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Hawai'i Marine

Sea Urchins to the Rescue

“What this shows is that you can use the Super Sucker to reduce the density of alien algae, and then use the urchins to keep it down”
-- Kim Hum, Hawai'i director of marine programs

Reef managers in Kāne‘ohe Bay are cheering the success of a new restoration technique that uses native species to combat alien invaders in the marine environment. 

At issue is the difficult problem of aggressive non-native algae that have been smothering reefs in several parts of the state, with one of the most significant invasions occurring in Oahu’s Kāne‘ohe Bay.

An early success was the development of the Super Sucker, an underwater vacuum system developed by the State Department of Land and National Resources, the University of Hawai‘i and The Nature Conservancy. It sucks invasive algae off the reefs, but it is not a permanent solution. They tend to grow back.

“Two years ago, we manually removed all the algae from a reef, and in five or six months it was back to baseline conditions,” said State aquatic biologist Tony Montgomery.

This past year, biologists tried a new experiment, clearing a 3,000-square-meter (about a half-acre) patch reef in Kāne‘ohe Bay. They left one half of the reef alone, while on the other half they placed 1,200 native sea urchins.

The experiment was a stunning success.

“The portion of patch reef without urchins grew back in five to six months, with 35 percent alien algae cover, while the urchin side has been kept to 3 percent cover for the past 12 months,” Montgomery said.

“What this shows is that you can use the Super Sucker to reduce the density of alien algae, and then use the urchins to keep it down,” said Kim Hum, the Conservancy’s Hawai‘i director of marine programs.

The State Division of Aquatic Resources is now working on raising sea urchins in captivity at its Anuenue Fisheries Research Center on Sand Island. According to Montgomery, the goal is to produce “tens of thousands” of urchins for outplanting onto reefs infested with alien algae.

Eric Conklin, the Conservancy’s Hawai‘i director of marine science, notes that Kāne‘ohe Bay has several troublesome algal species, but that the dominant one in the patch reef experiment was Eucheuma denticulatum.

“This and other invasive algal species— like the Avrainvillia found in Maunalua Bay—don't belong in Hawai‘i,” he said. “We are finding that they are aided by the flow of sediments and nutrients into the nearshore marine environment, and that there are not enough herbivores (plant-eating fish) to keep them under control.”

Overfishing for herbivores like parrotfish and surgeonfish, as well as for algae-munching urchins, help the alien seaweeds expand. Montgomery said the Kāne‘ohe patch reef experiment only brought the reef’s collector urchin population back to roughly natural densities.

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