Unnatural Selection

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 deliberately, 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 navigation 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 populations. 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.

Illinois Commercial Lake Trout Harvest 1930-1992

Source: Ecological Resources, Illinois Natural History Survey, 1994

Exotic species have assumed the roles of major predators and major forage species in the ecosystem, leading to unstable fish stocks with fluctuating population 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 outcompeting 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 lake proper by the early 1960s. The most recent of Lake Michigan's problematic non-natives is the zebra mussel, introduced to the Great Lakes in 1986. Astonishingly prolific breeders, these tiny mussels attach themselves to power plant and water system intakes, among other underwater structures. Costs of killing or screening larvae can be quite high. A recent survey of Chicago-area water treatment plants and industrial boilers found costs per facility averaged well over $500,000.

Zebra mussels are filter feeders that take microscopic particles from open water and expel undigested bits as pseudofeces. Their feeding thus speeds the transfer of food from open water to the lake bottom. Some benthic species--amphipods, planaria, chironomid larvae--benefit from what amounts to the manuring of the lake bottom, but the cleaning of lake water also has perverse effects. It rids open water of plankton that is the base of the food chain that ultimately supports sport and commercial fish, and by allowing more light to penetrate at depth it spurs growth of photosynthesizing macrophytes.

Tighter controls (including restrictions on the dumping of ballast water by ocean-going ships entering the Great Lakes) may help reduce the movement of new species into the Lake Michigan ecosystem. But the zebra mussel occurs in densities of several thousand per square meter, and already threatens to totally displace native mussel species in the Mississippi and Illinois rivers. Because of their sheer numbers and unique biological traits, zebra mussels may be able to reshape the whole lake ecology.

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