Huge sea-urchin populations are overwhelming Hawaii’s coral reefs

As coral reefs struggle to adapt to warming waters, high levels of pollution and sea-level rise, ballooning sea-urchin populations are threatening to push some reefs in Hawaii past the point of recovery.

The phenomenon is described in a new study that uses on-site field work and airborne imagery to track the health of the reef in Hōnaunau Bay, Hawaii. Overfishing is the main culprit behind the explosion in sea-urchin numbers, said Kelly van Woesik, Ph.D. student in the North Carolina State University Center for Geospatial Analytics and first author of the study.

“Fishing in these areas has greatly reduced the number of fishes that feed on these urchins, and so urchin populations have grown significantly,” van Woesik said. “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.”

Those urchins eat the reef, which is already not growing at a healthy rate, van Woesik said. Water pollution and overheated water created by climate change result in a poor environment for the coral to reproduce and grow, leaving the reef even less able to keep up with the pace of erosion caused by the urchins.

Reef growth is generally measured in terms of net carbonate production, which refers to the amount of calcium carbonate produced in a square meter over a year. Prior research in the 1980s found areas in Hawaii with carbonate production around 15 kilograms per square meter, which would signal a healthy, growing reef, van Woesik said. The reef in Hōnaunau Bay today, however, showed an average net carbonate production of only 0.5 kg per square meter, indicating that the reef is growing very slowly.

By combining data gathered through on-site scuba diving with images taken from the air, van Woesik determined that the reef would need to maintain an average of 26% coral cover to break even with the pace of urchin erosion, and a higher cover in order to grow. The average coral cover across all depths was 28%, she said, but areas in shallow depths with more erosion would still need nearly 40% cover to break even.

For the islands they surround, coral reefs like those in Hōnaunau Bay provide important coastal protection against erosion from waves, absorbing up to 97% of incoming wave energy. They are also often vital to the economies of those areas, which rely on the reefs and the fishes that live there. Van Woesik said the study highlights the need for more robust fisheries management in the area to bolster the populations of carnivorous fishes that eat the urchins.

“The reefs cannot keep up with erosion without the help of those natural predators, and these reefs are essential to protecting the islands they surround,” she said. “Without action taken now, we risk allowing these reefs to erode past the point of no return.”

The study, “Scaling-up coral reef carbonate production: sea-urchin bioerosion suppresses reef growth in Hawaiʻi,” is published in PLOS One. Co-authors include Jiwei Li and Gregory P. Asner of Arizona State University.

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Note to editors: The abstract of the paper follows.

“Scaling-up coral reef carbonate production: sea-urchin bioerosion suppresses reef growth in Hawaiʻi”

Authors: Kelly van Woesik, North Carolina State University; Jiwei Li and Gregory P. Asner, Arizona State University.

Published: May 28, 2025 in PLOS One

DOI: 10.1371/journal.pone.0324197

Abstract: Coral reefs provide essential social, economic, and ecological services for millions of people worldwide. Yet, climate change and local anthropogenic stressors are damaging reefs globally, compromising their framework-building capacity and associated functionality. A reef carbonate budget provides a quantitative measure of growth and functional status, but utilization of remote sensing to scale-up such a metric remains limited. This study used census-based field surveys across depths in Hōnaunau Bay, Hawaiʻi to examine rates of carbonate production, and scaled up estimates across the bay with high-resolution benthic-cover data derived from airborne imaging spectroscopy. Average net carbonate production was ~0.5 kg CaCO3 m-2y-1across the 2–17 m depth gradient, ranging from -2.1 to 2.4 kg CaCO3 m-2y-1 at 3 and 6 m, respectively. The scaling model with the lowest root mean square error was achieved using a 2-m resolution map of live coral cover. Sea-urchin densities averaged 51 individuals m-2, which were among the highest recorded densities on coral reefs globally. The subsequent high bioerosion from sea urchins suppressed estimated reef-growth potential, particularly in the shallow reef <6 m. Field estimates of net carbonate production translate to vertical reef accretion of ~0.5 mm y-1 across depths, indicating the reef in its present form is not keeping pace with the current rate of sea- level rise (3.55 mm y-1) in west Hawaiʻi. These results suggest a need for improved fisheries management in Hōnaunau Bay to enhance carnivorous-fish abundances, thereby helping to reduce sea-urchin densities and improve reef-growth capacity. Critically, an estimated threshold of ~26% live coral cover is currently needed to maintain positive net production across depths. This study demonstrates the utility of monitoring carbonate production by integrating field measurements and airborne imaging spectroscopy, and highlights the need for management decisions in west Hawaiʻi that enhance resilient carbonate budgets of coral reefs.