Sea urchins are destroying another coral reef – overfishing blamed

Another coral reef is in danger of being destroyed by sea urchins – this time, a population boom is threatening to erode coral reefs in Hawaii beyond the point of recovery.

In Hōnaunau Bay, researchers have combined underwater fieldwork with airborne imagery to reveal a reef in crisis, driven by the overfishing of carnivorous fish that typically prey on sea urchins––throwing the ecosystem out of balance and allowing urchin populations to surge.

“Fishing in these areas has greatly reduced the number of fishes that feed on these urchins, and so urchin populations have grown significantly,” says Kelly van Woesik, a Ph.D. student at North Carolina State University and lead author of the study. “We are seeing areas where you have about 51 urchins per square meter, which is among the highest population density for sea urchins anywhere in the world.”

Sea urchins, round and spiny animals found in oceans and on reefs around the globe, play an important role in coral reef ecosystems by grazing algae that would otherwise crowd out corals. But when their populations grow unchecked, they can strip reefs bare—killing coral and accelerating erosion.

Hōnaunau Bay’s reef is already vulnerable. Rising sea temperatures and pollution reduce the coral’s ability to reproduce and recover. Without enough predatory fish to control urchin numbers, the reef is now eroding faster than it can rebuild.

One key indicator of reef health is net carbonate production—a measure of how much calcium carbonate reef organisms deposit each year. In the 1980s, healthy reefs in Hawaii averaged around 15 kilograms per square meter annually. Today, Hōnaunau’s reef is producing just 0.5 kg per square meter.

An analysis published in PLOS One, shows the reef would need to maintain a minimum of 26% coral cover just to keep up with urchin erosion. While average coral cover across all depths is about 28%, shallow areas with more intense erosion would require nearly 40% to remain stable.

“Without action taken now, we risk allowing these reefs to erode past the point of no return,” says Woesik.

The implications are serious. Coral reefs protect shorelines by absorbing wave energy – up to 97% – and support local economies through fisheries and tourism. Their collapse would ripple across communities and ecosystems.

The researchers call for improved fisheries management in Hōnaunau Bay to boost populations of carnivorous fish, reduce urchin densities, and improve the reef’s ability to grow and recover.

Eat sea urchins instead?

Along Australia’s east coast, sea urchin populations are also surging—with more than 20 million long-spined urchins devastating kelp forests and coral reefs in Tasmania.

In this case, warming waters have driven sea urchins southward, while overgrazing has turned once-thriving reefs into so-called “barrens” – underwater deserts stripped of seaweed, biodiversity, and carbon-storing potential.

But scientists in Tasmania are testing a novel approach: eat the problem.

A recent study published in Nature Sustainability and featured in Cosmos highlights how establishing a targeted fishery to commercially harvest long-spined sea urchins can be a sustainable way to protect marine ecosystems.

“In Tasmania, we’re leading the way, with sectors working together towards an aligned goal,” says lead author, Dr Katie Cresswell from the University of Tasmania’s Institute for Marine and Antarctic Studies. “We have a unique and quite remarkable scenario here where commercial divers, researchers and government all work together to try to control the densities of this destructive urchin – and where government-supported incentives have driven the establishment of a commercial fishery targeting the species.”