Many species are released into new environments by shipping activities. Since 1959 almost 1/3 of the exotic species in the Great Lakes have been introduced as a result of shipping activities, coinciding with the opening of the St. Lawrence Seaway.
Ocean going vessels are likely the largest vectors of biological invasion. Ships take on ballast water for stability during an ocean crossing and then dump the water before they take on cargo. More than 1 million litres of water are can be discharged by a single vessel. Biological invasions occur when ships discharge ballast water that has been taken from foreign ports. If the introduced aquatic plant or animal species is compatible with the ecological conditions of the lakes, they survive, reproduce, and disperse through the environment. This invasion mechanism has become more significant, as transoceanic crossing times have decreased. The spiny water flea (Bythotrephes cedarstroemi), and the round goby (Neogobius melanostomus) are examples of ballast water introductions into the Great Lakes.