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Millionth Spiny Superhero Released to Devour Hawaiʻi's Coral-Smothering Algae

December 06, 2023

One million sea urchins have now been deployed through the Hawaii Division of Aquatic Resources’ work in partnership with NOAA to restore corals after the M/V Cape Flattery grounded on an Oʻahu reef.


A group of unlikely restoration heroes reached an impressive milestone this year: the millionth sea urchin is now working with its companions, helping struggling corals in Hawaiʻi. Since 2011, the State of Hawaii Division of Aquatic Resources has grown and released urchins in Kaneohe Bay in partnership with NOAA. The urchins eat invasive algae threatening corals and their ecosystems.

This prickly partnership began after M/V Cape Flattery grounded on a shallow reef off of Barbers Point on Oʻahu in 2005. Damage from the ship’s removal affected almost 20 acres of reef, scraping away approximately one million corals. 

a group of adults in wetsuits stand on a boat holding a signs that read 'one million urchins'
One million sea urchins have now been deployed (Photo: Hawai'i Division of Aquatic Resources)

Disasters like ship groundings, oil spills, or hazardous waste releases occur in marine and coastal environments. NOAA and other agencies, called Trustees, secure funds from those responsible for the environmental harm. We then use those funds to restore the damaged habitats and natural resources, making up for what was lost in the incident. 

In this case, there was some emergency restoration and reattachment at the site. The damaged corals and habitat were recovering naturally afterwards. The Trustees decided to monitor the reef as it healed, and instead looked into other restoration opportunities. Specifically, at Kāneʻohe Bay, on the east side of Oʻahu, which was struggling with invasive algae.


Without any natural predators, invasive algae can carpet Hawai‘i reefs. Photo: Hawaiʻi Division of Aquatic Resources

When the non-native algae was introduced to the bay in the 1970s, it found an ideal habitat with few natural predators. Like many invasive species, it ended up taking over. It smothered coral and blocked light from getting to them. Corals are the foundation of habitat for many fish and other species. This essential fish habitat was being suffocated out, which can have a trickle effect for the ecosystem relying on them as a base.

Luckily, we were able to identify something native to the area with an appetite for invasive algae. The problem, though, was that there weren't many sea urchins at the site. To help boost the population, our partners at the State of Hawaiʻi remodeled a hatchery previously used for growing prawns to grow sea urchins.


Diver collects algae with the Super Sucker hose before deploying urchins. Photo: Hawaiʻi Division of Aquatic Resources

The urchins are grown to a very specific size—about as big as a cherry tomato—before they're ready to release. This allows them to fit into the nooks and crannies of the corals and reef.

Prior to getting the sea urchins involved, we had to get rid of the large clumps of algae that had gotten out of control. To do this, we used another classic but innovative solution. We deployed a supersized vacuum called the “super sucker” to suck algae colonies off the reef. Then divers place urchins in targeted areas to get to work.

Cape Flattery Coral Restoration Sea Urchin Diver 3757x2825 NOAA Fisheries Matthew Parry.jpg

After growing in a hatchery, urchins are loaded on to shallow trays, then divers deploy them onto the reefs.

NOAA and partners, which include the State of Hawaiʻi, University of Hawaiʻi, The Nature Conservancy, and the U.S. Fish and Wildlife Service, are slated to continue this work through 2025.