Underwater aliens : quantifying propagule pressure of aquatic invasive species in Canadian shipping ports

Increased trading worldwide has created introduction pathways for aquatic invasive species (AIS), particularly through shipping activities. In our research, we used ballast discharge data and estimates of the wetted surface area of vessels to provide preliminary estimates of the potential propagule pressure ballast and hull fouling organisms on Canadian shipping ports from the commercial shipping sector. We compared total wetted surface area, vessel arrivals and ballast discharge across shipping ports and vessel categories in the Atlantic, Great Lakes-St. Lawrence and Pacific shipping regions. Using these potential propagule pressure estimates for ballast organisms, we developed a model, building on that of MacIsaac et al.’s (2002), to characterize the effective propagule pressure of aquatic non-native species to Canadian shipping ports. Our model includes the effects of environmental similarity between destination and source on mortality, which can be considerable. We parameterized the model using recent nationally and regionally collected databases on ship voyages, and abundance surveys yielding mortality rates of several zooplankton species. These empirically derived parameters were used in our model to predict abundances of live individuals after ballast is discharged, with estimates of uncertainty and sensitivity to key assumptions. Our results indicate that for our three shipping regions, aggregate wetted surface area, vessel arrivals, and total ballast discharge were significantly correlated across shipping ports (Spearman’s ρ ranged from 0.57 to 0.87, p