Undaria is a highly invasive seaweed that can form dense underwater forests, outcompeting native species for light and space. It poses a significant threat to marine ecosystems and can be a nuisance for marine farms, increasing labour and harvesting costs due to fouling issues. The seaweed spreads by producing millions of spores and is frequently transported via biofouling on vessels.

Undaria is regionally established in Otago from the Waitaki down to Otago Peninsula. It has not been established from Otago Peninsula south to the Catlins.

• Size: Grows between 1–2 meters long.
• Appearance: Brown, green, and yellow with a crinkly texture and a distinctive midrib running through the frond.
• Holdfast and Sporophyll: Attaches to surfaces via a holdfast and has a spiral reproductive sporophyll at the base.
• Similar Species: Looks similar to native kelp Ecklonia radiata but has a more prominent midrib and distinctive sporophyll.
• Habitat: Grows on hard surfaces including reefs, ropes, wharfs, vessel hulls, and moorings. It tolerates a broad range of temperatures and light levels and is prolific along the Otago coast.

Late summer to early autumn (January to April in Southern Hemisphere). This period often coincides with peak settlement and when individuals are most visible. Growth is typically rapid during warmer months, making colonies more prominent.

It is an offence to breed, knowingly communicate, exhibit, multiply, propagate, release, or sell, an unwanted organism (Biosecurity Act 1993).

ORC is currently developing a marine biosecurity programme and needs to obtain more information on species distributions before pursuing management options. If you think you have spotted Undaria please let us know by calling 0800 474 082 or emailing biosecurity@orc.govt.nz.