COORDINATED RESPONSE TO INVASIVE ANEMONE CONTINUES AT KĀNEʻOHE BAY

COORDINATED RESPONSE TO INVASIVE ANEMONE CONTINUES AT KĀNEʻOHE BAY

HONOLULU – The DLNR Division of Aquatic Resources (DAR) reports encouraging progress in efforts to contain and eradicate an invasive sea anemone in O‘ahu’s Kāne‘ohe Bay. While the Manjano anemone has been observed and monitored in the bay since 2020, DAR staff detected additional spread following the Kona low storms in March. 

Detection and eradication efforts are complex, requiring substantial planning and many hands to execute. DAR staff members have been working expeditiously with project partners to contain the introduced species.

“This isn’t a unilateral effort. There has been significant collaboration and input from the Hawaiʻi Departments of Health and Agriculture and Biosecurity, the United States (U.S.) Environmental Protection Agency, U.S. Army Corps of Engineers, research partners  and members of the Kāneʻohe Bay community through all steps of the process,” said Jesse Boord, aquatic biologist with DAR. “Additionally, Heʻeia National Estuarine Research Reserve has been actively engaged in this response as co-managers of the space.”

Current response efforts employ both physical and chemical control methods. Four impermeable tarps, measuring roughly 40 x 60 feet, are deployed and secured over the infested reef area forming isolated treatment zones. High concentration chlorine is dispensed below the tarps, and the perimeter of each tarp is sealed and held in place by 30-pound sandbags. Due to the large area of the infestation, teams are working in sections. So far, two of three sections have been completed and more than 600 sandbags used, in total.

Initial inspections indicate substantial mortality of the invasive anemone within completed treatment areas and staff members are pleased to report that monitoring has not identified any unintended impacts outside the treatment footprint.

Distinguished for its small size, rapid growth and few known predators, the Manjano anemone is poised to do significant damage to Hawaiʻi’s reef ecosystems. The anemones occupy pukas (gaps) within reefs, can colonize a variety of corals and easily fragment, with each piece potentially growing into a viable adult. Because conventional removal methods can promote fragmentation and spawning, the tarped treatment approach was selected and allows the team to contain and target the infestation.

The anemone was first detected in the bay in 2020, when local divers, Gabby and Hiʻilei Kawelo, noticed suspicious corals growing on the reef. They notified DAR staff who identified the non-native corals as a common aquarium species released into the bay. Only two Manjano anemones were seen on the coral at the time. Both the corals and the anemones were removed by DAR staff in 2020.

Subsequent surveys later identified additional anemones that expanded into the current infestation. This year’s March storms brought heavy rains which delivered more nutrients to the area, providing a potential contributing factor to its spread. What started as a problem area no larger than 300 square feet a couple of years ago, has grown to about one-third of an acre. The focused joint response is necessary to suppress the spread throughout the bay and into neighboring reef systems.

Adverse environmental impacts resulting from Hawaiʻi residents releasing pets and non-native species into the environment, both on land and into coastal ecosystems, continue to be felt statewide. The Manjano infestation is a prime example of how a seemingly harmless action, like releasing aquarium species into Hawaiʻi waters, can have widespread negative effects on our native marine life.

“We’ve had 20 aquarium-related responses in the last five years, a significant increase for the state,” Boord added. “Many of these responses have originated from intentional or accidental aquarium releases. Raising community awareness about responsible pet ownership, the dangers of pet release and diligence around preventing the spread of invasives is something that DAR and our partners take seriously.”

This extends to ocean users scanning the reef and reporting anything out of the ordinary or taking care to rinse and inspect fishing and diving gear before and after each use. The Manjano anemone can attach to almost anything, from rocks and debris to boats, anchors and fins. Prevention on the front end is more cost effective than the monetary resources and personnel required for a potentially lengthy repair and restoration effort.

The Manjano eradication project was supported by legislative funding through Act 104, Session Laws of Hawaiʻi 2025, due to the potential ecological risk posed by further spread of the species. Follow-up monitoring and surveys are ongoing to identify any surviving or satellite colonies outside the primary infestation area.