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 its ability to attach to boat hulls (fouling). The seaweed spreads by producing millions of spores and is frequently transported via fouling on vessels or on fishing gear (e.g. cray pots).

Undaria is established in Otago from Waitaki down to Otago Peninsula. It is not established between Otago Peninsula and The Catlins.

• Size: Grows to 1–2 metres long.
• Appearance: Brown, green, and yellow with a crinkly texture and a distinctive midrib running through the frond.
• Distinctive features: Undaria attaches to surfaces via a root-like feature and has a ruffled-looking reproductive structure that produces spores found at the base of the stem.
• Similar species: Looks similar to native kelp Ecklonia radiata but has a more prominent midrib and distinctive sporophyll — a leaf (or leaflike) that produces and carries spores.
• 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 widespread along the Otago coast.

In Otago, undaria is visible year-round, with density and reproductive peaks in spring (September–November) and autumn (March–May).

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

www.orc.govt.nz/undaria