Controlling Caulerpa in NZ oceans

First detected in New Zealand in 2021, the invasive macroalgae Caulerpa now has a significant presence in the Hauraki Gulf and Bay of Islands. The seaweed spreads rapidly, can smother sessile invertebrates and marine plants on the seafloor, and threatens to displace native habitat that supports taonga species such as snapper.

Doctoral candidate Michele Rogalin-Henderson hopes to change that. She is in the first year of a PhD supported by the Deane Endowment Trust, exploring the biological parameters of Caulerpa to gain an understanding of how it spreads and what the ecological impact might be.

It’s a dream project for the American-born researcher, who aspired to be a marine scientist as a child, diving and sailing Long Island Sound out of Westport, Connecticut.

“I did my first dive when I was ten and fell completely in love with it,” she says. “I’ve been a sailor and a diver most of my life – I actually sailed here to New Zealand from overseas.”

Having spent decades exploring the underwater world, she witnessed first-hand a decline in ocean health. Wanting to be part of the solution, she left her career as a nursing lecturer and enrolled in a Master of Ecology in marine science at the University of Auckland, followed by a PhD.

“Caulerpa started messing up my playground and I saw a lack of evidence-based decisions being made. I thought, ‘Somebody's got to do something … I’m somebody’. Making a meaningful contribution to the health of our oceans was mission critical for me at that point.”

During her masters Michele investigated Caulerpa fragments and found that fragments as small as two millimetres could grow and establish, with her findings directly informing public policy. She also looked at thermal shock treatment as an effective method for destroying Caulerpa, with promising results. But, as the species grew more pervasive, her focus shifted.

“Unfortunately, we aren’t going to get rid of Caulerpa; that ship has sailed. Now I’m focused on understanding it to help us make better management decisions.”

To do so, she led a series of experiments which have shed light on the biology of Caulerpa parvifolia, one of the two invasive exotic species in New Zealand. Data gathered highlights the seaweed’s resilience, finding it is much more cold-tolerant than previously thought and can withstand temperatures as low as six degrees Celsius. It can also survive in complete darkness for as long as 16 weeks and recover when returned to normal light.

Other findings revealed ways in which the seaweed is easily transported to new locations: it can survive on boat anchor chains for up to 12 hours, and fragments can float in moving water and reestablish once the current stills. Through the latter method, fragments in Australia moved several thousand kilometres and successfully reestablished.  

These observations will be crucial in supporting Caulerpa control efforts, emphasising simple management techniques such as advising boats to check and clean anchors before moving.

The research wouldn’t be possible without philanthropic support, says Michele.

“I am so grateful to the Deane Endowment Trust and really appreciate their support of my PhD. There isn't always enough government funding to go around, so it's important to acknowledge that funding like this is a serious part of why researchers can do this type of work.”

Michele is now replicating her experiments on the species Caulerpa brachypus and will compare findings with her existing work, while also investigating how both species interact with New Zealand’s native seagrass.

Additional Caulerpa research led by Caitlin Blain, Aven Zhuang and Cody Brooks and supported by the Chisholm Whitney Charitable Trust is also underway at the University.