handle. There is no obvious technology to control Bythotrephes, and there appears to be no predator in the Great Lakes capable of controlling it, except perhaps the alewife, itself an undesirable exotic species whose population is now controlled by large predatory nonnative fish (salmon) that have been stocked in the lakes.

Whereas the zebra mussel and Bythotrephes were introduced to the Great Lakes accidentally, introductions of nonnative species are often intentional. Stocking of fish, and in some cases fish food, in particular, is a common management practice. More than 25 percent of inland fish caught in the United States are nonnative stocks (Moyle et al., 1986). The effects of such introductions often are poorly anticipated and adverse to native populations. Spencer et al. (1991) described a case in which the negative effects of the stocking the opossum shrimp (Mysis relicta), a forage species for fish, extended out of Flathead Lake, Montana, one of the largest natural lakes in the western United States, to eagles and bears that inhabit the area around it. Eagle and bear populations declined markedly as the introduced shrimp reduced the number of salmon, on which the animals had depended as a major food source. The salmon decline was linked to direct competition between juvenile salmon and opossum shrimp, which ultimately depend on the same prey species.

Another major example of the purposeful introduction of nonnative species is the widespread stocking of salmonid fish into lakes in the mountains of western North America. Fishless alpine lakes were stocked with eastern brook trout, European brown trout, Atlantic salmon, arctic char, smallmouth bass, and many other species. Up to 80 percent of the fishless alpine lakes in the U.S. Rocky Mountains and 20 percent of the lakes in Canadian mountain national parks have been stocked with nonnative populations (Donald, 1987; Bahls, 1992). Limnologists have demonstrated that in fishless lakes, the original invertebrate predators, such as large calanoid copepods, benthic crustaceans, and midge larvae, were extirpated by stocked fish (Lamontagne and Schindler, 1994; Paul and Schindler, 1994); this set off changes in communities that increased algal biomass in lakes, even in pristine areas (Leavitt et al., in press). In lakes where fish were present, introduced species often displaced native stocks, in some cases eliminating them completely (see Box 3-10). In other cases, native species were eliminated prior to stocking by deliberately treating the lake with rotenone or toxaphene (Miskimmin and Schindler, 1993, 1994; Miskimmin et al., 1995).

Pacific salmon have also been introduced into the Great Lakes. The salmon were initially intended to replace native lake trout that were lost largely because of the effects of the sea lamprey, which entered the Great Lakes from the Atlantic Ocean through ship canals built in the nineteenth and early twentieth centuries (Christie, 1974). The salmon are maintained only through active management programs. A substantial portion of their

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