The coral reefs in Hōnaunau Bay, Hawaii, are facing an escalating ecological crisis as exploding sea urchin populations threaten to push them beyond the point of recovery, according to a report by NC State University.
A recent study led by Kelly van Woesik, a Ph.D. student at the Center for Geospatial Analytics at North Carolina State University, reveals that overfishing has drastically reduced populations of fish that naturally prey on sea urchins. This has triggered a dramatic rise in urchin numbers—up to 51 individuals per square meter—making it one of the highest sea urchin densities recorded globally.
“These urchins are essentially eating away at the reef,” said van Woesik. “And the coral isn’t growing fast enough to replace what’s being lost.”
The research combined scuba-based fieldwork with aerial imagery to assess the health of the reef. It found that the current average net carbonate production—the key measure of reef growth—is just 0.5 kilograms per square meter annually. For comparison, healthy reefs in Hawaii during the 1980s showed net carbonate production levels of around 15 kilograms per square meter.
This sluggish growth is compounded by other environmental pressures such as nutrient pollution and rising ocean temperatures driven by climate change. These factors impair coral reproduction and growth, weakening the reef’s ability to resist bioerosion caused by the overabundant urchins.
To remain stable, the reef in Hōnaunau Bay would need at least 26% coral cover, with shallower areas requiring up to 40% to counteract erosion. Although the study observed an average coral cover of 28%, this is not sufficient in high-erosion zones, signaling a precarious balance.
Coral reefs are not only biodiversity hotspots—they also play a crucial role in coastal protection. They can absorb up to 97% of incoming wave energy, shielding shorelines from erosion and storm damage. Additionally, they support local fisheries and tourism economies.
“The reefs simply can’t keep up without the presence of natural predators like carnivorous fish,” van Woesik said. “This study underscores the urgent need for improved fisheries management. Without intervention, these reefs may erode to the point of no return.”
The research, titled “Scaling-up coral reef carbonate production: sea-urchin bioerosion suppresses reef growth in Hawaiʻi,” appears in the journal PLOS One. Co-authors include Jiwei Li and Gregory P. Asner of Arizona State University.
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