. The Changing Illinois environment : critical trends : summary report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. 32 • The Changing Illinois Environment Unnatural Selection 1,200 - S 800 600 - - 400 - 200 At present 17% of the fish species in Lake Michigan are introduced species. Many of them, such as the Pacific salmon (coho and chinook) and brown and rainbow trout, were introduced dehber- ately, usually for sport. All but one of these species must be sustained by stocking, since they do not reproduce natu
. The Changing Illinois environment : critical trends : summary report of the Critical Trends Assessment Project. Man; Pollution; Environmental protection; Ecology; Environmental impact analysis. 32 • The Changing Illinois Environment Unnatural Selection 1,200 - S 800 600 - - 400 - 200 At present 17% of the fish species in Lake Michigan are introduced species. Many of them, such as the Pacific salmon (coho and chinook) and brown and rainbow trout, were introduced dehber- ately, usually for sport. All but one of these species must be sustained by stocking, since they do not reproduce naturally in Illinois' alien waters. Other exotics were introduced unintentionally. The opening of naviga- tion channels between Lake Michigan and the Atlantic Ocean via the St. Lawrence and Welland canals improved shipping at the cost of the lake's native fish popula- tions. Exotic species such as the alewife and the parasitic sea lamprey migrated into lake waters, where they either competed with natives for food or fed on them. These changes had sometimes catastrophic results; the sea lamprey, for example, is thought to have helped extirpate the lake trout by the mid-1950s. Exotic species have assumed the roles of major predators and major forage species in the ecosystem, leading to unstable fish stocks with fluctuating popu- lation densities. For example, the decimation of the lake's natural predators by the sea lamprey in the 1950s allowed populations of the introduced alewife to explode, and they in turn decimated native prey fish species such as the emerald shiner, both by out- competing them for food and by eating their larvae. Emerald shiner were so abundant in the late 1950s that the fish was a nuisance at power stations that drew upon the lake for cooling water, but they had disappeared from the Kike proper by the early 1960s. linois Commercial Lake Trout Harvest 1930-1992. WWII Sea Lamprey Impact Stocked Fisti 1930 1950 The most recent of Lake Michigan's problematic no
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