A new study suggests the removal of predatory fish such as sharks from coral reefs may be a key factor driving outbreaks of coral-eating crown-of-thorns starfish.
Dr Mark Meekan and Dr Emily Lester, from The University of Western Australia's Oceans Institute, co-authored the study led by the Australian Institute of Marine Science (AIMS), which was published in Communications Biology.
"Coral reefs around the world are in crisis and outbreaks of crown-of-thorns starfish are seen as one of the key contributors," Dr Meekan said.
"Recurrent outbreaks are adding to the decline of live coral cover across the Indo-Pacific."
Researchers looked at decades of coral reef studies and found a previously unsuspected link between fishing of sharks, the behaviours of smaller fish and outbreaks of crown-of-thorns starfish.
"Through 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," Dr Meekan said.
"Our insights show the presence of sharks might determine when this control occurs."
Findings suggested when the largest predators, notably sharks, in reef systems were removed it fundamentally alters the behaviour of these smaller fish predators – such as snappers and emperors.
"When sharks are present, smaller species are forced to feed close to the safety of coral on the seabed and as a result their diet is focused on things like crabs, worms and starfish," Dr Lester said.
"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.
"Fish that change their diet in this way grow faster – but the invertebrates they used to feed on have a chance to multiply rapidly."
Dr Diego Barneche from AIMS said studies across the Indo-Pacific have found correlations between fishing and crown-of-thorns starfish. In places where fishing pressure is absent, such as marine reserves, the crown-of-thorns starfish were less abundant, he said.
"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," Dr Meekan said.
"Reducing fishing so there are fewer starfish outbreaks may be a relatively cost-effective way to do this."
