Culling predatory starfish conserves coral on the Great Barrier Reef

A culling programme has succeeded in protecting key areas of the Great Barrier Reef from voracious coral-eating starfish. Scientists who analysed the outcome say the effort should be expanded to conserve more of the reef.

Crown-of-thorns starfish (COTS) are relentless feeders on nearly all species of coral within Australia’s Great Barrier Reef. Each starfish can reach 1 metre in diameter and eat 10 square metres of coral reef each year.

The starfish are native to the reef, but it is thought increasing nutrients pouring into the reef’s waters from agriculture and other human factors have increased their numbers and worsened the destruction of corals. Between 1985 and 2012, they accounted for 40 per cent of the region’s coral loss.

During a major reef-wide eruption of the starfish between 2012 and 2022, the Great Barrier Reef Marine Park Authority carried out a large-scale culling programme. Teams of divers inject the starfish with a single shot of either vinegar or ox bile, which kills them and prevents the release of larvae.

Roger Beeden at the Park Authority and his colleagues found that in areas where timely culling was implemented, the outbreaks were limited and coral cover recovered and increased by up to 44 per cent. Where no culling took place, the loss of coral was severe. The study also confirmed that by preventing outbreaks at strategically important reefs, larvae didn’t spread in currents to other reefs, meaning further outbreaks were reduced.

Until now, the programme has focused on 500 of the marine park’s 3000 reefs, which are spread throughout the park but were chosen because they have important value to the tourism industry or are known to be important in the spread of the starfish.

“The outcomes we have found in this study are a result of using integrated pest management to target [the starfish] on the right reefs at the right time – just like plague locust and other pest species management,” says Beeden.

But the researchers recommend that the programme should expand from its current fleet of five to seven ships to between 10 and 15 ships. “At any one time, out of that 500, about a third to a half are in play with current outbreaks,” says Beeden.

Terry Hughes, at James Cook University in Townsville, doesn’t agree that the culling programme is worthwhile. “It’s increasingly clear that attempts to protect corals on the Great Barrier Reef by culling crown-of-thorns starfish on a handful of reefs is just a drop in the ocean,” he says.

Hughes says geographical differences in the number of starfish and amounts of corals – which the study attributed to the level of culling in different parts of the Great Barrier Reef – could be explained by which regions have been affected most by recent cyclones and by mass coral bleaching events. Beeden acknowledges that these factors are hard to separate from the effects of culling, but says: “Our results are strengthened, and not confounded by the fact that the coral cover gains in the Townsville region were achieved despite two mass bleaching events in 2020 and 2022.”

Instead, the priority should be to tackle global warming, which is driving a rise in the frequency and intensity of coral bleaching, says Hughes. “After every bleaching event the Australian government announces more money for killing starfish on a small subset of reefs, shifting the focus from dealing with the causes of these outbreaks, or from reducing Australia’s emissions of greenhouse gases,” he says.

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