As coral reefs struggle to adapt to warming waters, high levels of pollution, and sea-level rise, ballooning sea-urchin populations are now threatening to push some reefs in Hawaii past the point of recovery, new research from the North Carolina State University Centre suggests.

The phenomenon has been described in a new study that uses on-site field work and airborne imagery to track the health of the reef in Honaunau bay in Hawaii, where overfishing has been pinpointed as the main culprit behind the explosion in sea-urchin numbers.

“Fishing in these areas has greatly reduced the number of fishes that feed on these urchins, and so urchin populations have grown significantly,” said Kelly van Woesik, Ph.D student in the North Carolina State University Centre for Geospatial Analytics and first author of the study.

“We are seeing areas where you have about 51 urchins per square metre, which is among the highest population density for sea urchins anywhere in the world.”

Those urchins eat the reef, which is already struggling under the weight of increased water pollution and overheated water created by climate change, resulting 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 metre over a year. Prior research in the 1980s found areas in Hawaii with carbonate production around 15 kilogrammes per square metre, which would signal a healthy, growing reef, van Woesik said.

The reef in Honaunau Bay today, however, showed an average net carbonate production of only 0.5kg per square metre, 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 island they surround, coral reefs like those in Honaunau 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,’ said van Woesik. “Without action taken now, we risk allowing these reefs to erode past the point of no return.”

The study – titled, ‘Scaling-up coral reef carbonate production: sea-urchin bioerosion suppresses reef growth in Hawaii’ – is published in PLOS One. Its co-authors include Jiwei Li and Gregor P. Asner of Arizona State University.

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