Ballast water transport of non-indigenous zooplankton to Canadian ports

Ballast water is one of the primary transport vectors for the transfer and introduction of non-indigenous zooplankton (NIZ). Regulations require vessels from overseas to conduct mid-ocean exchange before discharging ballast in Canadian ports. Intracoastal vessels from nearby ports may be exempt from exchange, whereas intracoastal vessels from more distant ports are required to exchange. Zooplankton in the ballast water of transoceanic exchanged (TOE), intracoastal exchanged (ICE), and intracoastal unexchanged (ICU) vessels arriving at Canada's west (WC) and east (EC) coasts were examined. NIZ density, propagule pressure, taxon richness, and community composition were compared among the three shipping classes. The WC ports received greater densities of NIZ and had greater NIZ propagule pressure than EC ports. Within WC vessels, NIZ propagule pressure and density were significantly greater in ICU vessels. TOE vessels on the EC had the greatest NIZ propagule pressure and density. ICU vessels entering Vancouver ports represented the greatest invasion risk to Canadian waters. These vessels likely mediate secondary invasions by facilitating the transport of unexchanged ballast directly from ports previously invaded, whereas short ICU voyage duration enhances organism survivorship and vessels transport NIZ over natural dispersal barriers.