World-first seaweed-eating machine launches in Bay of Islands

A $5 million remote-controlled submersible dredge planer began final sea trials at Omākiwi Cove on June 22, marking a new phase in New Zealand’s effort to eradicate invasive caulerpa seaweed. Developed by Ōpua-based Johnson Bros, the 16-tonne robotic vehicle is designed to clear nearly 1.5 rugby fields of the invasive pest daily, significantly outpacing previous manual suction methods.

Engineering a Local Solution

The project, funded by Biosecurity New Zealand, represents a four-year development effort following the first mainland detection of caulerpa in the Bay of Islands in May 2023. Designer Andrew Johnson, who holds an ocean engineering degree from the University of Tasmania, stated that the machine was built using a combination of local components and repurposed parts from a second-hand Scottish North Sea oil and gas vehicle.

The submersible is 13 metres long and 2.5 metres high, with a footprint of 52 square metres. It operates via a 650-metre umbilical cable, allowing surface operators to control the vehicle while viewing sonar data, camera feeds, and GPS positioning. The unit utilizes two seven-metre Archimedes screws—rotating steel cylinders with corkscrew blades—to navigate the seafloor at eight metres per minute.

Did You Know? The submersible’s onboard hydraulic power unit was repurposed from a decommissioned Scottish North Sea oil and gas vehicle, which was originally engineered to function at depths of up to 350 metres.

Operational Impact and Scale

The transition from manual diving to mechanical dredging represents a substantial increase in efficiency. While earlier manual suction methods removed roughly 70 square metres of seaweed, and initial mechanical trials cleared 2,500 square metres daily, the new prototype is capable of removing approximately 10,000 square metres in a single day.

The machine functions by shaving off the caulerpa plant, including its stolon and rhizomes, along with seven centimetres of surface sediment. This material is then vacuumed to a surface barge, where sand and water are separated and returned to the ocean. Northland Regional Council marine biosecurity manager Dr. Kaeden Leonard described the shift as a move from manual harvesting to large-scale mechanical removal.

Expert Insight: The deployment of this technology signals a shift in biosecurity strategy, moving away from labor-intensive manual removal toward industrial-scale mechanical intervention. By integrating underwater power units with specialized planing heads, the project aims to minimize the human risk associated with diving while maximizing the rate of eradication in dense meadow infestations.

Future Outlook

The long-term efficacy of the dredge planer remains subject to ongoing environmental monitoring and performance assessments. While large areas of caulerpa in the Bay of Islands and around Great Barrier Island have experienced significant dieback—partially attributed to sediment from Cyclone Tam—project lead Derek Richards noted that the seaweed has begun to regrow.

Future operations may depend on how the machine performs in varying seafloor conditions, as its ballast system is designed to balance traction with the need to minimize sediment disturbance. As trials progress, the data collected will likely determine whether this mechanical approach can be scaled to address the broader, multi-site infestations currently facing New Zealand’s coastline.

Frequently Asked Questions

How does the submersible remove the seaweed?
The vehicle uses a planing head similar to a wood planer to shave the caulerpa, including its roots and seven centimetres of sediment, before vacuuming the material to a surface barge for separation.

What powers the vehicle?
It is a tethered robotic submersible that receives power, control signals, and data through a 650-metre umbilical cable connected to a control room on a surface barge.

Why is this machine considered unique?
According to designer Andrew Johnson, the vehicle is a world-first large-scale submersible dredge planer specifically engineered to remove invasive caulerpa without the use of divers, built at a fraction of the cost typically associated with such specialized marine technology.

How might the success of these sea trials influence the management of other invasive marine species in New Zealand?