Scientists are testing rubble stabilization as a coral restoration technique in the coastal waters of Hawai‘i for the first time, with promising results. It involves securing loose, broken parts of the reef to the seafloor so corals have a stable surface to grow. The process has given a disintegrated reef system a new chance to recover.
In November 2022, NOAA and State of Hawai‘i scientific divers were on a monitoring mission off the southwestern coast of O‘ahu. They discovered that the coral habitat at the site of a ship grounding 12 years prior was not recovering in the way they'd expected and hoped. One of the main issues was that sections of the pulverized reef were still covered by loose rubble. Determined to figure out another approach, they started researching. They came across an innovative method that had not been used in coastal Hawai‘i yet. Questions arose: Would it work in our high energy wave systems? Would it improve the chances that new corals will survive and grow? And how would they learn how to do it?
Restoration in the Wake of Ship Grounding
The area was damaged in 2010 by the M/V Vogetrader cargo ship when it ran aground. It damaged almost 3,500 square meters of vibrant coral reef—important habitat for fish and other marine species. When events like this happen, NOAA’s Damage Assessment, Remediation, and Restoration Program evaluates harm to marine resources, coastal habitats, and recreation before putting forth plans for restoration.
The damage assessment documented the loss of an estimated 100,000 coral colonies. The ship’s hull had gouged and disintegrated the reef framework itself into a field of rubble. In 2013, habitat restoration began with the removal of 354 square meters of rubble and outplanting of 643 coral colonies. We expected this would kickstart a natural recovery process that wouldn’t require future intervention.
Monitoring funded by the settlement showed that only 45 percent of the outplanted coral colonies survived. More importantly, the team discovered that there was a significant amount of rubble that remained rolling and tumbling across the seafloor, almost a decade after the ship grounding. Although there were baby corals recruiting to the site, they could not survive the constant movement of the rubble they were attached to. Clearly a passive approach to recovery wasn’t going to work—they needed a new plan.
Corals Play a Critical Role for Fish and Communities
Fish rely on healthy coral reefs for food and shelter, while corals rely on fish to clean off algae and provide nutrients. When pollution or grounding events injure corals, fish populations are damaged too. Fish are also critical to Native Hawaiian communities’ cultural practices, which have been passed down for generations. Corals themselves are regarded as the beginning of life, as described in the Hawaiian creation chant, which states that life began with the coral polyp.
Healthy coral reefs also absorb wave energy, buffering shorelines from damage and threats to human life during storms and floods. Wave energy is high in Hawai‘i, making this particularly important. The reefs also attract visitors from around the world, who come to see flourishing and vibrant fish and other marine species swimming in and out of coral–directly contributing to local economies and livelihoods.
New Restoration Methods Come to Hawai‘i
NOAA Fisheries and Cooperative Institute for Marine and Atmospheric Research staff were focused on finding a solution to the Vogetrader site recovery’s biggest issue: the rubble. Coral Reef Researcher Courtney Couch was involved in the 2022 surveys and started researching manual ways to tackle the issue. She discovered an emerging technique used in other coral reefs around the world—rubble stabilization—which looked promising. Marine Habitat Resource Specialist Shannon Ruseborn learned we had been employing this successfully after ship groundings in Puerto Rico.
There, rubble stabilization efforts were introduced in an area with zero live coral cover. After the process, 14 percent of the reef was covered by live coral. It also increased the reef ruggedness index by more than 100 percent.
It seems simple: Loose, tumbling live rocks are secured on the seafloor, creating stable surfaces for coral to grow. But the waters of Hawai‘i are rougher than tropical reefs. Whether it could even be done was an open question. Ruseborn organized a workshop in partnership with the Coral Restoration Foundation. Trainers from the Caribbean educated NOAA’s staff and Hawai‘i coral restoration practitioners about the method. It took months of planning to secure permits, charter vessels, and obtain permission for multiple organizations to dive together.
“The decline of our reef’s 3D structure—either through events such as mass bleaching or ship groundings—will continue to threaten our reef communities. I think manual rubble stabilization has exciting potential to give these degraded reefs a leg up on recovery. But no one method is going to work for all reefs. So we were extremely fortunate to have the opportunity to bring experts together during our recent workshop to discuss and test different approaches together. We couldn't have begun this process without the training,” said Couch.
Participants spent time in the classroom before going underwater to set up pilot plots at the Vogetrader site to test four types of mesh and multiple ways of attaching them to the seafloor. They also learned how to install boulder piles. The approach needed to adapt to Hawai‘i’s rougher environment if it was going to work.
Spreadsheets, Bags of Cement, and Reignited Hope
After the training, the Pacific Islands Fisheries Science Center scientists monitored the plots for several months to figure out which approach and materials worked best. NOAA Fisheries and Hawai‘i State Division of Aquatic Resources began a complex planning process to prepare for a larger experimental study at the site. There were spreadsheets, whiteboards, multiple trips to the hardware store, day-long meetings, multiple run-throughs of the entire mission, and consultations with local stakeholders.
To install the treatments, they needed to:
- Choreograph teams of divers on the sea floor
- Determine who would be on the boat mixing the cement and how much time they had to get it to the divers below
- Identify how much time each person could do each job
- How and when to rotate positions
The site is adjacent to an active shipping channel, so the U.S. Coast Guard and other partners would need to be in the loop as well.
The team knew they would approach the mission with a scientific question in mind. Couch would be investigating the rubble stabilization plots with funding from NOAA Coral Reef Conservation Program. Before the mission began, they conducted baseline surveys that they will compare against future conditions. Do these methods perform better than no intervention at all? How much better? Over what time period? This made the planning even more complex and of critical importance.
A Complex Mission
When the day finally arrived, the team was anxious but excited that they’d be bringing a new tool to Hawai‘i. The day marked a new phase of restoration for the area. It also reignited a sense of hope that this habitat could again be home to the corals and fish that were once abundant.
They loaded up two boats with scuba gear, painters’ buckets, PVC-coated mesh, steel pins, a mud mixer, and bags of cement and sand.
The 3-week mission was a huge success, but no small amount of work.
To build the boulder piles, first, they mixed cement in a bucket on the boat, then lowered and dumped before it cured. A snorkeler communicated between the boat and divers on the seafloor. They made sure divers were well clear of the incoming bucket of cement, and signaled when to move to the next plot. Divers on the seafloor retrieved the buckets and attached them to a lift bag, which looks like a small parachute. Next, they’d inflate the liftbag using a scuba tank—but the proper amount of air was tricky. Too little, and it wouldn’t lift enough; too much, and the cement would float to the surface.
The piles were built like layer cakes: first cement, then pieces of broken reef framework, and repeat once cured. Because this is an experimental study, the plots all needed to be the same size, so divers took measurements each time they added another layer. Dumping cement out on the ocean’s floor was difficult due to the water’s resistance, and the plume that immediately erupted brought visibility to about zero. So diver teams stayed close to each other and worked together to pass materials while they built each plot.
To install the mesh plots, the team unrolled small sections of PVC-coated mesh onto the seafloor and pinned it down with stainless steel rods. To secure the pins in place, cement was patted into place around each pin.
In the end, the teams:
- Installed 10 mesh plots
- Cemented 10 boulder piles
- Set up 10 control sites
Next Up: Monitoring
The team will head back to the site in the spring of 2025 to check on the plots. Despite testing the mesh during the training, they need to ensure they are still secure after Hawai‘i’s winter swells. If they are, the team plans to partner with Kuleana Coral Restoration to outplant corals onto the mesh and boulder piles. The hope is that these outplanted corals will grow and add more stability to the plots. From there, we will monitor the experiment every 6 months. This will help us figure out whether new corals are settling and surviving and what tweaks to make in the future to ensure its success.
Ruseborn is hopeful about the future. “After the last few years and all of our efforts that got us to this point, it’s going to be hard to wait until the spring to see what’s happening down there. I am excited that we’re trying something new. The project will determine if this can be a tool in the restoration toolbox in Hawai’i. We’re all going to remain cautiously optimistic that we can help bring this reef back to life.”
