Vector control reduces the rate of species invasion in the world's largest freshwater ecosystem

Abstract

The Great Lakes-St Lawrence River basin is the world's most invaded freshwater system. Ballast water release from transoceanic shipping is deemed responsible for 65% of invasions in the basin since the modern St. Lawrence Seaway opened in 1959. Regulations requiring mid-ocean exchange of ballast water applied in 1993 failed to stem ship-mediated invasions because the procedure was not mandated for all ships. In 2006 and 2008, Canada and the United States, respectively, mandated that all transoceanic ships should conduct open ocean flushing to ensure that partially filled ballast tanks intended for discharge into the Great Lakes contained water of salinity ≥30 ppt before entering the Seaway. These regulations have been strictly enforced through record inspections and tests of ballast tank salinities of inbound ships. Before-and-after comparisons of total organismal abundance and species richness in ballast tanks revealed a substantial reduction in invasion risk from ships that conducted saltwater flushing. Since 2006, the rate of discovery of newly established non-native species in the Great Lakes declined by 85% to its lowest level in two centuries. While multiple factors could plausibly contribute to this decline, empirical evidence supports the 2006/2008 ballast water regulation as the primary cause, highlighting the benefit of internationally coordinated vector control.