Winning the war on Great Barrier Reef crown-of-thorns starfish

By Amy Edwards

21 July 2020

3 minute read

Close up of a crown of thorns starfish on great barrier reef coral

Crown of thorns starfish on coral at Rib Reef near Townsville. Credit: David Westcott

It’s taken a well-coordinated army, but researchers and reef managers are finally toppling the crown.

The crown-of-thorns starfish that is.

These hungry critters have been a long-time predator of coral. Their tasty battle ground is the beautiful Great Barrier Reef.

They feed by extruding their stomach out of their bodies and onto the coral reef. Then they use enzymes to digest the coral polyps.

Crown-of-thorns starfish (COTS) are a native coral predator. But when populations reach outbreak status (about 15 starfish per hectare), they eat hard corals faster than they can grow. During an outbreak, crown-of-thorns starfish can eat 90 per cent of live coral tissue on a reef. This puts added pressure on the reef on top of threats like bleaching and climate change.

Scientists don’t know what causes outbreaks of COTS. But they think ocean ‘stressors’ play a part. This includes spikes in ocean nutrients caused by coastal and agricultural run-off into the ocean as well as a loss of predators due to overfishing.

Now for the good news. An adaptive approach to managing the coral-eating starfish has reduced their numbers at key reefs. So much so they can no longer consume coral faster than it can grow back.

You and what army?

a scubadiver collecting a starfish with tong-like instrument underwater

A diver collects a starfish for research. Credit: David Westcott

The Great Barrier Reef is currently in the midst of its fourth major COTS outbreak since the 1960s.

This current outbreak has been underway since 2010. Researchers, working with the Great Barrier Reef Marine Park Authority, have been testing their new management strategy on more than 160 priority reefs across the Great Barrier Reef. Professional COTS divers (including Indigenous and youth trainees) and control vessel operators have culled more than 700,000 starfish.

The new process means the team can adapt quickly based on up-to-date information. It also considers research on the starfish’s biology and reef ecology.

The secret weapon against crown-of-thorns starfish on the Great Barrier Reef

A top down view of a crown of thorns starfish surrounded by the white dead coral it has eaten.

Crown of thorns starfish surrounded by the white dead coral it has eaten. Credit: David Westcott.

And what does this army use to slay the crown? Vinegar. Yep, the same trusty condiment found in your kitchen cupboard.

Divers inject the starfish with either vinegar or bile salt solution and leave them in place on the reef. These techniques kill quickly and effectively with no negative impacts on the marine environment. Within 24 hours there’s basically nothing left of the starfish, who go into an autoimmune self-destructive process.

Victory for the taking

The Expanded Crown-of-Thorns Starfish Control Program’s success marks a huge milestone for scientists, managers and on-water control teams. They have shown their approach to culling is effective at reef and regional scales.

As a result, the Federal Government has awarded contracts worth $28.6 million to help win the war against COTS.

The government has committed five, fully-crewed boats over the next two years. This will address what remains one of the most significant threats to the reef.

Researchers and managers from a range of institutions including us have been instrumental in addressing the starfish problem. It’s thanks to a coordinated control program and strategy funded by the Australian Government’s National Environmental Science Program Tropical Water Quality Hub (NESP TWQ).

More information on the crown of thorns starfish great barrier reef research is in the recently published technical report.