Connected by maritime traffic: Network modelling to support understanding and management of marine biosecurity risk

Invasive species pose a serious risk to the biological, cultural, and economic value of marine ecosystems, a threat that is increasingly driven by maritime traffic as vessels transport species between domestic and international locations in our connected world. Network models are a useful technique for characterizing this functional connectivity in complex transport systems, and subsequent risk of invasion through these connections. Using New Zealand as a case study, we developed an 'epidemiological' decision-support model to simulate marine invasive species spread under various incursion and management scenarios. This process-based network model estimates relative incursion risks to domestic ports, aquaculture facilities, and coastal areas, factoring in vessel types (commercial, recreational, aquaculture), species growth, in-transit mortality, and recruitment. It tracks the flow of propagules through invasion stages—entrainment, transport, introduction, establishment, and population growth. We used the model to evaluate the effectiveness of pest control and vessel cleanliness on managing several marine pests over a 10-year period, assessing (1) how initial incursion sites affect spread, (2) dominant pathways and downstream risks, and (3) the impact of management interventions on detection and containment. Our findings highlight that vessel type and network position significantly influence translocation distance and intensity. Early simulations suggest that even localized measures, like vessel cleanliness or pest control, can reduce nationwide outbreak severity. The web-based tool, marPEST, enables stakeholders to explore and compare incursion scenarios, guiding biosecurity monitoring and intervention strategies at national scales. This model can be adapted for other regions and supports proactive management of invasive species, preserving ecological, cultural, and economic values.