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Underwater Intruders: New Zealand's Challenge with Exotic Caulerpa Seaweeds

Green toxic seaweed floats underwater surface, small fish swimming around, with sun light beams casting down through the water.
Toxic marine species can lie just beneath the surface and can spread easily without notice.

Summer is just around the corner in New Zealand, and there's a real need for us Kiwis to understand the big threats to our seas and marine life. Many communities are directly affected by these issues, and learning about them is a big step towards stopping the spread. Because Naut is passionate about the health of our oceans, in this blog we've broken down the key points you need to know.

Known for their pristine beauty, New Zealand’s waters are facing a new challenge with the arrival of exotic Caulerpa seaweeds. These foreign seaweeds, found in places like Great Barrier Island (Aotea), Great Mercury Island (Ahuahu), Te Rāwhiti Inlet in the Bay of Islands, Kawau Island, and Waiheke Island in the Hauraki Gulf, have the potential to spread quickly and threaten our native species.

Identifying Exotic Caulerpa:

The exotic seaweeds, Caulerpa brachypus and Caulerpa parvifolia, share a close resemblance and can be tricky to identify. They have distinctive green fronds, up to 10 centimetres long, shaped like oar blades, rising from long runners or roots known as stolons. Found growing below the tideline, anywhere between 2 and 30 metres, on both hard and sandy surfaces, they can spread rapidly, forming dense mats covering large areas of the seafloor.

Why These Seaweeds Are a Concern:

The rapid spread of exotic Caulerpa can lead to dense underwater fields, competing with other marine species, including our native Caulerpa species, for space. This competition can upset the balance of our local ecosystems, impacting recreational, cultural, and commercial marine activities.

Our Collective Response in Aotearoa:

In a combined effort, Biosecurity New Zealand, mana whenua, local authorities and other partners are working tirelessly to control this pest where possible. Since its detection at Aotea Great Barrier Island in 2021, consistent efforts have been made to understand the pest, trial treatments, and work towards preventing its spread. This includes implementing legal restrictions on certain boating and fishing activities, as detailed in a Controlled Area Notice (CAN). Rāhui have also been placed over areas with known invasive seaweed presence.

Exploring Solutions:

Experts are desperately looking at ways of eradicating or at very least managing the seaweed. The task, as international experience shows, is quite challenging due to the lack of control tools and the large scale of some of the infested areas in New Zealand. In June 2023, an advisory group assembled to explore the potential of diver-controlled suction dredging to remove exotic Caulerpa.

Educational Campaigns:

Campaigns are rolling out to educate the public about exotic Caulerpa and how to halt its spread. Ongoing surveillance to track the presence of the seaweed is crucial for understanding and controlling its spread.

Call to Action: Find out more information on Exotic Caulerpa

For further information and to stay updated on the situation with exotic Caulerpa seaweeds in New Zealand, you can explore the following resources:

Information Hotline:

Reach out to Biosecurity New Zealand at 0800 80 99 66 to report any sightings or for further information.

How to identify Exotic Caulerpa:

Key Source: Ministry for Primary Industries

- Naut


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