonesia in particular, is home to the largest concentration of reefs and coral habitat in the world. Yet many of these vibrant ecosystems were pulverized by decades of fishers dropping explosives into the water to concuss fish they could then scoop out of the sea. With loose rubble then left to tumble in the currents, corals had little hope of recovering on their own. Any coral spawns that might settle and grow were liable to be crushed by errant rocks.

To overcome this, the Mars Coral Reef Restoration Program — a non-profit funded by the Mars corporation known for M&Ms, Twix and Snickers — brought together restoration experts who developed what they call the reef star: a six-legged steel spider coated in sand, to which coral fragments harvested from nearby healthy reefs or found rolling with the tides are strapped. Restoration workers, often members of local communities, deploy them across dozens of sites. These webs provide the protection and stability the transplants need to grow, while also settling the debris created by blast fishing. Without such help, researchers believe that corals — those strange yet essential sea creatures — might never have returned to the damaged areas.

Within a year of placing the reef stars, the fragments grew into colonies. By year two, the branches of neighbouring colonies knit into a marine embrace. By 2023, the former fragments had grown into orange bushels, broad yellow pads and twisting pink tentacles that trains of fluorescent fish explore.

A diver installs a reef star in a degraded coral reef to stabilize loose rubble and kickstart rapid coral growth. Photo by The Ocean Agency

Scientific analysis confirmed what the eye could see. By measuring something called a carbonate budget — a way of understanding how well a colony can grow its limestone skeleton in the face of erosive forces like fish, divers and passing vessels — researchers found that the rate of growth for sites established just four years before matched that of healthy, undamaged coral growing nearby.

‘Reef stars’ restored #Indonesia’s blast-damaged corals in just four years. #CoralRestoration #CoralReefs #DynamiteFishing #MarsCoralReefRestorationProgram

Studying this growth helps scientists to understand how well a reef fulfils its role as the star of a healthy ecosystem providing habitat for marine life. “The 3D structure of the reef is basically the city where these animals live,” said Ines Lange, a coral reef ecologist and lead author of the paper. “So providing an actively growing three-dimensional structure is the basis for this whole ecosystem.”

The rate and state of growth also reveals whether the reef can be expected to once again protect coastlines from storm surges and coastal erosion — and grow quickly enough to keep up with rising seas to continue doing so. The results show that won’t be a problem around South Sulawesi. Other restoration efforts, like those in the Florida Keys, tend to string up a few strands of coral fragments or pepper the seafloor with them in a way that felt, for Lange, “like a little tiny garden.” But at the Mars program sites, “It’s like they put a forest there.”

“I think it was the first time I saw a restoration site that was a proper reef,” she said.

These sea groves are populated primarily by branching, arborescent coral sprouting from the reef star arrays in the coastal shallows. They’ve created a terrain flourishing with life that turns the aquamarine waters into a Technicolor dreamscape. Overall, the method has proven itself even to those watching it unfold from afar.

“The Mars project has set the bar really high for how you can do evidence-based reef restoration,” said Lisa Boström-Einarsson, a coral reef ecologist with the University of Exeter.

Though not affiliated with the study, Boström-Einarsson has collaborated with two of its authors on a previous paper. Unsurprisingly, the world of coral reef conservation remains small, despite the great need for its work.

Four years ago, Boström-Einarsson compiled a systematic and comprehensive review of reef restoration projects, which she is in the process of updating based on the progress made in such efforts globally in the intervening years. That background led her to conclude, after reading Lange’s paper, that “it’s a gold-standard study on a gold-standard project.”

A healthy coral reef in Sulawesi, Indonesia. Photo by Ines Lange

Still, Mars’ reef stars are suited best to sites like South Sulawesi, where the trauma is physical. When reefs have been broken by widespread blast fishing or gored by ship groundings — of which there are hundreds every year — the study shows the devices can help heal those injuries. But in areas like the Great Barrier Reef, which has been marred by recurrent bleaching events that offer little of the reprieve reefs need to recover, they can only do so much; the repeated heat waves spurred by elevated temperatures make the water itself hostile to coral. Nonetheless, the Mars program launched an effort late last year to adapt its approach for Australia’s iconic reef. The kinds of coral most sensitive to warming are also those best fit for the Mars method.

In the waters of South Sulawesi, the restoration team favoured branching corals, both because they make up the bulk of the healthy reefs in the region and because they grow quickly — Boström-Einarsson called them “weedy coral.” But the treelike Acropora can’t stand the heat the way their massive, slow-growing cousins the brain coral can; Acropora are among the first to bleach when temperatures climb. So, while the marine meadows at the restoration sites have prospered in recent years, more remains to be done to make them resilient to warming seas.

“You can put a bunch of coral back out into place, but that doesn’t mean you’re building a resilient reef,” Albright said. “You have to have diversity.”

Lange said the Mars program is bolstering the ecosystems’ resilience, transplanting massive corals and providing the surfaces they need to establish, settle, and mature. This is just one area that reflects the responsive approach Boström-Einarsson said the Mars program has brought to its efforts by listening to scientists, considering their evidence, and tapping their expertise.

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