COTS: how to stop crown-of-thorns starfish destroying reefs

On the thorn of a dilemma as reef saviours allocate scarce resources to take on the crown

This is the final installation in a three-part series. Part 1 and Part 2 appeared in last week’s Cosmos Weekly.

In far north Queensland, coral researcher Dr Morgan Pratchett saw something gathering last month that sent shivers down his spine. Crown-of-thorns starfish. Big ones. In numbers he’d not seen in years. He’d ventured to a remote reef in search of the elusive “source” of this arch-enemy of coral; a killing machine so efficient, it’s been dubbed an “autonomous chainsaw.” Each organism is capable of consuming 10 square metres of coral a year. What Pratchett saw convinced him not only had he found the source, but that the next big outbreak of crown-of-thorns is coming.

“If we can get to it this Christmas, when they’re breeding, we can put out the spot-fire before it’s a raging wildfire,” he tells me, sounding genuinely afraid. “I’m trying to convince people we need to cull in the north right now!”

It’s been a while since we’ve heard much about crown-of-thorns starfish, but when I was a young science reporter, so called “COTS” stories were everywhere. I’ve often wondered what happened. Are they still a serious threat? And what about those COT-killer robots? In this final part of my Cosmos Investigation into the Great Barrier Reef, my search for answers reveals some truly remarkable breakthroughs in our anti-COTS weaponry … and if Pratchett is right, we’ll urgently need them.

“If we can get to it this Christmas, when they’re breeding, we can put out the spot-fire before it’s a raging wildfire.”

Dr Morgan Pratchett

Pratchett, from James Cook University, is the expert on crown-of-thorns starfish. As he explains, despite their monstrous reputation, COTS are native to Australian waters. Outbreaks naturally occur in 15 to 17 year cycles, which start in the northern reef, and advance slowly south as baby COTS travel with currents, leaving a wave of coral devastation behind them.  We are currently on the “fourth wave” which started in the north in 2010, and has gradually made its way down the coast.

So if they are natural – why worry? Well, in the absence of global warming it might not be such a problem, but with relentless temperature rises now posing an annual game of bleaching “Russian roulette”, Pratchett fears a concurrent COTS outbreak could tip reefs into functional collapse.

When COTS send their army of babies into the world, the dominant currents are sometimes reversed, flowing from the north down toward Lizard Island.

“It’s the combination of the two that scares me,” he tells me. “The likelihood of a big bleaching event is increasing. I think we have to worry about every summer. But I’m really afraid for this summer – I think we’re about to have a massive spawning of COTS across the whole reef!”  

Pratchett believes our best hope is to attack the starfish at their ultimate source – an area known to scientists as the “initiation box.” And that was long thought to be Lizard Island – from which ocean currents flow north and south. But Pratchett began suspecting the real initiation box was much further north. To test this theory, his team developed a new hydrodynamic model of reef currents – and discovered that in summer, when COTS send their army of babies into the world, the dominant currents are sometimes reversed, flowing from the north down toward Lizard Island.

Crown-of-thorns starfish underwater
A crown-of-thorns starfish on the Great Barrier Reef, off the coast of Mackay. Credit: © Ryan McMinds (CC BY)

Late in 2021, Pratchett organised an expedition to check out the under-researched reefs between Lizard Island and far north Australia, homing on an area off Cape Grenville. Sure enough, crown-of-thorns were amassing – bigger animals than at Lizard Island – suggesting the outbreak was initiated up there.  A follow up trip October 2022 revealed COTS numbers were rising – and that’s when Pratchett sounded the alarm, begging for a cull team to be sent.

“I believe I’m seeing the beginning of the fifth wave,” he says.

If this news had come prior to 2018, the options for effective action would have been limited. But the last ten years have seen two extraordinary breakthroughs which have transformed the fight against COTs.

Killing them was like trying to kill a zombie. Hack off some of its up-to-20 arms, and each arm could form a new starfish. Try killing it with poison, and a diver would have to deliver 20 to 40 lethal injections, making sure every arm was hit, or the starfish would just break up and… grow more starfish.

“Before, each person could only kill 50 to 100 crown-of-thorns in a single dive. Now it was more than 500.”

Dr Morgan Pratchett

But by chance, in 2012 when Pratchett’s team was conducting unrelated lab experiments, their crown-of-thorns starfish all unexpectedly died. They’d been injected with a bacterial mix which included bile salts – which it turns out, COTS are fatally allergic to. Later, other scientists discovered vinegar had a similar effect. COTS could now be killed with a single shot.

“It was a gamechanger,” says Pratchett. “Before, each person could only kill 50 to 100 crown-of-thorns in a single dive. Now it was more than 500.”

The second gamechanger was… robots. Kidding. The much-hyped COT-bots of my science-reporter youth are a long way from matching a human diver – if they will ever. No – the breakthrough was the use of a supercomputer after 2015. For the first time, scientists could model the complexity of the 3,000 or so reefs that make up the Great Barrier Reef and map how they are connected by currents. It allowed them to identify source “hubs” – reefs that are disproportionately important for crown-of-thorns starfish outbreaks.

“It’s like a network of airports,” explains Dr David Wachenfeld, Chief Scientist for the Great Barrier Reef Marine Park Authority (GBRMPA). “Like Chicago or Denver in America. Some hubs are super-connected. So if they fail, it’s a disaster.”

But it also means those hubs can be targeted, potentially suppressing the crown-of-thorns’ advance.

After the shock mass bleaching of 2016 when 30% of large corals across the Great Barrier Reef died, managers at GBRMPA got serious about controlling COTS.

“I’ve got to wargame the [Great Barrier Reef] park.”

Dr Roger Beeden

Since 2018, they’ve had the resources to target around 100 of these “super spreader” hubs a year, using a fleet of seven boats, each with a team of eight to 12 divers who don’t move on to the next reef until COT densities get below a critical threshold.  

“We would have expected a major decrease in coral in the last five years from crown-of-thorns if we weren’t doing this,” says Dr Roger Beeden, the man tasked with running the COTS campaign for GBRMPA. “Compared with what happened at this stage of the third outbreak in the 1990’s to early 2000’s, we have suppressed the peak and slowed the spread.

Roger beeden smiling at camera
Dr Roger Beeden.

“I think the fact we have had an increase in Acropora coral cover (as reported in part one of this Investigation) supports that. It’s one of the most hopeful things we are doing on the reef. It’s one thing we can do to stop coral dying in the first place.”

So will he be sending one of those boats up to Cape Grenville to put out the COTS spotfire, as so passionately requested by Pratchett?

“Here’s my challenge,” says Beeden, sounding like a war-weary general. “I’ve got to wargame the [Great Barrier Reef] park. We still have active COTS ‘fire-fronts’ going through to the south, including heading for the Whitsundays. Do we abandon some of them and send resources to the remote north? And it’s incredibly difficult logistically up there. We have to carefully balance the potential future fire-fronts versus fighting the fire-fronts we’ve got.”

He explains there are no plans currently to send a ship to Cape Grenville – though that could change in the future. However, their field reconnaissance did pick up the emerging northern outbreak in 2021, and they have since sent tactical control teams to the reefs between Cape Melville and Cairns to suppress it as it moves south.

“It will certainly undo the recovery we’ve had so far on the reef if we get a COTS outbreak and a bleaching this year.”

Dr Morgan Pratchett

“I still think our best hope is to contain that outbreak at the source,” says Pratchett, “stop it before it becomes wildfire. But I can completely understand the tensions. What will happen if we don’t attack the source this year? Well, I worry it gives us an even bigger problem for next year.”

“It will certainly undo the recovery we’ve had so far on the reef,” he adds, “if we get a COTS outbreak and a bleaching this year.”

This whole story is emblematic of everything mind-bending about trying to preserve our glorious, irreplaceable, embattled coral reef. The task is riddled with uncertainty, overwhelming in scale, limited by resources and populated by passionate people trying to do their best, even when they don’t agree, in a situation that has already seen global temperatures pass a threshold where mass bleaching is an annual risk.

If we don’t rapidly reduce emissions, nothing the thousands of people trying to protect the reef can do will save it from fundamentally shifting from the Great Barrier Reef as we know it.

But that won’t stop them trying.

“We have to keep fighting this fight,” Pratchett tells me quietly but determinedly. “That’s what keeps me motivated. In the hope that the world gets activated. And we manage to stop emissions soon enough – well before we get to 2 degrees.”

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