Marine species are in constant motion in the ballast water and on the hulls of the ships that ply the world's oceans; ships serve as a major vector for biological invasions. Despite federal and state regulations that require ballast water exchange (BWE), particular trade routes impose geographic and temporal constraints on compliance, limiting whether a ship can conduct BWE at the required distance (>= 200 nautical miles) from shore to minimize transfers of coastal organisms. Ships moving across the Americas are largely unable to conduct open-ocean BWE, but instead often conduct exchanges inside coastal waters. Overall, strong differences exist in volumes, geographic sources, and the use of BWE for ballast water discharge among the three major coasts of the contiguous United States. Such patterns suggest important geographic differences in invasion opportunities and also argue for more effective alternative ballast water treatments that can be applied more evenly.
