Hakai Magazine

Coastal science and societies

shipwreck on atoll in Saint Brandon
In 2012, this ship ran aground on a coral atoll located far off the coast of Madagascar, killing and blackening the reef. Photo by Mascarene Coral Island Expedition

A Shipwreck’s Incalculable Toll on a Remote Reef

After a fishing boat ran aground on a fragile atoll, the consequences for the ecosystem were alarming—and curious.

Authored by

by Doug Johnson

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In 2012, a tuna longliner ran aground on a coral atoll on the southeastern edge of Saint Brandon, a remote archipelago in the Indian Ocean. Two years later, a team of researchers measuring the area’s pollutants stumbled upon the wreck. Currents and storms had torn the ship into roughly three pieces: stern, aft, and wheelhouse. As the researchers probed the wreck, they quickly realized something was off: toxic metals had leached from the shallow shipwreck, and the effects were alarming.

Upon impact, the boat crushed and killed the corals beneath it. When the corals died, they changed color—not to the characteristic bone-white of bleaching, but to a somber shade of black.

“[It] was the first time any of us had seen coral in that kind of state,” says Veronica van der Schyff, a doctoral candidate in ecotoxicology at North-West University in South Africa and lead author of a study exploring the wide-ranging effects the wreck had on the reef ecosystem.

Metals from the ship likely leached into the water and caused the blackening, she says. Olivier Tyack, a contractor who has surveyed Saint Brandon shipwrecks for a maritime insurance company, and who holds a master’s degree in marine environment protection, offers a different explanation: he suggests the color change could have resulted from cyanobacteria moving into the coral skeletons after they died, preventing new coral growth. Van der Schyff says this is a possibility. Although this isn’t the first time scientists have documented corals turning black after a shipwreck, the exact mechanism behind it is still poorly understood.

Regardless of whether metals in the ship caused the blackening, some of the reef’s corals and algae were contaminated with heavy metals like lead, mercury, chromium, and vanadium. That’s a problem, van der Schyff says, because some of these metals can accumulate up the food chain.

This potential for damage was magnified beyond the initial collision because the hazardous wreck attracted many species. And down current from the ship’s wheelhouse, large numbers of sea cucumbers collected around a 40-hectare algal bloom that was likely fed by rusting iron and rotting fish from the ship’s cargo. Although the bloom site had low biodiversity compared to elsewhere in the reef, a few species of fish fed on the sea cucumbers and strands of algae, as did some critically endangered hawksbill turtles.

The whole process, from impact to algal bloom, took a few months, says Julian Merven, the captain who took van der Schyff and her colleagues to the site. Merven travels to the area frequently, and says that now, eight years later, the corals appear to be back to normal. According to van der Schyff, this is likely because of new coral growth.

From oil to sewage, ships emit many hazardous and toxic materials when they run aground. Lost nets, too, can entangle wildlife and divers. Older ships may be particularly dangerous, as some are coated in lead paint or other chemicals that have since been banned, like tributyltin, which was previously used in paint to prevent barnacles.

For a wreck to be more of a boon than a threat, hazardous materials must be removed, says Doug Helton, regional operations supervisor for the emergency response division of the US National Oceanic and Atmospheric Administration. The vessel also needs to be sunk far from reefs, and deep enough that tides and storms won’t cause it to thrash around, Helton says.

Wrecks, however, can also present an opportunity. In some cases, ships are scuttled on purpose to create diving terrain or a hard surface on which corals can encrust, creating an artificial reef.

Considering its position in the shallow water and the toxicity of its contents, the ship that ran aground in Saint Brandon eight years ago isn’t likely to serve as an artificial reef. There is also little chance that it will get removed, as doing so could potentially damage the reef even more, says Helton. The same is likely true for countless other wrecks that linger in the ocean, their impact on marine life gone largely unnoticed.