have transplanted rusty crayfish to new lakes to increase the harvestable resource.

Exotic fish have displaced native species, contributed to the collapse of fisheries, and even led to water quality problems (Magnuson, 1976; see Lake Michigan case study, Appendix A). The common carp, Cyprinus carpio, is not native to this country but was introduced to many northern lakes and rivers in the late 1870s by the U.S. Fish Commission in response to requests from European immigrants. Carp are widely eaten in European countries but are rarely consumed in this country and are not a sought-after game fish. Because carp are benthivorous (bottom feeders) and stir up bottom sediments, they accelerate nutrient recycling from sediments, destroy spawning areas for other fish, and cause turbidity problems in lakes and rivers.

The Great Lakes have a long and unfortunate history of invasions by exotic species. The sea lamprey (Petromyzon marinus), a large parasite of game fish, is a native of the Atlantic Ocean that made its way into Lake Erie through the Welland Canal in 1921. It gradually worked its way as far as Lake Superior, where it remains a significant cause of fish mortality (especially for lake trout). The lamprey has been controlled (but not eliminated) by applying a "lampricide," 3-trifluoromethyl-4 nitrophenol (TFM), to tributary streams where adult lamprey spawn. The TFM selectively kills young lamprey. The alewife, a small forage fish, was also introduced into the Great Lakes inadvertently, as a result of development of the St. Lawrence Seaway. The fish grew to great abundance in the 1960s, and episodes of massive mortality in alewife populations caused problems along urban beaches. The fish was controlled in the Great Lakes primarily by stocking the lakes with other exotic fish, coho, and Chinook salmon.

The latest in a series of exotic species to invade the Great Lakes, and potentially the most devastating, is the zebra mussel (Dreissena polymorpha). First found in Lake St. Clair in 1988, this rapidly spreading species was found throughout the western basin of Lake Erie in 1989 and as far as the Duluth-Superior harbor in western Lake Superior in 1990. The organism was most likely introduced to the Great Lakes by discharge of ballast water from oceangoing vessels. A native of Asia, the zebra mussel has been a problem in European waters for more than 100 years. It is already causing obstruction problems with water intake for power plants and municipal and industrial water treatment plants in Lake Erie. Because fouling organisms historically have not been a problem in inland waters of the United States and Canada, most facilities have not been designed to control or compensate for these problems, and the potential costs are enormous (Mackie et al., 1989). The zebra mussel has become abundant enough that it



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