A small clump of suspected Caulerpa brachypus spotted on a New Zealand beach is prompting Biosaecutiy New Zealand to issue a warning to boaties and fishers.

That warning is to clean their anchors and fishing gear and reduce the risk of introducing or spreading any exotic seaweeds.

Biosecurity New Zealand director of readiness and response John Walsh says a local resident who had been at a beach at Te Rāwhiti, near Russell, did the right thing by taking a photograph of the 20cm seaweed clump they found and sending it in for visual identification.

"We did provisionally identify the seaweed as Caulerpa brachypus based on the photograph, but unfortunately the seaweed was no longer there when the person returned, and no more has been found on the beach, so we have been unable at this stage to collect a sample for confirmation testing," says John.

"We do have a small team up there today doing beach-based surveillance.

"We know that exotic Caulerpa can be spread through breaking into little pieces and can potentially spread over large distances by rafting on floating debris.

John says this can happen by wave action, or when anchors and fishing gear are moved into or through weed beds.

"Which is why keeping your boat and equipment clean is the best thing you can do to avoid spreading marine pests and diseases, including exotic Caulerpa.

"We have the strictest biofouling requirements for vessels in the world, as shown over the last summer cruise season, to help protect our marine ecosystems but it's important everyone plays their part."

John says if people think they've found this species of Caulerpa, they should not move it elsewhere but let Biosecurity New Zealand know.

"We thank the person who notified us in this instance."

John says Biosecurity New Zealand has spoken to the Northland Regional Council and local iwi and will work closely with them and others to determine next steps in Te Rawhiti, including having divers in the water later next week looking for any further signs of Caulerpa brachypus, weather permitting.

"Exotic Caulerpa was first found at Aotea Great Barrier Island in July 2021, and then, later, at Ahuahu Great Mercury Island. It is very difficult to remove as it travels and grows easily, and there are limited treatment options.

"To minimise the spread, a controlled area notice (CAN) remains in place on three affected harbours at Great Barrier Island and over an area of the south western coastline of Ahuahu Great Mercury Island, which bans activities likely to result in its movement (fishing and boating).

"Mana whenua for the islands have imposed a rāhui on the same areas.

"At the original finds, we worked closely with Aotea and Ahuahu mana whenua and the local communities, along with Auckland Council, the Waikato Regional Council, and the Department of Conservation to collectively decide a course of action."

John says if Biosecurity New Zealand establishes through surveying that exotic Caulerpa is in Te Rāwhiti, then a CAN may be an appropriate response.

"But we will work with the community and seek advice from experts here and overseas.

"We continue to investigate possible treatment options for Caulerpa and are seeking advice regarding suction techniques and whether they could be viable in our conditions."

John says Caulerpa was included in Biosecurity New Zealand's national marine high-risk site surveillance programme, which surveys locations where there is a high volume of vessel traffic every six months.

Whāngarei Harbour and Ōpua Marina/Waikare inlet in Northland are locations in this programme.

People can report suspected finds to Biosecurity New Zealand on 0800 809 966 or by completing the online reporting form at report.mpi.govt.nz .

Help stop the spread:

1. Check your gear – anchors and chains especially. If you see any seaweed on your equipment, chuck it straight back.
2. Know where Caulerpa is
3. Know the rules about anchoring and fishing there

Caulerpa brachypus and Caulerpa parvifolia

Caulerpa brachypus and Caulerpa parvifolia are seaweeds exotic to New Zealand. Exotic means they have come from overseas. They are native to the Indo-Pacific region, ranging from Africa to Australia, the Pacific Islands, and southern Japan. Caulerpa brachypus is considered an invasive pest in Florida, the United States, and Martinique in the Caribbean.

Both seaweeds are closely related and appear identical. They have fronds up to 10 centimetres long that rise from long runners or roots known as stolons.

They can be found growing below the tideline at between two metres and 30 metres on both hard surfaces and in sandy areas.

In favourable conditions, they can spread rapidly, forming vast, dense beds or meadows.
How exotic Caulerpa species can spread

The two seaweeds can be spread through breaking into little pieces. This can happen, for example, by wave action or when anchors and fishing gear are moved into or through weed beds.

Fragments are also carried easily on coastal currents.

Pieces can get tangled in or stuck on equipment (for example, nets, dive and fishing gear, and crayfish pots). It can survive out of water for up to a week or more if it's in a moist location - like in an anchor locker or a bunched-up fishing net.