Abstract
The invasive kelp Undaria pinnatifida was first detected in the South Island of New Zealand in 1989 and 36 years on from its initial discovery it is now widespread throughout shallow reef habitats. In recent years, U. pinnatifida has become a dominant species within southern New Zealand shallow subtidal reef communities, leading to concerns among local communities around potential impacts. In response, broad-scale control has begun within high-value marine areas, presenting an opportunity to conduct research on U. pinnatifida at a spatial scale never previously done in research on invasive macroalgae. The aim of this thesis was to map the distribution of U. pinnatifida biomass at a high resolution over broad spatial scales throughout different shallow subtidal reefs around southern New Zealand while providing insight into responses to control from manual dive removal. This was achieved by mapping the removed U. pinnatifida biomass from control sites of the Ngāi Tahu Undaria Control Programme. Across two years, 54,733.1 kg of U. pinnatifida biomass was removed from 47 control sites around southern New Zealand allowing distributions to be plotted at a high resolution over 11.3 hectares, providing the first record of invasive U. pinnatifida distribution at a broad spatial scale. Control of U. pinnatifida biomass was conducted by divers manually removing sporophytes into catch bags. To record the location of removed U. pinnatifida biomass divers worked within a five-metre radius of a GPS unit attached to a float, recording the diver location every ten seconds. The wet weight of removed U. pinnatifida biomass was recorded along with the time divers began and finished filling each catch bag. Analysis of the removed biomass showed U. pinnatifida was ubiquitous throughout control sites, being found at levels that make it a dominant component of macroalgal communities within southern New Zealand. The observed distribution of U. pinnatifida across wave-exposed reefs shows that wave action has not limited invasion to wave-sheltered areas and may enhance invasion by frequently removing native macroalgae, creating available space for further invasion. To assess the impact of control on U. pinnatifida at a population scale five repeated control events occurred within Pūrākaunui Inlet in August/September 2022, November 2022, January 2023, November 2023 and January 2024. A rapid response to the initial control event by the U. pinnatifida was observed, as following the removal of 4,768.9 kg in only 55 days an additional 5,141.8 kg of U. pinnatifida biomass was removed from Pūrākaunui Inlet. An additional 1,827.6 kg was then removed in January 2023 with recruitment and growth having occurred during a period when growth of native macroalgal species is typically limited. The observed response in January 2023 provides evidence of U. pinnatifida recruitment and growth during marine heatwave events. Within Pūrākaunui Inlet, a small decline of 7% was observed within areas where U. pinnatifida biomass was removed in both years. Overall, this thesis contributes to the understanding of invasive U. pinnatifida within southern New Zealand, providing insight into the distribution of biomass and response to broad-scale control. The work of this thesis is valuable to the future management of invasive species as it shows how the integration of research alongside broad-scale control programmes can provide valuable insights to improve the effectiveness of future control.