Published in Conservation Letters earlier this year, the study noted that the Great Lakes-St. Lawrence River basin is the world’s most invaded freshwater system, with ballast water from transoceanic shipping responsible for 65% of invasions in the basin since the modern St. Lawrence Seaway opened in 1959.
It also pointed out that early ballast-water regulations applied in 1993 failed to stem the influx of invasives because they were not mandated for all ships.
That changed in 2006 and 2008, however, when Canada and the US, respectively mandated that all transoceanic ships should conduct open ocean saltwater flushing to ensure that partially filled ballast tanks intended for discharge into the Great Lakes contained salt water before entering the seaway. Both governments have strictly enforced the mandate since.
The study’s 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.
According to the study, “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.”
The authors went on to say, “To our knowledge, the 2006/2008 regulation is the only case of a policy intervention that is linked to a massive reduction of the invasion rate of a large aquatic ecosystem… No other equivalent period of time in the documented history of the Great Lakes basin since 1835 has had fewer invaders discovered than the period of 2007−2019, and not since the Second World War has there been as few ballast-water invasions recorded over a 13-year period.”