Shark fishing is making coral reefs collapse in an unexpected way

Coral reefs – vivid, bustling cities beneath the sea – are in trouble. Warming oceans, pollution, and human interference have put them on the brink. But a quieter, less obvious threat is making things worse. The loss of sharks, the ocean’s apex predators, may be triggering outbreaks of coral-eating crown-of-thorns starfish.

These outbreaks, in turn, accelerate the destruction of already struggling reef systems.

Scientists from The University of Western Australia’s Oceans Institute and the Australian Institute of Marine Science (AIMS) took a deep dive into this mystery. They found a surprising link between shark fishing, shifting fish behavior, and surging starfish populations.

The study sheds new light on the fragile balance of reef ecosystems – and the consequences of disrupting it.

Rise of a coral killer

Crown-of-thorns starfish aren’t new to coral reefs. In small numbers, they help maintain balance, preventing a single coral species from taking over.

But when their numbers explode, these starfish become a menace, devouring coral at an alarming rate and leaving behind skeletal, lifeless reefs.

“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.”

For decades, researchers have searched for reasons behind these population booms. Climate change, nutrient pollution, and natural cycles were suspected culprits. Now, there’s another piece to the puzzle – one that begins with sharks.

Sharks keep the food chain in check

Sharks rule the reef, keeping everything beneath them in line. Their presence forces smaller predatory fish like snappers and emperors to stick close to the seabed, where they hunt crabs, worms, and, crucially, young crown-of-thorns starfish.

When sharks disappear, those smaller fish move up into the open water, chasing different prey.

“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.”

Take sharks out of the equation, and the entire system shifts. The once-grounded hunters now roam freely, snatching squid and smaller fish instead. The invertebrates they used to eat are left unchecked.

Diet shifts lead to starfish explosions

The change happens quickly. Sharks vanish, and smaller predators find themselves free to venture beyond the reef floor.

Their diet changes and they grow faster. Meanwhile, crown-of-thorns starfish, no longer a staple in their meals, thrive. More larvae survive. More starfish mature. Soon, they blanket the reef, stripping it of coral.

“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,” said Dr. Lester.

“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.”

It’s a chain reaction – one change leads to another. A shift in diet creates a gap in predation, and before long, an outbreak emerges.

More shark fishing, weaker reef ecosystems

Dr. Diego Barneche from AIMS pointed out a telling pattern: areas where fishing pressure is low, like marine reserves, tend to have fewer outbreaks of crown-of-thorns starfish.

Where fishing is rampant, starfish numbers surge. The connection is clear – overfishing, particularly of sharks, makes reefs more vulnerable.

This isn’t just about one species of starfish. The removal of sharks disrupts the entire ecosystem, leading to cascading effects that are difficult to reverse. Coral reefs, already fighting climate change and pollution, now have to battle an overpopulation of coral predators as well.

Protecting sharks to save reefs

“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.

One practical step? Reduce shark fishing. By keeping their populations stable, reefs may have a better chance of regulating starfish numbers naturally.

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

Coral reefs are among the most threatened ecosystems on Earth. Climate change may be the biggest villain in their story, but human-driven disruptions – like the removal of sharks – are making things worse. The ocean works in delicate, intricate ways.

When one piece is removed, the effects ripple outward. In this case, fewer sharks mean more starfish. More starfish mean fewer corals. And fewer corals mean the loss of entire marine communities.

The solution might be as simple as letting sharks do their job.

The study is published in the journal Communications Biology. 

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