$650,000 Caulerpa Removal Trial Begins in Bay of Islands

Johnson Bros’ Angus Johnson on the barge with the new Ōpua-designed mechanical caulerpa suction dredge at Omākiwi Cove. Photo / Northland Regional Council

By Susan Botting, Local Democracy Reporter

New Zealand’s first mechanised caulerpa suction dredge will tackle the invasive pest seaweed in the Bay of Islands before Sunday.

The new mechanical suction dredge is at the centre of a $650,000 trial expected to be a game changer in the fight against caulerpa.

“We’re effectively moving from the equivalent of weeding by hand, to a mechanised approach for removing caulerpa from the sea floor,” Northland Regional Council (NRC) marine biosecurity manager and national marine invasions specialist Dr Kaeden Leonard said.

The Government-funded Omākiwi Cove caulerpa trial is an NRC-led joint project with the Ministry for Primary Industries (MPI) plus Te Rawhiti hapū Ngati Kuta and Patukeha.

If successful, its mechanical harvesting will potentially expand to bigger areas of the Bay of Islands, Aotea/Great Barrier Island and around the world.

The Ōpua-designed dredge will be used in mainland eastern Bay of Islands’ Omākiwi Cove, where caulerpa was first identified on May 5 last year.

It will dredge about a hectare or 1.4 rugby fields equivalent of caulerpa from the seabed, working inwards from the infestation’s outside edge.

Leonard said there was a wide range of tools available to use in addressing land-based biosecurity threats. That was not the case for the marine world.

“We’re 100 years behind the terrestrial ecosystem with our tools for marine biosecurity. We need advancements like this.”

The dredge sits on a barge in Omākiwi Cove. It gently sucks up the invasive plant pest off the seafloor.

Counteracting hydraulic brushes at the end of a suction hose, attached to a long-reach excavator arm, lightly travel over the top of the seabed, to a depth of about 30cm, shaking caulerpa’s anchoring Kikuyu-like horizontal spreading roots.

Caulerpa plants, their loosened roots and surrounding muddy or sandy seabed are gently sucked up the long hose to the barge above into a large specially-designed filtering bag.

Leonard said the mechanisation transformed what would otherwise have involved divers manually dragging heavy suction hoses underwater to caulerpa spots.

It would allow for the removal of significantly more caulerpa over a significantly bigger area in the same amount of time.

“It’s quite heavy work for a diver to be hauling a suction hose around the sea floor to the exact location required. There’s also more potential for fragments of caulerpa to break off,” he said.

The new dredge has been designed by Ōpua-based dredging and marine construction company Johnson Bros.

Te Rawhiti resident Michelle Elboz (Ngāti Kuta, Patukeha) said the New Zealand-first mechanical suction dredging trial at Omākiwi was an important next step.

“We need to eradicate caulerpa and we will do everything we can to do so. It’s not in anyone’s best interests to sit around on our hands doing nothing.

“The moana is our backyard and kai cupboard. It’s important, not just for us but for everybody who uses the Bay of Islands. This place has been throbbing with visitors over Christmas and summer.”

Elboz said left unchecked, caulerpa would take over, meaning habitat loss for creatures that lived in the areas it invaded, hitting kaimoana food sources including for pipi, oysters, mussels, kina and snapper.

Dredged material in the barge’s large sealed geotech filtering bag is drained of sea water that slowly seeps out through the extremely fine-meshed porous material the bags are made from.

Sea water slowly seeps out of the dredged material through the porous walls of the bag.

Leonard said the wall material’s very fine .2mm filter size was small enough to block the escape of viable caulerpa fragments that might spread the pest.

Once drained, the remaining mixed seafloor and caulerpa solids are transferred to a new specially-designed dredge spoil area on private land at the head of Omākiwi Cove, where they will be buried.

Leonard said trial results may indicate the need for a second sweep of the same area and/or other follow-up tools such as chlorine mats.

The trial is taking place inside the Government’s caulerpa controlled area notice that bans anchoring, fishing and diving. This is supported by a rāhui laid down by Te Rawhiti hapū Ngāti Kuta and Patukeha.

Leonard said getting the necessary permissions for the trial had involved a high degree of scrutiny during gathering MPI permits, NRC resource consents and hapū agreement.

Caulerpa has now been found at Te Rāwhiti Inlet in the Bay of Islands, Great Barrier Island (Aotea), Great Mercury Island (Ahuahu), Kawau Island and Waiheke Island in the Hauraki Gulf.

- LDR is local body journalism co-funded by RNZ and NZ On Air.