Undaria seaweed

What does it look like?

Undaria seaweed grows from the low intertidal area to subtidal depths of around 15 metres. It grows on any hard surface including shells, reefs, ropes, wharf piles, vessel hulls, moorings and other artificial structures. It can form dense "forests" in sheltered reef areas.

The appearance of Undaria pinnatifida differs depending on its maturity. Mature plants are a brown, green and yellow and grow up to one to two metres. They have a very visible midrib up the plant. They have a holdfast (which anchors them), a stipe (or stem) and a sporophyll (a spiral shaped reproductive structure which produces spores) found at the base of the stipe.

Juvenile plants have a holdfast and stem and an undivided blade which appears as a single leaf. The distinctive midrib starts becoming apparent once the plant grows over five centimetres. The species can look similar to the New Zealand kelp Ecklonia radiata, however Japanese kelp has a distinctive midrib up the middle of the blade plus the distinctive sporophyll (spores on underside of leaves).

Why is it a problem?

• The impacts of Japanese kelp are not well understood and are likely to vary considerably depending on the location. It can change the structure of ecosystems, especially in areas where native seaweeds are absent and potentially impact on biodiversity.
• The weed also has the potential to become a nuisance for marine farms by increasing labour and harvesting costs due to fouling problems

Control methods

You can avoid spreading marine pests by:

• Regularly cleaning your boat’s hull – keep fouling growth to no more than a light slime layer.
• Applying a thorough coating of antifouling paint and keep it in good condition.
• Ensure your hull is clean and free of fouling before you travel to a new area.
• Clean and dry any marine equipment (e.g. ropes, lines and pots) before using in a new area.
• Inspect areas on your boat that retain water for signs of marine life.
• Check for aquatic weeds tangled around anchors, trailers and other equipment.

Related links

• Marine biosecurity
• Ministry for Primary Industries
• New Zealand Plant Conservation Network
• Unwanted Organisms Register