Crown-of-Thorns starfish (Acanthaster planci) are coral-hungry starfish that are able to eat up to six square meters of living coral a year. When you compare this to a Bumphead Parrotfish (Bolbometapon muricatum) that is capable of eating up to 5 tonnes of coral a year, this seems like only a small inconvenience to the reef.

However, since 2012, the Great Barrier Reef has been subject to a population explosion of Crown-of-Thorns (COTS), with densities of up to 25,000 individuals on one single reef! Now, when you consider that each of those individuals are able to eat six square meters of living coral, suddenly the problem is a whole lot bigger! This is the third such documented outbreak in the last forty years.

So, what are we doing at Lizard to help the reef here?

The Station has had a targeted COTS reduction program in place since the beginning of the second outbreak in 1993. It concentrates on removing individuals from specific reefs around Lizard. Since July 2012, over 4000 individuals have been removed from only a few small reefs near the Research Station.

How do we remove the COTS?

Until recently, the standard strategy was to inject the animal with sodium bisulphate: approximately 12 ml is injected into the body of the animal in each of about eight different places within the disc. This kills the animal within around 24 hours.

Recent research into COTS control mechanisms has developed a new solution made out of ox bile. It is more effective than previous methods because it requires only one injection into the body of the animal.

This was tested at LIRS in 2013 and found to be effective at killing COTS and harmless to other reef life. This method is now being used at LIRS and elsewhere, along with the older method which is still effective at low COTS densities. However, both methods require high numbers of diver hours. Luckily, researchers are currently working on new solutions to contain populations of COTS.

Mass spawning of crown-of-thorns starfish
Mass spawning of crown-of-thorns starfish at Lizard Island Image: Lyle Vail and Anne Hoggett
© Australian Museum

What causes population explosions of COTS?

COTS are highly fecund animals – a single female spawns tens of millions of eggs into the sea, probably during multiple spawnings in summer. With numbers like that, anything that affects the success of fertilisation and survivorship of the resulting larvae has a big impact on adult populations down the track.

COTS larvae eat microscopic plant plankton while drifting in the sea before they settle to the bottom and change into tiny starfish. Research suggests that increases in COTS populations might be correlated with an increase in nutrient run-off from land adjacent to the GBR.

High-intensity fertiliser-rich farming on the Queensland coast has caused an increase in the nutrients flowing through rivers and out into the ocean. This increase in nutrients is thought to enhance the survival rate of COTS larvae which in turn results in more adults. Good for COTS but bad news for healthy coral populations.

What can we expect in the future?

Healthy, pristine reefs are able to recover well from COTS outbreaks. However, with human-induced climate change affecting the Great Barrier Reef, coral reefs are becoming more and more stressed andrecovery from events like a COTS outbreak is increasingly difficult.

Therefore, we all need to do our bit to help with climate change: walk or cycle your bike to work; reduce, reuse, recycle; and support the excellent research taking place in coral reef ecology so we can understand how best to save our GBR.

Genevieve Phillips
PhD student intern
University of Queensland

Further information