A NOAA / University of Michigan study released in May 2005 found that:
While carrying goods and raw materials to the Great Lakes, international ships, even though fully loaded with cargo and not carrying pumpable ballast water (no ballast on board or “NOBOB”), still carry aquatic species that can be released into the lakes, … .
NOBOB ships are loaded to capacity with cargo and carry no declarable ballast water on board. However, once they unload their cargo, they take on Great Lakes water for stability. If they then load cargo at another Great Lakes port, they must discharge the ballast water, which now is a mix of Great Lakes water and residual foreign water and sediment and the organisms therein.
About 90 percent of the saltwater ships entering the Great Lakes are NOBOB vessels and are not covered by the ballast water exchange regulations implemented in 1993 by the U.S. Coast Guard. The exchange system requires ships to replace pumpable ballast water at sea with open-ocean water.
It seems from this study that requiring ballast exchanges before international ships can enter the Great Lakes basin can have little to no affect preventing the introduction into the basin of invasive species.
Source: NO Ballast ON Board Final Report(pdf)