<P> Since the 19th century an estimated 160 new species have found their way into the Great Lakes ecosystem; many have become invasive; the overseas ship ballast and ship hull parasitism are causing severe economic and ecological impacts . According to the Inland Seas Education Association, on average a new species enters the Great Lakes every eight months . </P> <P> Introductions into the Great Lakes include the zebra mussel, which was first discovered in 1988, and quagga mussel in 1989 . The mollusks are efficient filter feeders, competing with native mussels and reducing available food and spawning grounds for fish . In addition, the mussels may be a nuisance to industries by clogging pipes . The U.S. Fish and Wildlife Service estimates that the economic impact of the zebra mussel could be about $5 billion over the next decade . </P> <P> The alewife first entered the system west of Lake Ontario via 19th - century canals . By the 1960s, the small silver fish had become a familiar nuisance to beach goers across Lakes Michigan, Huron, and Erie . Periodic mass dieoffs result in vast numbers of the fish washing up on shore; estimates by various governments have placed the percentage of Lake Michigan's biomass, which was made up of alewives in the early 1960s, as high as 90% . In the late 1960s, the various state and federal governments began stocking several species of salmonids, including the native lake trout as well as non-native chinook and coho salmon; by the 1980s, alewife populations had dropped drastically . The ruffe, a small percid fish from Eurasia, became the most abundant fish species in Lake Superior's Saint Louis River within five years of its detection in 1986 . Its range, which has expanded to Lake Huron, poses a significant threat to the lower lake fishery . Five years after first being observed in the St. Clair River, the round goby can now be found in all of the Great Lakes . The goby is considered undesirable for several reasons: it preys upon bottom - feeding fish, overruns optimal habitat, spawns multiple times a season, and can survive poor water quality conditions . </P> <P> Several species of exotic water fleas have accidentally been introduced into the Great Lakes, such as the spiny waterflea, Bythotrephes longimanus, and the fishhook waterflea, Cercopagis pengoi, potentially having an effect on the zooplankton population . Several species of crayfish have also been introduced that may contend with native crayfish populations . More recently an electric fence has been set up across the Chicago Sanitary and Ship Canal in order to keep several species of invasive Asian carp out of the area . These fast - growing planktivorous fish have heavily colonized the Mississippi and Illinois river systems . The sea lamprey, which has been particularly damaging to the native lake trout population, is another example of a marine invasive species in the Great Lakes . Invasive species, particularly zebra and quagga mussels, may be at least partially responsible for the collapse of the deepwater demersal fish community in Lake Huron, as well as drastic unprecedented changes in the zooplankton community of the lake . </P>

The great lakes and the finger lakes in new york are the result of