State enlists sea urchins to fight invasive seaweed

DLNR Sea Urchin Out-Planting
DLNR Sea Urchin Out-Planting

The state Department of Land and Natural Resources is planting hatchery-raised native sea urchins in Waikīkī as part of an innovative algae control strategy.

Eventually, 100,000 native sea urchins (Tripneustes gratilla) will be positioned in the Waikīkī Marine Life Conservation District (MLCD) to help control over four acres of invasive algae. The project builds upon the success of collector urchin plantings in Kāneohe Bay.

According to the DLNR Division of Aquatic Resources (DAR), sea urchins are vital to aquatic life in Hawaii as they are consumers of invasive algae (seaweed). Invasive seaweed alters coral reef ecosystems by overgrowing and starving corals of light and can eventually kill the coral colony. When urchins eat invasive seaweed, the coral can regrow, which opens up spaces for fish, native seaweed (limu) and invertebrates to utilize these areas that were previously overgrown by algae.

This method of sea urchin biocontrol of invasive seaweed has been used in Kāneohe Bay for over nine years, where the state has seen great success in controlling several species of invasive seaweed.

Invasive seaweed used to dominate corals in Kāneohe Bay, but are now reduced to manageable levels, which has greatly reduced the impact to corals in the Bay.

“We hope to see the same level of success in Waikīkī,  to improve coral habitat and expand healthy reef coverage in the most visible [marine conservation district] in the state,” said Wesley Dukes, DAR Habitat Monitoring Coordinator.

Wild urchins are collected from local reefs, spawned, and then raised for out-planting at the Ānuenue Fisheries Research Center urchin hatchery. This is where urchins will be raised for the three-year project in the Waikīkī conservation district. To date, the Ānuenue Fisheries Research Center has produced over 500,000 sea urchins.

“We pick the urchins out of barrels with the hatchery crew and then load 200 to 400 urchins on individual trays,” Dukes explains. “From there, to a truck, to the beach, to boogie boards and floats and to their new homes and jobs grazing away patches of invasive seaweed.”

During the first out-planting this week, the team out-planted 7,000 collector urchins. It takes four to six months at the hatchery for urchins to get big enough to go into the ocean.  

“It is a proven, natural way to combat invasive species and we hope to enhance collector urchin populations at other locations statewide in the near future,” said DAR Administrator Brian Neilson. 

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