Some nations, and the International Maritime Organization, are moving towards requirements for managing ballast water to reduce the number of alien species transported and released. These and other measures will be most efficient when targeted at ships posing the greatest risks. Here, we analyse world-wide ship movements and port environmental conditions to explore how these risk components differ across arriving ships. Global, with a case study of the Laurentian Great Lakes. We gathered salinity and temperature data for all global shipping ports, and data for all global ship movements during a 12 -month period. We applied these data to the Laurentian Great Lakes to determine which global ports may donate new species to the Great Lakes via ship traffic, and which are most environmentally similar to the Great Lakes. We show that ships regularly travel to the Great Lakes from all major coastal, and many inland, regions of the world. Most global ports, and thus the species in them, are separated from the Great Lakes by no more than two ship voyages. Combined with a measure of environmental similarity among global ports, we identify ship routes likely to transport species adapted for survival in the Great Lakes and global regions that may be the source of increasing future invasions. The Great Lakes account for a small fraction of global shipping yet are closely connected to all other ports, and the species in them, by the shipping network. Our methods and data allow risks from individual ships to be ranked so that management activities can be targeted at ships most likely to introduce new invaders. Because our data sets are global, they could be applied to ship arrivals at any global port.