On Sept. 25, the state Board of Land and Natural Resources approved a one-year special use permit to Mark Hixon, a professor of marine biology at the University of Hawaii, for the deployment and study of artificial reef modules in the Hanauma Bay Marine Life Conservation District and Waikiki.

If the modules successfully recreate coral reef habitats, Hixon told the board, they could prove to be valuable tools in mitigating the impacts of climate change and intensive fishing.

Artificial reefs are nothing new and, in fact, the state has constructed a number of them. But according to Jack Randall, senior ichthyologist at the Bishop Museum, those reefs “have not provided the appropriately sized shelter for fishes.”

Hixon’s work could remedy that.

A holed reef module in the Bahamas is surrounded by fish and overgrown with coral and sponges.
A holed reef module in the Bahamas is surrounded by fish and overgrown with coral and sponges. Environment Hawaii

A year ago, Hawaii suffered the greatest coral bleaching event in its history, he told the Land Board.

“We were lucky that time. Most of those corals recovered. … We’re now on a coral bleaching watch well into the fall. We’ve been lucky so far, but the projections are that the intensity and frequency of events will be increasing. Our luck will not last forever,” he said.

Indeed, shortly after his presentation to the Land Board, the state experienced another unprecedented bleaching event. By early October, the Department of Land and Natural Resources’ Eyes of the Reef Network had more than 100 bleaching reports in a matter of days.

Bleached corals can recover if larvae are able to settle. Otherwise, they are overcome by seaweed. Hixon said he’s seen reefs die from coral bleaching, taking with them the biodiversity and ecosystem services the reefs support.

A key factor in reef recovery is the presence of herbivores, such as parrotfish and sea urchins. If they’re present in reasonable numbers, dead corals are kept clean and larvae can settle and grow.

But the problem on Oahu, he said, is that intensive fishing has severely reduced the abundance of herbivorous fishes, especially parrotfish, otherwise known as uhu.

According to a reef assessment by the National Oceanic and Atmospheric Administration, Oahu has by far the lowest abundance of reef fish of any of the Hawaiian islands.

On the deep sand flats of Hanauma Bay, Hixon plans to build six fish houses to locally enhance herbivores and help corals persist as they suffer the effects of bleaching. Each house, or “artificial coral module,” would consist of 48 concrete blocks. He plans to install the same number of modules in the Apuakehau or Halekulani sand channels in Waikiki.

Because fishing is prohibited in Hanauma Bay, the modules there will serve as a control site to determine the effects of fishing. Some of the blocks will have holes in them, some will not. Those without holes are meant to simulate dead reefs that have lost their structural integrity.

Each house, or “artificial coral module,” would consist of 48 concrete blocks.

“The proposed deployment location in Hanauma Bay is well beyond the sight of the vast majority of the visitors to the MLCD and will therefore have little impact on the visitor experience. In addition, this research can be used as a valuable opportunity to educate the public and visitors on the benefits of healthy reefs and how we can manage for resilience,” UH scientist Alan Friedlander wrote in an April 14 letter of support to the DLNR’s Division of Aquatic Resources.

Hixon said he will monitor the modules to determine whether the fish that aggregate around them are local or are transplants from other areas. Hixon has already built 52 such structures in the U.S. Virgin Islands and has done similar work in the Bahamas. In both cases, he said, the modules attracted fish and spurred coral colonization.

“They eventually grow into natural structures. When organisms settle and grow, you get coral growth where there wasn’t before,” he said.

The modules to be placed around Oahu could become natural features, but they can also be removed, Hixon said.

At DAR’s request, the university’s Office of Research Services has entered into a Memorandum of Agreement to ensure that Hixon remains responsible for the maintenance or removal of the modules. Should the modules be removed, he would also be responsible for the cost of returning the sand flat to its original condition.

Land Board member Keoni Downing expressed some concern about the stability of the modules during a hurricane. Hixon replied that the modules in the Bahamas have been hit by a number of hurricanes and are still standing.

He added that the modules are the first step in a two-pronged approach to restore reef ecosystems. Researchers are also working to identify corals that are resistant to bleaching and transplant them.

“If all there is out there is seaweed, they’re not going to grow. We have to have an acceptable environment. The danger we face on Oahu is a phase shift,” he said, referring to what happens when a reef ecosystem becomes dominated by algae. And because Oahu has such low fish abundance, that’s a real danger.

Unless bleached corals are kept free of seaweed so they can be recolonized, worms and other boring organisms break the corals down.

“I can’t tell you how ugly it is to see that,” he said.

Reprinted with permission from the current issue of Environment Hawaii, a non-profit news publication. The entire issue, as well as more than 20 years of past issues, is available free to Environment Hawaii subscribers at www.environment-hawaii.org. Non-subscribers must pay $10 for a two-day pass.

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