Using network analysis to quantify the spread of non-indigenous species through New Zealand's maritime vessel network

The intensification of global trade and travel has resulted in marine non-indigenous species (NIS) being introduced to areas outside their native ranges at increasing rates. Understanding the movement patterns of marine vessel networks can go a long way to predicting sites potentially at risk of NIS incursions in order to help inform management intervention. This study presents an adaptable model that quantifies the invasion risk using proxies for the potential propagule pressure (PPP) of NIS. I used two complementary proxies, one to quantify NIS movement on ship hulls based on total wetted surface area, and a second based on movement within ballast water. Focusing on the New Zealand seascape, I used datasets containing commercial and recreational vessel movements standardised to a one-year period, conducted a network analysis aimed at quantifying the relative spread of NIS through New Zealand as a whole, and a targeted approach at the high-value and important natural heritage site, Great Barrier Island. To account for speciesspecific influences on invasion, key biological traits and associated vessel characteristics were used to parameterise a predictive model of species-specific spread. Across New Zealand, sites with the highest relative risk to future NIS incursions are Auckland, Tauranga, Lyttelton, Wellington and Napier. Importantly, my analyses identified two sites that have a high relative risk to incursions but are currently not prioritised for biosecurity management: Timaru and Whanganui. This approach can be adopted in any jurisdiction across the globe with access to vessel movement information, and used to inform prioritisation for surveillance and pathway management.