The origin and identity of invertebrate organisms being transported to Canada's Pacific coast by ballast water

We assessed the colonization risk of invertebrate nonindigenous species (NIS) in ballast water being brought into Canada's Pacific coast by indirect and direct methods. Initially we mapped the locations in the northern Pacific Ocean where ships coming into Vancouver Harbour had performed mid-ocean exchange (MOE). Exchange transects (the distance between the start and end positions for the onset and end of MOE) were on average about 400 km long. Samples were also taken from ballast tanks after filling at a northwestern Pacific port and then again after MOE in the mid-Pacific. Invertebrate communities were different pre- and post-MOE, but some coastal organisms were still present after flushing. In addition, samples were obtained from ballast tanks aboard ships in Vancouver Harbour. In low salinity samples (<25e), 13 different taxa were found, and 52 taxa were found in high salinity samples (>25e). Risks could be reduced if MOE was performed in the southerly subtropical domain, south of the subarctic domain and transition zone in the mid-Pacific, or well offshore for north-south shipping routes. For voyages on the west coast of North America, colonization risk of invertebrate NIS is likely higher for ballast water from harbours already colonized by NIS.