Undaria pinnatifida


Photo credit: ©Kathryn Birch - CCW
Undaria pinnatifida
  • Opportunistic seaweed able to rapidly colonise new or disturbed substrata and artificial floating structures. It occurs in dense, vigorous stands on benthic shores.

  • Undaria is a large brown species of kelp. The maximum length of Undaria fronds in the environment is 1.5m, but have been known to reach 3m. The seaweed has a branched holdfast (for attachment to the substrate) giving rise to a stipe (stem) just above the holdfast. The stipe has very wavy edges, giving it a ridged appearance. A broad, flattened leaf shaped blade is conspicuous. It has a distinct midrib and the margins of the blade are wavy. Found from low tide level down to 15 or 20 m in clear waters, but the highest biomass is typically between 1 to 3 m in depth.
  • Download N.I.E.A. ID guide

Origin and Distribution:
  • Originally native to Korea, Japan, and parts of China.  The species has been recorded in geographically distant places such as Argentina, Australia, Belgium, California, England France, New Zealand, Netherlands, Mexico, and Spain. The species is also recorded in Britain.
  • Now has been found in Northern Ireland at Carrickfergus marina and in Ireland in Carlingford Lough on the County Louth side.

  • Undaria pinnatifida is described as an opportunistic seaweed and has been observed to colonise new or disturbed substrata and artificial floating structures. It occurs in dense, vigorous stands, forming thick canopy over the biota in a wide range of shores and exposure.
  • With its rapid growth rate, this species can colonise areas not typically inhabited by native seaweed species. This can drive ecosystem changes and Irish species may not be able to adapt to this change, however, the impacts of this species are, at present, poorly understood.
  • It is possible that the presence of Undaria pinnatifida may alter the food resources of herbivores that would normally consume native species. Replacement of native seaweed species that have been grazed (or removed by seaweed harvesting) has also been suggested

Is it found in Ireland or Northern Ireland?
  • It is present in Ireland and Northern Ireland.
  • More information can be found at NBDC and NBN Atlas NI.

Static Distribution Map as of December 2019 - Courtesy of CEDaR

How did it get here?
  • Fouling of sea going vessels and marine aquaculture equipment.
  • Contamination of oyster spat and ballast water. 
  • Potential for intentional introduction.

Prevent Spread:
  • Prevent fouling of boats and equipment
  • Clean all equipment including boats before moving to new waterbodies
  • Report all sightings.
  • Ensure that water equipment (e.g. kayaks, fishing rods, Wellington boots) are clean and dry before using in another water body. See Check Clean Dry for further information.

You can help by reporting any sightings:
Current Legislation Position:
  • Undaria pinnatifida is listed in Schedule 9 of the Wildlife (Northern Ireland) Order 1985 and as such, under Article 15 it is an offence to plant or otherwise cause this species to grow in the wild.

Species Related Files:

Invasive Species Ireland

Invasive Species Ireland