Sea urchin protection plan helps Kaneohe reef thrive

Tens of thousands of native sea urchins have been marshaled in the fight against invasive seaweeds threatening patches of reef in Kaneohe Bay.

The sea urchins, Tripneustes gratilla or hawa‘e, serve as a biological control after most of the invasive seaweed is removed by a suction machine.

The hawa‘e are deposited on the reef as juveniles and grow to maturity to eat whatever is left of the alien seaweed.

Scientists said the result has been a more balanced ocean environment with more diversified marine life.

The sea urchins are produced at a hatchery on Sand Island. “What we do in the hatchery is we try to improve the urchins’ odds of survival,” hatchery manager David Cohen said. “Maintaining healthy populations of native sea urchins is vital in restoring the health of the reef.”

William Aila Jr., director of the state Department of Land and Natural Resources, said the alien seaweed otherwise would smother Hawaii’s reefs.

Cohen, who works under contract with the state Division of Aquatic Resources, said his sea urchin work began about four years ago. He said some scientists were moderately successful in growing the creatures but not on a mass scale.

In the first release, in 2011, 1,000 sea urchins were placed on the reef. Cohen said the hatchery released 60,000 sea urchins in 2012 and this year he’s hoping to release 80,000.

Cohen, a biologist who has grown clams and oysters commercially, said sea urchin larvae are fragile and require the right water motion. Cohen and others at the state’s Anuenue Fisheries Research Center also grow single-cell phytoplankton, known as microalgae, to feed the sea urchins.

It takes four to five months for sea urchin larvae to grow into urchins about the size of a nickel, at which point they are released onto the reef.

An adult hawa‘e is 3 to 4 inches in diameter.

The smothering seaweeds, which grow in the Philippines and Indonesia, were introduced in the 1970s as part of a research project to explore potential use for commercial purposes.

Cohen said the seaweed species — Kappaphycus alvarezii, Kappaphycus striatum and Eucheuma denticulatum, used as emulsifiers in foods such as milkshakes — proved not feasible for commercial purposes.

“They didn’t think they would grow pretty well here, but they did,” he said.

Cohen said it took about four years for the sea urchin program to gain the required support from the Hawaii Invasive Species Committee, National Oceanic and Atmospheric Administration, the Nature Conservancy, Kaneohe Canoe Club, University of Hawaii and a number of nonprofit groups.

“I feel pretty good about it,” he said.