We might have been able to say 20 years ago that it was unanticipated consequence, but we know now that a shipload could bring in a bunch of invasive species into the Great Lakes.
These species are not inherently bad. They're just in the wrong place.
The damages are far reaching - from the shoreline, to the pipes of power plants and municipal waterworks, to the many other lakes and rivers that are under threat and indeed under harm as zebra mussels and many other species spread from the Great Lakes across the continent.