Abstract
Undaria pinnatifida (Harvey) Suringar is an invasive kelp which was first discovered in New Zealand in 1987 in Wellington Harbour. U. pinnatifida is already well established with harbours in New Zealand and is now spreading onto open coastlines in areas of cultural significance and high conservation value. This study focused on the spread of U. pinnatifida during its establishment phase after an introduction by hull fouling at new locations, the expansion of a population along a fjord, and a wave-exposed coastline 12 years after introduction and a recent invasion north.
Surveys of a recent invasion of U. pinnatifida into a remote mooring site at Abrahams Bosom, Rakiura (Stewart Island) demonstrated that within a year of introduction U. pinnatifida had become the third most dense large brown species of seaweed present in the subtidal zone of the mooring site. It has spread ~500m from the mooring site, starting its spread into Port Adventure. U. pinnatifida was found as deep as 6.8 m and was the densest in the shallow intertidal (2 m) within dense native stands of Carpophyllum flexuosum and Cystophora scalaris. U. pinnatifida was also seen attached to those native macroalgae as an epiphyte.
The distribution of an older incursion of U. pinnatifida was determined for 11 sites in Breaksea Sound, Fiordland. U. pinnatifida was first found in Breaksea Sound in 2010 and was found up to 8 m deep in older sites, spreading and establishing in the top layer in new sites. Epiphytic growth was found to be a new invasion strategy for U. pinnatifida, it was commonly seen around Otago and Stewart with U. pinnatifida attached to large brown native seaweeds and other key endemic biotas. There is potential for an increase in harm from shading and dislodgement.
Surveys of U. pinnatifida show the extension of U. pinnatifida in the East Otago Taiāpure between 2009 and 2021 and into the North Coast. Changes in population densities in the depth studies suggest that the low tidal zone is the area of initial invasions with populations extending into greater deeper depths over time. In 2021 populations were found to have homogenised equally over the depths with density ranging between 1-3 plants per m2 after a 12-year invasion period.
Results from this thesis show that U. pinnatifida can expand and establish quickly in pristine environments. When U. pinnatifida invades an environment, it settles in the shallow subtidal and once established settles in the lower subtidal zone. In pristine environments with a healthy dense canopy, U. pinnatifida has shown common epiphytic behaviour. This could facilitate the loss of native seaweed and create gaps in the canopy for further invasion, placing further stress on the native ecosystem.