Relative risk assessment for ballast-mediated invasions at Canadian Arctic ports

Vector-based risk assessment is a powerful and efficient management approach for nonindigenous species (NIS). By managing a vector, an entire assemblage of associated NIS is simultaneously considered. The majority of current risk assessment frameworks have been conducted for a single, or selected few, target species and thus are not useful for managing vectors transporting a large number of potentially unknown species. Here we develop a predictive framework to assess relative invasion risk for a vector (ballast water) transporting an unknown species assemblage, using the Canadian Arctic as a case study. Ballast water discharge is a known high-risk vector globally, but its magnitude in the Arctic has not been well characterized. Our framework determined relative invasion risks between different transit pathways by quantifying the probability of NIS successfully transiting all stages of the invasion process and the magnitude of consequences of introduction to those ports. Churchill, Manitoba was ranked at 'higher' invasion risk via ballast water discharged by international merchant vessels than any other recipient port studied. The overall pattern of ballast water discharge suggests that water originating from coastal domestic sources carried by international merchant vessels may be important for dispersal of NIS. In addition, ballast-mediated NIS are more likely to arrive to the Hudson Bay region during summer months. These results can be useful for developing prevention and early detection programs for the region. Our risk assessment framework is not limited to ballast water and could be applied to other vectors for effective management of NIS.