Regional spread of marine non-indigenous species, a pathway modelling approach in New Zealand
The devastating ecological and socio-economic impacts of non-indigenous species (NIS) have been documented worldwide, highlighting the need for effective management. In contrast to terrestrial and freshwater ecosystems, management of marine NIS is relatively new and usually focused on preventing initial introductions at national (e.g., in New Zealand) and state (e.g., in the United States, Australia) borders. However, the intrinsic ‘leakiness’ of these borders and thus, inevitable arrival of new NIS, in addition to the potential spread of those already established, makes regional management an integral component of marine biosecurity programmes.
Even in New Zealand, a leading country in marine biosecurity, the implementation of regional strategies has been difficult, putting New Zealand’s iconic values such as marine biodiversity and aquaculture, at risk. This thesis aims to model and analyse recreational boating, aquaculture and natural currents as pathways for the regional spread of NIS within New Zealand. It uses Golden Bay and Tasman Bay as a case study and applies existing and new modelling and prioritisation approaches.
Chapter 1 presents a succinct introduction to some basic concepts of marine invasions and risk assessment. The term biosecurity is presented, followed by an outline of this system in New Zealand, with an emphasis on the post-border component. The chapter describes Golden Bay and Tasman Bay, including aspects related to its marine biosecurity.
Chapter 2 develops a comprehensive conceptual model representing a range of sequential events that could lead to the release of a NIS into the environment when transported by a recreational vessel to a new area. The model was developed using fault tree analysis and expert input. The results show the complexity of the marine invasion process via recreational vessels, even when only one step of the process is considered. They also highlight the role that other components of the vessel besides the hull could have in the spread of NIS and identify user awareness as a determining factor in the release of a NIS into a new area.
Chapter 3 characterises recreational boating in Golden Bay and Tasman Bay. It introduces the connectivity ranking value and priority ranking value concepts to identify areas within the study region where marine biosecurity surveillance should be a priority. The information required for this analysis was generated through a mail survey with recreational boat users and estimates from a group of experts, which were combined using interval type–2 fuzzy logic. The results show that areas such as Nelson and the Abel Tasman National Park would be comparatively more important than other regions in the spread of NIS within the region.
Chapter 4 uses the Greenshell mussel aquaculture industry to model its potential role as a pathway for NIS. The results identify the components (e.g., farms, hatcheries) and processes, as well as potential vectors (vessels, gear, spat) that define mussel aquaculture as a potential pathway for NIS in this region. The results show that, based on the likelihood of 1) movement between locations, 2) retention of fouling, sediment and/or water, and 3) cleaning between locations, spat is the most important vector of this pathway and thus, a management priority. This chapter also describes some overlaps between mussel aquaculture and other potential pathways such as commercial shipping, recreational boating, public aquaria, and coastal currents.
Chapter 5 presents a 2D advection-diffusion model that was used to investigate the role of currents in the spread of NIS within Golden Bay and Tasman Bay. A current field of 13 years of hourly data is used to analyse the dispersal patterns for four planktonic propagule durations (PPD) between 1–30 days released at 11 different locations. High variability in the results associated with release location, PPD and time period, indicates that the fate of propagules will be species and spatio-temporally dependent. The results present the connectivity pattern for each release location, identifying the role of these locations as source and/or recipients.
Chapter 6 presents a brief summary of previous chapters, highlighting their main aspects. It conducts an initial integral assessment across the pathways recreational boating, aquaculture, and natural currents in the study region by combining the connectivity patterns identified in previous chapters. The results show that when considering all these pathways together, the entire region is interconnected creating a potentially ‘efficient NIS pathway’ (i.e., with the ability to spread a NIS from any region throughout the entire region). Future application of the concepts and results are suggested.