They are just doing what comes naturally in exploiting the environment. Certainly, the most prominent invader in the Great Lakes has been the zebra mussel, a one-inch mollusk first detected in Lake St. Clair in 1986 that now infests waters from Vermont to Oklahoma. The zebra mussel arrived from Europe in the ballast water of oceangoing freighters. With no natural predator to control its proliferation, zebra mussels have caused massive damage to the Great Lakes ecosystem by eliminating native mussels and toxic algae blooms. An almost equal number of plant species also have invaded the Great Lakes watershed, he said.
Normal migration happens all the time.
While normal migration usually allows the local ecosystem time to absorb the newcomers and adjust to their presence, the artificial movement perpetuated by people generally overloads the ecosystem and results in problems, Monostory said. Two other aquatic species that have led to a decimation of the Great Lakes' commercial and recreational fisheries are sea lampreys and ruffes. Although an historical native of Lake Ontario, sea lampreys moved into the Upper Great Lakes after the Welland Canal opened in 1929, causing a near collapse of lake trout and whitefish populations, which at one time were the lakes' two largest commercial fish. The ruffe, another European fish, was first spotted in Duluth Harbor in the early 1980s, also finding its way to the United States in ballast water. It has been blamed for the precipitous decline of nine native fish species and is now more abundant than all other fish species combined, Monostory said.