Japanese kelp
Alariaceae - Undaria pinnatifida

What does it look like?

Japanese kelp 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/yellow colour 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 (they appear 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 – ideally keep fouling growth to no more than a light slime layer.

• Applying good thorough coatings of antifouling paint and keep it in good condition

• Ensuring your hull is clean and free of fouling before you go travel to a new region

• Cleaning and drying any marine equipment (e.g. ropes, lines, pots) before using in a new location.

• Inspecting areas on your boat that retain water in case they’re harbouring marine life.

• Checking anchors, trailers and other equipment for tangled weeds.

Related Links