$6 million caulerpa removal tool shows promise in Northland

A giant underwater dredge device designed to remove large areas of exotic caulerpa seaweed has performed well in recent testing at Omākiwi Cove in Northland, Biosecurity Minister Andrew Hoggard says.

“Known as the submersible dredge planer (SDP), the remotely operated equipment was developed for Biosecurity New Zealand by Northland Regional Council, which contracted local company Johnson Brothers Ltd to design and build it.

I’m thrilled that field tests over the past few weeks have shown the machinery works as expected and will be a useful addition to our toolbox of treatment methods.”

The SDP is the last of three removal tools developed using the $10 million we allocated in 2024 to tackle this invasive seaweed. The other two are a UV-C light treatment to irradiate caulerpa, and a scalable enclosure system (the ‘Rehabitat’) that contains and seals chlorine over affected areas.

The trials, supported by the Cawthron Institute and Ngāti Kuta and Patukeha hapū, demonstrated the SDP’s mechanical performance, manoeuvrability and functionality. Environmental impacts are still being assessed.

“Developing the SDP has been a mammoth effort. Two key components - the remotely operated vehicle (ROV) and the control room had to be sourced from Scotland. There was no off-the-shelf solution available anywhere in the world, so it’s been custom designed for New Zealand conditions. Its unique screw propulsion system, for example, allows it to operate effectively on soft, sandy seabeds like those in the Bay of Islands.

It’s driven by remote control and fitted with blades to cut through caulerpa mats and a large dredge system to suction material to the surface for disposal. It’s a great example of Number 8 wire Kiwi ingenuity.”

A challenge for testing has been the absence of caulerpa at the trial site. The SDP was conceived when the weed blanketed Omākiwi Cove, but recent dieback events have left only sparse populations.

“Despite this, we now have a good understanding of the speed and area the SDP can cover in a day, and it’s ready to go should dense caulerpa return. It can also be modified for use on other species in both salt and freshwater environments.

Having a range of tested treatment tools - including large-scale removal technology like the SDP is important so we can respond quickly to control or eliminate invasive species in future.” ...