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

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," said van Woesik, who worked on the project while at Arizona State University. "We are seeing areas where you have about 51 urchins per square meter, which is among the highest population density for 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. The corresponding author of the study is Gregory P. Asner of Arizona State University. Co-authors also include Jiwei Li of Arizona State University. The study was made possible through funding from the Arizona State University Center for Global Discovery and Conservation Science. The ASU Global Airborne Observatory supplied the imaging spectroscopy data pivotal to the study.