Pilot Program: Early Signs of Life

Just 12 weeks after being deployed on the seafloor near Onekaka, the Mohua Marine Trust's reef ball pilot program are already showing promising signs of life, offering an exciting glimpse into their potential to restore marine habitat in Golden Bay.

Close-up on one of the Reef Balls - showing star fish, red filamentous algae and encrusting bryozoans.

In February, with the expertise of the mussel vessel Clearwater Resolution, the Trust deployed 20 reef balls in around five metres of water off Onekaka. The project aims to create hard substrate where marine life can settle and thrive, helping rebuild biodiversity in an area where soft sediment and fine silt have limited natural reef formation.

On Monday 18 May, trustee and experienced diver John Davis returned to the site for the Trust's first detailed inspection since deployment. Launching from Tarakohe with Gene at 9am, they crossed the bay to arrive at the reef site ahead of the 10.30am high tide.

"I made my way down to the centre cluster," says John. "The reef balls have been in the water for almost exactly 12 weeks, and they're already showing really encouraging signs of colonisation."

His observations revealed small grazers including starfish and snails, along with early growth of sponges, seaweeds and bryozoans. Every reef ball showed healthy signs of marine life establishing itself.

"The most exciting thing I saw was what had happened inside the base of each reef ball," John says. "The swirling currents and tides have flushed out the fine silt, exposing the original seabed beneath. Old oyster and scallop shells are now visible, and in the future this shelly substrate could give scallop spat and other shellfish larvae something to cling to."

For years, much of this seabed has been buried beneath layers of fine sediment, making it difficult for shellfish and other marine organisms to establish.

The photographs and observations from John's dive delighted the Trust's scientific adviser, Dr Emilee Benjamin, who identified an impressive range of species already making themselves at home.

"There are tube worms, cushion sea stars, 11-armed sea stars and spotties," says Emilee. "I also spotted a triplefin fish and some filamentous red algae, which is exciting because it helps facilitate mussel settlement. Most importantly, after such a short time, there are already encrusting bryozoans."

The appearance of bryozoans is particularly significant. These tiny colonial animals are ecosystem engineers that create complex habitat for countless other marine species.

Local bryozoan expert Dr Sean Handley travelled from Nelson to share his knowledge at a recent Mohua Marine Trust meeting.

"The main bryozoan species from Nelson Bays is Celleporaria agglutinans," Sean explains. "It's special because it can build hard, coral-like reef structures above soft sediment. These living reefs create nooks and crevices that provide shelter and food for juvenile fish such as tarakihi and snapper, while also allowing shellfish like scallops and oysters to settle."

Bryozoan reefs were once abundant around Separation Point, where they played an important ecological role as fish nursery habitat. Sean believes artificial reef structures such as reef balls could one day help support the recovery of these remarkable ecosystems.

"The Separation Point Power Fishing Exclusion Zone was established 46 years ago to protect this important nursery habitat, but it appears to have been severely affected by Cyclones Fehi and Gita in 2018. The seabed is now much more turbid, and it's unclear how much recovery has occurred. Reef balls could provide elevated hard surfaces above the silt, giving these habitats the leg-up they need to regenerate."

For the Mohua Marine Trust, the early results are both encouraging and exciting. Although the reef balls have only been underwater for three months, they are already beginning to function exactly as intended—providing hard surfaces for marine life to colonise and creating the foundations for increasingly diverse habitat over time.

"I'm really looking forward to the next dive," says John. "It will be fascinating to compare the progress and see how much more life has established itself."

As the reef continues to mature, each dive is revealing another chapter in what the Trust hopes will become a long-term success story for marine restoration in Golden Bay, Mohua.