A newly established link between the role sharks play upon the coral reef and its ability to contain outbreaks of the coral-killing starfish, the crown-of-thorns, should be used to ‘rapidly improve’ the resilience of coral reef ecosystems around the world by tightening restrictions on shark fishing, scientists have said.

Investigating the effect of shark fishing and depleting numbers of predatory fish within the coral reef ecosystem has had on the stability of the environment, scientists from the Australian Institute of Marine Science (AIMS) have unearthed new findings to suggest the role they play has been vastly underestimated.

It’s no secret that coral reef ecosystems are in a state of emergency around the world right now, suffering the impacts of climate change, heat marine waves, and the effects of overfishing. Chief among the contributors to their current plight are routine outbreaks of coral-munching starfish.

The crown-of-thorns starfish is such a species, which in moderation, has its own role to play in the finely balanced marine environment. When those environments are subjected to sustained and recurrent outbreaks of the starfish, however, it can lead to a rapid decline in live coral cover across swathes of the ocean.

In the Indo-Pacific, for instance, outbreaks of crown-of-thorn starfish have been discovered at the centre of large areas of declining live coral cover.

Searching for an answer to the crisis, researchers started by looking at decades of coral reef studies. And it’s here they found a rather unsuspected pattern; and one that links the fishing of sharks with the behaviours of smaller fish and outbreaks of crown-of-thorns starfish.

The findings have been published this month in the scientific journal, Communications Biology, in which researchers from AIMS as well as marine scientists from the University of Western Australia have pieced together, through a series of DNA studies, the complexity of life on the coral reef.

“Through these recent DNA studies, we knew lots of smaller predatory fish sometimes consumed crown-of-thorns starfish, but we didn’t know how frequently this happened and if it could control outbreaks,” said Dr Mark Meekan, scientists from the University of Western Australia’s Oceans Institute.

“Our insights show the presence of sharks might determine when this control occurs.”

Detailed in their study, titled Predator removals, trophic cascades, and outbreaks of crown-of-thorns starfish on coral reefs, are findings that suggest when the largest predators – notably sharks – in reef systems were removed, the behaviour of these smaller predatory fish (such as snappers and emperors) was altered fundamentally.

“When sharks are present, smaller species are forced to feed close to the safety of coral on the seabed,” said the University of Western Australia’s Dr Emily Lester, co-author on the study. “And as a result, their diet is focused on things like crabs, worms, and starfish.

“But when sharks are removed by fishing, they are able to safely venture up into the water column and mostly feed on pelagic prey, like small fish and squid.”

This presents a bit of a dilemma. According to Dr Lester, fish that change their feeding behaviour and their diet in this way tend to grow faster. But it also leaves the invertebrates they once fed on with the chance to multiply free from predation.

Since the discovery of this link, correlations supporting the hypothesis have been found across a number of AIMS-led studies throughout the Indo-Pacific. Dr Diego Barneche, a researcher from AIMS has since confirmed that in places where fishing pressure is absent – such as marine reserves – the crown-of-thorns starfish have been less abundant.

“We need to rapidly improve the resilience of coral reefs at a time when long-term threats, such as climate change and nutrient inflows from catchments, are causing damage that may become irreversible,” said Dr Meekan.

“Reducing fishing so there are fewer starfish outbreaks may be a relatively cost-effective way to do this.”

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