Gorse of the Sea:

The ecology and impact of the invasive kelp Undaria pinnatifida in the Hauraki Gulf

By Kate James, PhD Candidate, Institute of Marine Science, University of Auckland

Undaria pinnatifida is an edible kelp species native to the Japan Sea. In its native range it is commercially grown to produce popular sea vegetable products known as Wakame. Undaria has become established in ten countries outside its native range and has been nominated as one of the one hundred “World’s Worst Invaders” by the International Union for the Conservation of Nature [IUCN]. Undaria has also been named one of the most hazardous seaweeds in Europe and dubbed “the gorse of the sea” here in New Zealand. Since its detection in Wellington in the late 1980s Undaria has become an invasive pest around much of our southern coastlines. Undaria is classified as an Unwanted Organism in New Zealand and is currently the subject of local (experimental) eradication programmes in Fiordland and on the Chatham Islands.

Undaria is a highly adaptable and plastic species which can tolerate a wide set of environmental conditions. It behaves as an opportunistic weed and can rapidly colonise new or disturbed substrata and artificial floating structures such as marinas and marine farms. Undaria is highly fecund, has a fast growth rate and (in New Zealand) multiple reproductive cohorts per year. Undaria can displace and dominate native marine flora during the peak of its growth season. Changes in native community dynamics and trophic food webs occur when native seaweeds are replaced by Undaria and when dense patches of Undaria establish in areas previously devoid of large seaweeds. Undaria can disrupt aquaculture activities and can affect the cultural and recreational (including tourism) values of marine sites. Despite its invasive traits and a desire to keep Undaria out of “high value” marine sites in New Zealand the Ministry of Primary Industries, in 2012, announced three “heavily infested” sites around the South Island where farming of Undaria is to be allowed.

The warmer waters around the upper North Island were thought to be outside the optimal range for Undaria colonisation. But in 2002 Undaria was discovered on mussel farms in the Firth of Thames, in 2004 it was found at Westhaven Marina in the Waitemata Harbour and it has since become well established on both natural and artificial substrata around the Hauraki Gulf. This could mean Undaria farming will be permitted in the Hauraki Gulf in the future.

My work aims to provide a description of the temporal and spatial distribution of Undaria in the Hauraki Gulf as well as gain an understanding of the ecological impacts of its establishment in this warmer region. My survey work has begun to classify the habitats in which Undaria occurs, determine the extent to which Undaria is spreading into natural reef systems (often from mussel farms) and identify the potentially important environmental factors influencing its distribution and spread in the Hauraki Gulf. Undaria population monitoring has been underway since mid-2011 at Westhaven Marina and in the Coromandel Harbour, these studies provide information on the life and reproductive cycles of Undaria on both artificial and natural habitats in this region. Undaria population structure and growth data will be related to local sea surface temperature and seawater nutrient content information and will help identify sites likely to be at risk of Undaria infestation and the potential extent of impacts on ecological and environmental values.

Data on the distribution and biology of Undaria in the Hauraki Gulf is vital if we are to keep marine reserves and conservation sites free of Undaria. This information can also be used to inform policy around the possible commercial farming of Undaria in this region.

I have prepared a report for the Waikato Regional Council which includes some preliminary findings from my work in the Hauraki Gulf. This can be found at: http://www.waikatoregion.govt.nz/PageFiles/26055/TR201315.pdf

General information on Undaria can be found on the New Zealand biosecurity website: http://www.biosecurity.govt.nz/pests/Undaria or on the Global Invasive Species Database at: http://www.issg.org/database/species/ecology.asp?si=68&fr=1&sts=sss&lang=EN

Please feel free to contact me for more information on my work or if you have any information on locations where Undaria is growing in the Hauraki Gulf or further north in New Zealand.

Kate James | This e-mail address is being protected from spambots. You need JavaScript enabled to view it | 0212406442

Thank-you to the Waikato Regional Council and the Auckland Council for supporting this research.