Building Reef Resilience Through Invasive Algae Removal and Urchin Biocontrol in Kāne‘ohe Bay
Location
Kāne‘ohe Bay, O‘ahu, Hawai‘i
The challenge
Kāne‘ohe Bay lies below the dramatic Ko‘olau mountain range on the windward side of O‘ahu. It is the largest sheltered body of water in the main Hawai‘ian Islands, and is surrounded by numerous freshwater streams and wetlands. It is the only bay in Hawai‘i that includes fringing reefs, patch reefs, and barrier reefs. It has significant cultural and ecological value, and has long been a rich resource for commercial, recreational and subsistence uses. The bay has more than 40,000 people living near its shores or in the mountains above it, as well as a U.S. Marine Corps Base, the University of Hawai‘i Institute of Marine Biology on Moku o Lo‘e (Coconut Island), and a public pier that provides daily access to hundreds of tourists and fishers.
Invasive alien algae are a significant threat to Hawai‘i’s nearshore coral reef ecosystems. In ocean ecosystems that are already threatened by land-based pollution and overfishing, they can easily take over and smother reefs causing death. Unfortunately, the reefs in Kāne‘ohe Bay are suffering from an overgrowth of invasive algae that forms thick, tangled mats. In the early 1970s, Gracilaria salicornia (gorilla ogo) and Kappaphycus/Eucheuma spp. (smothering seaweed) were introduced to the bay for aquaculture purposes.
These fast-growing algae have now spread throughout the entire bay, where they outcompete native seaweeds, smother and kill coral reefs, cover up native fish habitat, prevent new corals from attaching to the reef, and reduce the overall health and biodiversity of the entire bay. Fortunately, native “collector” sea urchins (Tripneustes gratilla) like to eat these algae, but their populations in Kāne‘ohe Bay are sparse (it has not been determined whether populations were ever more common in the past).
If nothing is done to stop the spread of these invasive algae, they will continue to move northward, spreading from the bay to reefs along the rest of O‘ahu’s shoreline.
Actions taken
To restore Kāne‘ohe Bay coral reefs and prevent the further spread of invasive algae, the State of Hawai‘i’s Division of Aquatic Resources (DAR), in partnership with The Nature Conservancy (TNC) and the University of Hawai‘i (UH), embarked on a dual-phased restoration project to control the invasive algae by:
- Removing invasive algae to allow coral reefs and native fish habitat to thrive
- Restocking the reefs with native sea urchins that eat the invasive algae and keep it from growing back
Invasive Algae Removal
DAR, TNC, and UH developed the first Super Sucker barge in 2005 to “suck up” invasive algae. The Super Sucker is a suction pump on a barge with hoses that are used by divers to vacuum the invasive algae off the reefs. The invasive algae are then delivered to local farmers who use it for compost.
Divers using the Super Sucker to remove invasive seaweed. © Hawai‘i DLNR
The Super Sucker is capable of removing anywhere from 600-1,000 pounds (270-450 kilograms) of algae per hour depending on location and conditions. In 2013, TNC built a second Super Sucker, along with a Mini Sucker to be used in shallower water, to help speed up the operation. By 2016, manual removal was no longer needed due to decreases in invasive algae cover and successes in biocontrol through the outplanting of Tripneustes gratilla sea urchins, and the Super Sucker project was closed.
Native Sea Urchin Biocontrol
Although the Super Sucker was effective at removing the bulk of the invasive algae, the algae can return within six months if nothing is done to stop it. In order to prevent the algae from growing back, DAR operates a collector urchin hatchery at their Anuenue Fisheries Research Center.
The state has successfully reared T. gratilla from the larval stage all the way through to adulthood. The hatchery has been producing approximately 5,000 urchins each month, and they are working to dramatically increase that number to keep pace with the rate of invasive algae removal. The urchins are transplanted directly onto the reefs to graze after the Super Sucker has removed the majority of invasive algae. Because the urchins will not cross the sandy areas between reefs, they are likely to stay put; they can also be re-collected and moved to other patch reefs if necessary. As of 2023, DAR is still managing the hatchery, and recently outplanted their millionth collector urchin.
Juvenile collector urchin being outplanted to the reef. © Hawai‘i DLNR
Keeping the reefs clear of invasive algae opens up new space for coral recruits and native seaweeds, and helps restore natural habitat for fish and other sea creatures, making Kāne‘ohe Bay more resilient to future threats.
How successful has it been?
Between October 2012 and August 2013, the coordinated TNC/DAR crews removed 250,000 pounds (114,000 kilograms) of invasive algae from 20 acres (8 hectares) of reef. The algae were given to local farmers for compost. Research efforts determined that around 2 urchins per square meter are needed to keep the algae in check. DAR conducts annual snap-shot surveys on the reefs in Kāne‘ohe Bay to Field monitor urchin density, coral cover, coral recruitment, and algal density and diversity.
While the complete removal of invasive algae may never be possible, DAR aims to keep the invasive algae cover below 5%, and is succeeding at that through on-going collector urchin outplanting. That urchin outplanting has supplanted the Super Sucker is itself an incredible success.
Lessons learned and recommendations
- Preventative actions combined with active invasive algal monitoring and management is an ideal strategy for mitigating the impacts of invasive algae.
- Preventative actions (such as proper cleaning of boating, fishing, and dive gear, limiting take of native species, and restoring and maintaining watersheds) should involve engagement with local fishers and community members to communicate about best practices.
Funding summary
State of Hawai‘i
The Super Sucker barge and urchin hatchery have been funded through the state’s Hawai‘i Invasive Species Council, in addition to federal grants from the National Oceanic and Atmospheric Administration. Mitigation funds are also a source of current and future support. The Department of Aquatic Resources continues to run the urchin hatchery.
The Nature Conservancy
The Nature Conservancy raised US $2.5 million to pay for the construction of Super Suckers and three years of operation.
Lead organizations
Hawai‘i Department of Land and Natural Resources, Division of Aquatic Resources
Partners
Hawai‘i Department of Agriculture
Harold K.L. Castle Foundation
Hawai‘i Invasive Species Council
Kākoʻo ‘Ōiwi
Kama‘aina Kids
Kāne‘ohe Canoe Club
National Oceanic and Atmospheric Administration
The Nature Conservancy
University of Hawai‘i Institute of Marine Biology
Resources
Hawai‘i Invasive Species Council
Hawaii Coral Reef Strategy 2030
Sea Urchin’s Helping to Control Invasive Seaweed
Hawai‘i Aquatic Invasive Species Management Plan
Invasive Algae of Hawai‘i Poster
