How did round goby get into great lakes?

Round goby were likely introduced to the Great Lakes via ballast water discharge from transatlantic ships. Ballast water is used to stabilize ships during transport and is pumped into and out of the ship's ballast tanks. Round goby eggs or larvae may have been present in the ballast water and subsequently released into the Great Lakes when the ballast water was discharged. Once introduced, round goby quickly established themselves and became a dominant species in the Great Lakes.