Shark Loss Is Causing Coral Reef Destruction — Here’s How

How overfishing sharks sets off a chain reaction that leaves coral reefs vulnerable

Ecosystems could be considered superorganisms, as all its components are necessary for it to survive in its most healthy stage. Today’s story exemplifies this so beautifully. 

The biggest challenge I faced as a conservation biologist, especially during my appointments at conservation agencies, was making people understand the hidden ecological relationships between all species in an ecosystem. How could wolves affect grasses? How could sharks affect corals?

If you’ve ever spent time on a coral reef, you know it’s a world in motion. Fish dart in and out of coral crevices, sharks glide through the water, and the reef itself buzzes with life. But beneath that beautiful seascape, coral reefs are in trouble, and one of the culprits is something most wouldn’t expect: the crown-of-thorns starfish (CoTS). 

What most don’t know is that these spiky, coral-devouring predators can destroy entire reefs when their populations explode. For years, scientists have debated what drives these outbreaks, and new research suggests the answer might have more to do with sharks than anyone expected.

I know; I’m so excited to talk about sharks!

The Chain Reaction of Shark Loss

Coral reef ecosystems are complex, shaped by countless interactions between species. So much so that the whole system shifts when one piece of the puzzle is removed. 

That’s exactly what researchers from the Australian Institute of Marine Science and The University of Western Australia found when they looked at the relationship between shark fishing and CoTS outbreaks.

They discovered something that most wouldn’t expect — that removing sharks from coral reefs disrupts the behavior of smaller predatory fish, which in turn allows crown-of-thorns starfish populations to grow unchecked. I know, it sounds complicated, but it’s actually quite straightforward. 

As Dr. Mark Meekan, one of the study’s authors, put it, “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.” 

Their findings suggest that the presence of sharks might determine when this natural control occurs.

How They Figured It Out

The researchers used a very sophisticated analysis. They combined decades of coral reef studies, long-term monitoring, and modern DNA analysis to track the diets of smaller predatory fish. 

They also used behavioral studies and underwater camera surveys to see how fish responded to the presence (or absence) of sharks. The key question was whether sharks indirectly control CoTS populations by influencing what smaller predators eat — and the answer turned out to be yes.

Why Shark Loss Changes Everything

This is where things get interesting. What do sharks have to do with this? 

When sharks are present, smaller predatory fish — like snappers and emperors — stay close to the reef to avoid their predators. There, they hunt benthic prey, including crabs, worms, and young crown-of-thorns starfish. 

However, when sharks are removed, these smaller fish feel safe venturing into open water, where they switch their diets to prey swimming freely in the more open water like squid and small fish. That shift means fewer little CoTS get eaten, allowing their populations to grow rapidly and devastate coral reefs.

As Dr. Emily Lester, another study author, explains, “When sharks are present, smaller species are forced to feed close to the safety of coral on the seabed. But when sharks are removed by fishing, they are able to safely venture up into the water column and mostly feed on pelagic prey.” 

In short, shark removal releases CoTS from natural predation, making outbreaks far more likely. Quite a spin of events, isn’t it?

What This Means for Coral Reefs

For decades, coral reef conservation efforts that keep CoTS in check have focused heavily on bottom-up factors, such as nutrient pollution from land runoff that fuels CoTS larval survival while other species die off. But this study represents a perfect example of the importance of top-down control: the role predators play in keeping ecosystems balanced.

One of the most striking findings was that CoTS outbreaks are far less common in marine reserves, where shark populations remain intact. “Studies across the Indo-Pacific have found correlations between fishing and crown-of-thorns starfish,” Dr. Diego Barneche explains. “In places where fishing pressure is absent, such as marine reserves, the crown-of-thorns starfish were less abundant.”

Fun fact: Dr. Barneche and I started our PhDs the same day, in the same university, and I was always fascinated by his brilliant research ideas. 

But back to the study, this suggests a relatively simple and cost-effective way to reduce CoTS outbreaks: protect reef predators, protect the sharks. The good thing? Well, compared to tackling massive issues like climate change or global ocean acidification, enforcing fishing regulations in key reef areas is achievable in the short term.

A Broader Conservation Lesson

As usual, this study isn’t just about coral reefs. As I mentioned earlier in this story, this study reminds us that ecosystems are deeply interconnected. Whether you’re looking at reefs, forests, or grasslands, predators shape the world around them in ways that aren’t always obvious. 

Remove them, however, and the effects ripple outward in often unexpected ways.

For those of us who have spent our careers working in conservation, this is a familiar pattern. The loss of wolves led to deer overpopulation and degraded forests. The decline of large herbivores in Africa changed entire landscapes. And now, we see that removing sharks — a species most people don’t associate with coral health — could be tipping the balance in favor of a coral-killing predator.

Quite a twist.

If there’s a takeaway from this research, it’s that conservation isn’t just about saving individual charismatic species; it’s about protecting the relationships that make ecosystems work. If you fail to protect a single element, the system will collapse.

Sharks, snappers, and crown-of-thorns starfish aren’t just coexisting on reefs; they’re shaping each other’s populations in ways we’re only beginning to understand. Just by keeping those relationships intact, we might just give coral reefs an extra fighting chance.

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Best,

Sílvia P-M, PhD Climate Ages

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