Turning the tide: Innovative research reveals path to restoring Hawaiʻi’s coral reefs
An overabundance of one species of sea urchin is dismantling the coral reef in Hōnaunau Bay, Hawaiʻi, through bioerosion (live and dead large corals shown here). But a new ASU study shows how much live coral cover would allow the reef to outpace the urchins and sea-level rise in order to thrive. Researchers are growing and planting coral to boost the Hōnaunau Bay reef to a healthier state. Photo courtesy of Greg Asner
Hawaiʻi’s coral reefs are essential to the islands’ communities and economy, but they are struggling against climate change, coastal pollution, overtourism and overfishing. However, a new Arizona State University study, conducted with Hawaiian cultural practitioners, offers a ray of hope by identifying a critical threshold for successful reef restoration efforts.
This latest research, published May 28 in PLOS One, focuses on Hōnaunau Bay. It provides a crucial piece of the puzzle, revealing the significant impact of an increasing number of sea urchins on the reef's structure.
While urchins are a natural part of Hawaiian reefs, an overabundance of one species of urchin is thought to be caused by a combination of recent ocean warming, heavy tourism and overfishing. Fish that eat these urchins such as triggerfish, parrotfish and rays are in low abundance in Hōnaunau Bay. As a tourism hot spot, these predators are scared away on a day-to-day basis by swimmers. Overfishing in the area also puts pressure on predators that eat the urchins.
The result is an increase in urchin density to the highest levels ever measured on a reef in Hawaiʻi. This explosion of the urchin population is dismantling the reef through bioerosion as they grind away on coral skeletons, particularly in areas weakened by marine heat waves in 2015 and 2019. Bioerosion poses a significant threat to the reef's ability to grow coral fast enough to keep pace with rising sea levels.
Using a combination of underwater measurements and high-resolution spectroscopy from the ASU Global Airborne Observatory, the team discovered an important threshold: Maintaining approximately 26% live coral cover would allow the reef to outpace the destructive effects of the urchins and sea-level rise.
"This 26% target gives our restoration team a clear goal," said Greg Asner, senior author of the study and director of the Center for Global Discovery and Conservation Science in the Julie Ann Wrigley Global Futures Laboratory. "We are actively growing and planting coral in the Hōnaunau Bay to boost these numbers and push the reef toward a healthier state. If we can increase live coral cover above 26%, this may give these reefs a fighting chance to grow and thrive.”
The average net carbonate production — how reefs build their physical structure — in Hōnaunau Bay is significantly lower than healthier reefs, and the vertical reef accumulation rate isn't keeping pace with the current rate of sea-level rise.
“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,” former ASU student and first author Kelly van Woesik said. “Without action taken now, we risk allowing these reefs to erode past the point of no return.”
And while the high densities of sea urchins are concerning, this knowledge informs targeted management strategies to support the reef's recovery. The study not only identifies a critical challenge, it also provides a pathway toward a solution. Control of overtourism, reduction in chemical pollution, and education on overfishing are essential to bringing the urchins back into balance. The 26% live coral cover threshold empowers restoration efforts with a measurable objective, offering hope for the future of Hōnaunau Bay and other struggling reefs in Hawaiʻi.
“Armed with the scientific knowledge, our team can push forward with coral rescue and recovery, alongside efforts to reduce overtourism and pollution,” said Jeana Kelekolio, a Hawaiian lineal descendent of Hōnaunau, ASU outreach coordinator and a cultural advisor for ʻĀkoʻakoʻa, a collaborative effort that fuses state-of-the-art science with the cultural knowledge of Hawaiʻi’s community partners. “This is a great pathway for bringing back our beloved reef that we all depend upon.”
Asner’s ASU research team, including co-author and School of Ocean Futures Assistant Professor Jiwei Li, partnered with the state of Hawaiʻi and local communities through the ʻĀkoʻakoʻa program, which takes a multipronged approach, tackling challenges from land-based pollution to climate change-related coral bleaching.
"The key is understanding what's out of balance," said Asner, also a professor with the ASU School of Ocean Futures in the College of Global Futures. "We need to identify the specific problems affecting each reef and develop tailored strategies to restore that balance."
The ʻĀkoʻakoʻa partnership is supported by private donations, the state of Hawaiʻi and ASU.
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