Efficacy of 'saltwater flushing' in protecting the Great Lakes from biological invasions by invertebrate eggs in ships' ballast sediment

P>1. Mid-ocean exchange and saltwater flushing were implemented as management practices to reduce the likelihood of new biological invasions in the Laurentian Great Lakes associated with ships' ballast water and sediments. Despite this, there has been no formal assessment of the efficacy of these procedures. Here, we conduct a comparative analysis of community composition of dormant taxa transported by ballast sediment before and after regulations came into effect in 2006. 2. Ballast sediment samples were collected from 17 ships during the post-regulation interval of 2007 and 2008. Invertebrate eggs were counted, hatched and species identified in the laboratory. Results were compared to similar samples collected from 39 ships between 2000 and 2002, prior to implementation of saltwater flushing regulations. 3. The estimated amount of residual ballast sediment transported by vessels was significantly lower during the post-regulation period, ranging from < 1 to 45 tonnes per ship, with an average of 5 tonnes. Mean density and number of dormant viable eggs per ship declined 91 and 81%, respectively. 4. Community composition also changed through time, with Rotifera accounting for 78% of taxa transported prior to regulation, whereas Cladocera and Copepoda each accounted for 38% of abundance post-regulation. Although the number of non-indigenous species (NIS) declined 73% per ship after 2006, the reduction was not statistically significant; however, the number of freshwater NIS - which pose the greatest risk of invasion for the Great Lakes - was significantly lowered. 5. Our comparative analysis suggests that ballast management regulations enacted in 2006 markedly reduced the probability of introduction of NIS via dormant eggs carried in ballast sediments.