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There was a time – not that long ago – when the iconic colours and ecological majesty of the Great Barrier Reef weren't readily available at the push of a button.

Before the 5G convenience of smartphones and social media, the only way to really experience the Reef was to visit the Reef.

And it was one of these visits, as part of small cohort of fresh-faced university students in 1993, that chartered the course of Dr David Williamson's life.

"I grew up Western Australia and, in that of time of pre-internet, the Reef was still sort of a mythical place where you only saw the odd photo and media snippet," he says.

"Although it's a very different marine system on the West Coast, we do have beautiful reefs, and I was involved in diving and fishing from a very young age.

"I remember thinking, 'I've just got to get over to the Great Barrier Reef at some point to check it out'."


Fast forward three decades, a marine science degree and doctorate later, and David is still here, still enraptured by the Reef, and still devoting his professional life to ensuring it remains one of the world's iconic natural wonders.

His role as the Reef Authority's Assistant Director of the Crown of Thorns Starfish (COTS) Control Program pits David and his colleagues against one of the most divisive albeit misunderstood threats to the Great Barrier Reef. 

Contrary to popular belief, Pacific Crown-of-thorns starfish (Acanthaster cf. solaris) are native to the Great Barrier Reef. They feed exclusively on live coral and have the ability to consume an area equivalent to half of their body size – about the size of a dinner plate – every day.

And herein, David says, lies the problem.

"We definitely don't hate crown-of-thorns starfish," he says resolutely.

"They're an amazing species, and they've evolved on reefs with the coral and with the fish and with everything else. They're a natural part of the Great Barrier Reef ecosystem, but they do cause substantial loss of live coral when they reach outbreak numbers."

During outbreaks, COTs can multiply in plague proportions and compound upon the damage caused by tropical cyclones, coral bleaching events, coral disease, and flooding events, which can lead to widespread reef degradation if outbreaks are left unchecked.

It has been estimated that COTs are responsible for approximately 40 per cent of total coral loss across the Marine Park.
"You can have thousands of COTs on an individual reef, and we've got over 3000 reefs out there, so when we're talking outbreaks, we're literally talking in the millions!" he says.

What's more, the exact cause of COTS outbreaks is not known, but David says it is generally acknowledged that a combination of factors can create a perfect storm for outbreak initiation and spread. 

"One of the main mechanisms (that contributes to COTs outbreaks) may be that fishing has reduced numbers of predatory fish species that feed on the crown-of-thorns starfish, particularly as juveniles," David says. "We see reduced numbers of COTs and fewer outbreaks on Marine National Park (green zone) reefs than on reefs that are open to fishing."

And while David notes that the Reef Authority and its counterparts have worked tirelessly to protect the Reef's environmental, cultural and heritage values through proactive management, including the introduction of the Great Barrier Reef Marine Park Zoning Plan 2003, directly controlling COTs outbreaks is one of the most effective management actions being deployed at scale to protect coral.

Rising sea temperatures, driven by increased greenhouse gas emissions and a changing climate, may increase opportune spawning conditions for COTs, which generally breed through the warmer summer months, where a single female can release over 100 million eggs.

Huge egg production, coupled with the fact that COTS larvae can clone themselves; juveniles will often delay feeding on coral until conditions are favourable; as well as the rapid growth of adults and their ability to regenerate lost arms, means that targeted control is critical to keep numbers in check.

And perhaps what's even harder to believe, is that the future sustainability of one of the most complex ecosystems on earth relies, in part, on household vinegar. A single 20ml injection is enough to safely cull COTs within 24 hours without harming other reef inhabitants. 

The Reef Authority's Crown of Thorn Starfish Control Program has culled more than a million COTs since its inception in 2012 - protecting over 700,000 hectares of coral reef habitat - and directly employs almost 150 people.

"In the current financial year, we have five COTS Control Program vessels, each with a team of up to eight divers, delivering 220 to 260 days on water per year and deployed to hundreds of target reefs distributed across the Marine Park," David says.

"By protecting those reefs, we're able to secure important sources of coral larvae, which disperse and replenish surrounding reefs."

And, in testament to the Great Barrier Reef's unique biodiversity, the key is simply finding the right balance.

"The main objective of the cost control program is to protect coral," he says.

"It's not to eradicate the crown-of-thorns starfish from the park or to reduce their numbers to zero. We're only reducing COTS numbers to the point where coral growth can outpace the rate at which it is being eaten.  And we're achieving that across hundreds of reefs!" 

Listen to the podcast now.


Dr Dave Williamson Crown-of-Thorns Starfish Director 

Updated 21 Dec 2023
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