The perfect invasion

It’s all eyes on the coastline to find and stop the invasion of an ecologically dangerous seaweed. We follow the exotic caulerpa story so far.

This article was originally published in the September issue of Yachting & Boating Quarterly, Yachting New Zealand's new e-magazine. Click here to read more.

On a cold winter’s day in 2021, an ecologist named Jack Warden was fishing with his father in Okupu/Blind Bay, Aotea Great Barrier Island when, thanks to a lower-than-usual tide, he spotted some unusual weed he hadn’t seen before. Curious, Jack loaded photos of it to an online community called iNaturalist where it was quickly identified as an invasive species of exotic (new to New Zealand) seaweed called caulerpa. This set in motion a response led by Biosecurity New Zealand and Iwi that ultimately resulted in a Rāhui and Controlled Area Notice (CAN) for several of the Hauraki Gulf’s most popular bays to try to prevent the pest seaweed spreading. 

Despite the response, a few months later exotic caulerpa was found in two more harbours on Aotea Great Barrier, followed by a discovery of a small amount at Ahuahu Great Mercury Island. This May it was found in Te Rāwhiti Inlet in the Bay of Islands, followed by the Northern Channel of Kawau Island and Waiheke Island in the Hauraki Gulf.  

New Zealand also has two native species of caulerpa: Caulerpa articulata and Caulerpa brownii. This is why the invasive species, Caulerpa parvifolia and Caulerpa brachypus are referred to as ‘exotic caulerpa’ or ‘invasive caulerpa’.

Described as the ‘perfect invader’ Caulerpa parvifolia and Caulerpa brachypus are native to Africa, Australia and the Pacific Islands, and can thrive in New Zealand conditions. 

The impacts are not only ecological. For the communities on Aotea Great Barrier, the reality of living in the hot zone of an exotic caulerpa response means that because of the biosecurity controls, access to the water is limited, as is the ability to anchor. It’s at best inconvenient, at worst economically challenging for those that make their living from the sea, and heartbreaking for those that love and care for those bays.

We are fortunate that one of the worst species of caulerpa, Caulerpa taxifolia, dubbed the ‘killer algae’ is not in New Zealand so far.

Genetically engineered by the aquarium trade to survive and grow fast in colder water, it was released into the Mediterranean in the 1990s through an outfall from an aquarium in Monaco where it was discovered by a passing diver. It has caused serious environmental problems in waters between France and Croatia. Closer to home, Caulerpa taxifolia was found in New South Wales in 2000 and is now present in 14 estuaries and coastal lakes in the state – where fortunately its impacts are said to be less than first feared. 

 We may never know how Caulerpa parvifolia and Caulerpa brachypus arrived in New Zealand – but there is plenty of speculation in both the scientific and boating communities. It’s plausible that they arrived as a fragment on an anchor or fishing equipment, potentially from the Pacific Islands or Australia where it is native, but not prolific. One theory recently reported in the media is it may have entered our waters in an aquarium on a superyacht. While MPI has confirmed that the species of concern are not sold in New Zealand by the aquarium trade, one expert believes it’s possible that a fragment could have slipped in with the species that are permitted. 

Even before the Waiheke Island discovery, the Ngāti Manuhiri Settlement Trust and the Waiheke Marine Project had worked with Biosecurity New Zealand to bring a group here who had achieved some success in controlling another caulerpa species (Caulerpa proliferia) in California to share their knowledge and expertise.  While their use of suction-dredging achieved some success at one Californian location, this was a smaller incursion in a small, contained body of water, unlike open coastal environments. However, the experience is informing current trials here to see if techniques such as suction dredging and other treatments that are being piloted may be of assistance in New Zealand.  

Like with Mediterranean fanworm, which has been suppressed and even eliminated in many New Zealand harbours, a combination of early detection through surveillance and rapid response is a proven strategy for pest management. It not only provides protection to marine environments by suppressing the population of the pest species, it also buys scientists time to not only develop new techniques to control these species, but to determine the extent of their impacts on our native ecosystems.  

The hunt for exotic caulerpa has so far followed the routes and anchoring spots that are most commonly used by New Zealand’s vessel fleet. While exotic caulerpa can spread via currents and waves, the reality is that vessels can take it to new places more quickly than natural dispersion. That is the reason why so much focus is placed on the checking and cleaning of equipment. 

There is hope. While it’s possible that exotic caulerpa will now always be a part of our ecosystem, in New South Wales the invasive species has not affected native seagrass as much as first feared. Scientists explain that introduced marine species tend to boom then bust on the invasion population curve. ‘Boom’ when they first establish themselves in a new environment and over time there is a decline in abundance.

Key questions for the scientific community and authorities in New Zealand are: how far and fast will it spread? What will its impacts be? And can we control it? Only time will tell. In the meantime, as boaties, playing our part and remaining vigilant is essential.