Invasive kelp in Rangaunu and Houhora Harbours

11 Jan 2018

Boaties and cray fishers are being asked for their help to stop a highly invasive kelp – Undaria pinnatifida – spreading into other parts of Northland beyond the Rangaunu and Houhora Harbours, where it has unfortunately already made itself at home. 

Don McKenzie, Biosecurity Manager for the Northland Regional Council, says Undaria (also known as Japanese or Asian kelp) was first detected in Rangaunu and Houhora Harbours about three years ago. 

“It’s a serious biosecurity issue because it can be hard to distinguish from native kelps, is fast-growing (up to one centimetre per day) and in other parts of New Zealand has already impacted on the aquaculture industry, fouling mussel lines and other equipment.” 

Mr McKenzie says the kelp’s rapid growth rate means that as a marine pest it’s every bit as bad – if not worse – than Mediterranean fanworm, another unwanted marine pest currently confined to Whangarei Harbour. 

Undaria’s presence has prompted a renewed call by the regional council for boaties to keep their hulls clean and for cray fishers to check their pots and other equipment if they are in the Rangaunu or Houhora Harbours over summer. 

He says both recreational and commercial vessels (including cray fishers, especially if they’re laying pots) are at risk of unknowingly picking up undaria on pots, ropes and other gear if they’re in either harbour over summer. 

“Both harbours are home to the only known populations of this opportunistic kelp in Northland and the challenge now is to contain it and keep it out of other harbours in the region.”

Mr McKenzie says that’s no easy task as vessels based in the Bay of Islands often travel to and from the Houhora and Rangaunu Harbours, with many commercial fishing vessels also operating out of Houhora. 

“Some of these vessels could well travel to the abundant waters around The Three Kings Islands (Manawatawhi), heightening the biosecurity risk to this high value marine ecosystem.” 

Mr McKenzie says vessel owners have a vital role to play in stopping the spread of marine pests like Undaria and is urging them to clean their boat hulls before they leave infected harbours.

“Council – and the wider community it represents – is reliant on boaties to be vigilant and act responsibly to ensure our marine environment is protected from invasive pests.  I can’t stress this enough; we really do need vessel owners to clean their hulls before they leave Rangaunu and Houhora Harbours.” 

Mr McKenzie says a soft cloth clean would suffice for vessels with a slime layer only. 

“Anything more than that – such as evidence of actual Undaria growth – should be reported to the regional council.  We’ll work alongside vessel owners to deal with this problem on a case-by-case basis.” 

The regional council, along with a number of other regional councils, has also introduced a multi-pronged ‘Clean below? Good to go’ summer awareness-raising campaign. Visit for more information. 

“The inspection of a planned 2000 boat hulls in Northland over summer has been underway since late last year and divers are due to begin vessel inspections in the Houhora Harbour from about 20 January. 

Mr McKenzie says council is also in the process of adopting a new Marine Pathway Plan that introduces new rules on hull-fouling to prevent the spread of marine pests. 

The guidelines are: 

  • Notify the Northland Regional Council if you find Undaria or other marine pests on your hull. Don’t try to remove them yourself
  • Clean your boat hull regularly; keep fouling growth to no more than a light slime layer that can be removed with a soft cloth
  • Apply antifouling paint, ensuring all ‘niche’ areas such as the base of the keel and any hard to access places, are coated and kept in good condition.
  • Make sure your hull is clean and free of fouling before you travel to a new region
  • Clean and dry marine equipment such as ropes, lines or pots, before using them in a new location
  • Inspect areas of your boat, such as live bait tanks that retain seawater, in case they’re harbouring marine life
  • Check anchors, trailers and other equipment for tangled weeds. 

Mr McKenzie says information on a variety of marine and land-based pests is available online via