Hawaiian sea urchins have an appetite for invasive algae

Let’s say you are trying to remove tiny piece of invasive plant material from an area 8 miles long and almost 3 miles wide. If you miss any, it will take over again. And you are working under water.

This is the situation the Department of Land and Natural Resources’ Division of Aquatic Resources faces at Kaneohe Bay on Oahu. An invasive algae, Kappaphycus spp., also known as smothering seaweed, has overrun the reef causing a cascade of impacts.

“They’re so aggressive they outcompete native limu (seaweed); they grow so dense they kill coral and dominate habitat for tako (octopus) and small fish,” said Jono Blodgett, aquatic species project leader with the Aquatics Resource Division.

First, division officials tackled the alien algae with the Super Sucker, a giant vacuum mounted on a boat. Divers worked their way across the reef, sucking up the algae. Technicians would sort through it, tossing native limu back to colonize the reef, and bagging the invasive algae to give to local farmers to be used for compost. Unfortunately, gains were temporary – fragments of invasive algae left behind would regrow. Then they called in native Hawaiian sea urchins with an appetite for invasive algae.

If the Super Sucker acted like a giant vacuum, native collector urchins (Tripneustes gratilla) are more like a Roomba, that small robotic vacuum that automatically cleans house floors. Once released, these spiny herbivores move across the reef munching algae as they find it. They would eat native limu as well, if the invasive algae hadn’t outcompeted all the natives.

The first urchins were released in January of 2011, and they’ve proven their worth. Although this urchin is native to Hawaii, it wasn’t abundant enough on the reef to control the algae in Kaneohe Bay.

Their numbers had been much greater in the bay, but unknown reasons have caused their populations to decline. By elevating the urchin populations, resource managers are getting ahead of the invasive algae.

The urchins used are a native species, selected because they stay on the reef and munch algae day and night. Adult urchins are collected from the wild and bred in captivity. After juvenile urchins reach 15 millimeters, typically within five to six months, they are sent to work: released in Kaneohe Bay to settle out over the reef and eat to their hearts’ content. Currently 5,000 urchins are released each month in Kaneohe Bay.

Density numbers are still being analyzed, but initially, urchins are released at a density of two per square meter and once the algae are under control, one urchin per square meter keeps the reef maintained. With some patch reefs as large as 30,000 square meters, it will take a lot of urchins. “We’re hoping to double production by next year,” Blodgett said.

Monitoring in Kaneohe has shown that the collector urchins are effectively controlling invasive algae. Based on the success of the cleanup in Kaneohe Bay, collector urchins could become invasive algae cleanup crews across the state. Trials have been done to see if the urchins will control another alien alga: the gorilla ogo (Gracilaria salicornia) that plagues Waikiki and much of Molokai.

In the meantime, Blodgett advises leaving urchins and other herbivores alone and making sure dive and snorkel gear is free of any pieces of algae when you leave a site. Invasive algae spread through fragmentation, Blodgett said. “Rinse gear before leaving a site.”

* Lissa Fox Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. Kia’i Moku, “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.