Prioritising domestic locations for marine biosecurity management within a regional vessel network

Growing economic development is dramatically increasing international and domestic maritime trade. A network of vessel traffic facilitates this trade, but also the movement and introduction of marine invaders. Each vessel movement can transport organisms beyond their natural range, threatening destination ecosystems and economies. Understanding vessel movements is key to predicting and managing potential incursions and the continued dispersal of non-indigenous species (NIS). Here, we present a powerful, transferable framework for quantifying the relative likelihood of NIS incursion and vessel-mediated dispersal through international and domestic maritime networks. We used two proxies of potential NIS transfer, derived from the movements of commercial and recreational vessels. One for NIS movements via a ship's wetted hull surface area, and a second via ballast water. We demonstrate our framework with a case-study of New Zealand's marine transport system. Using network analysis, we quantified relative incursion likelihood for each site from 1) international and 2) domestic vessel arrivals. Lastly, we quantified the extent of ‘secondary exposure’ for sites one more stop downstream of the high-risk sites identified in 1 & 2. Sites with the highest relative incursion likelihood were Auckland, Tauranga, Bluff, and New Plymouth. Several domestic sites were critical stepping-stone sites (e.g. Auckland and Tauranga) or local ‘spatial super-spreaders’ of NIS (e.g. Wellington, Lyttelton). We identified several locations experiencing elevated secondary exposure (e.g. Whanganui and Ravensbourne) – sites that may not be flagged by traditional biosecurity prioritisation approaches. This network-based approach is transferable to any jurisdiction with vessel data, to support NIS surveillance and pathway management.