Since the opening of the St. Lawrence Seaway in 1959, ballast water release from ocean vessels is the most probable vector for almost two-thirds of invasions, including some of the most notorious Great Lakes invaders: zebra and quagga mussels and round gobies. Strict control measures have been enacted by the U.S. and Canadian governments to eliminate the ballast water release invasion pathway from transoceanic vessels.
- Ballast water exchange (BWE), the exchange of fresh or estuarine water with sea water to purge or kill organisms in ballast tanks, has been required since 1993.
- Saltwater flushing instituted after 2006 further requires sea water rinsing of ballast tanks of all Great Lakes-bound ships, even ships reporting no ballast on board.
- Current regulations do not apply to ships operating exclusively in the Great Lakes (Lakers), which account for 95% of the volume of all ballast water discharges and may facilitate the secondary transport of species that have already been introduced.1
Mapping ballast water invasion risk as a Great Lakes stressor
We mapped the total volume of ballast water discharged at each Great Lakes port as an estimate of propagule pressure. The data were provided by Fisheries and Oceans Canada, which completed a ballast water risk assessment for the Great Lakes in 2012.2 Correction factors of 0.1 and 0.01 were applied to discharge volumes from foreign and coastal vessels, respectively, to account for the reduction in propagules that occurred as a result of ballast water management activities such as BWE or saltwater flushing.
Propagules of invasive species introduced from ballast water release are assumed to be concentrated in the vicinity of the port. Their densities were assumed to become reduced to 10% of their initial abundance within 2.5 km of their point of release and to 1% at 5 km from the point of release.
Spatial distribution of ballast water discharges as a stressor in the Laurentian Great Lakes.