and few generalizations can be derived from them. Control or management of the exotic species is usually the only practical alternative. Most attempts even to control exotic species in lakes fail. For success, control measures must be specific for the nuisance species and highly effective. The requisite combination of specificity and effectiveness is rarely found. The exotic sea lamprey (Petromyzon marinus) in the Great Lakes has been suppressed (but not eliminated) by the chemical 3-trifluoromethyl-4-nitrophenol (see Lake Michigan case study, Appendix A). In Australia and Papua New Guinea, infestations of the exotic kariba weed (Salvinia molesta) have been controlled (but not eliminated) by the herbivorous beetle Cyrtobagous salviniae (Barrett, 1989; also see discussion of control of aquatic macrophytes, above).

In small lakes and ponds, exotic or nuisance fish are sometimes removed by applying rotenone to kill all fish and then restocking with the desired species (Magnuson, 1976). The risks that accompany this drastic approach make it controversial among lake users, including anglers. The fish community after such treatment has fewer species than the system can support and thus is highly susceptible to invasion. The most likely invaders are undesirable species that lead to long-term degradation of the fishery (Magnuson, 1976). The result is a perpetual cycle of fish removal and restocking, rather than a restored, self-sustaining community.

In some cases, the invading species declines naturally in population after some years, eventually becoming a subdominant member of the community. Natural declines are known for the macrophytes Elodea canadensis in Europe (Hutchinson, 1975) and Myriophyllum spicatum in North America (Carpenter, 1980; painter and McCabe, 1988; Nichols et al. 1991). In the case of M. spicatum, management by dredging, drawdown, mechanical harvesting, and herbicides may actually prolong infestation (Smith and Barko, 1990). In general, the long-term community consequences of macrophyte control are poorly known.

In other cases, species invasions accompanied by extirpations of native species have permanently altered lake ecosystems. Depending on the outcome of efforts to establish reproducing populations of lake trout, Lake Michigan may be an example of a permanently altered ecosystem (see Lake Michigan case study, Appendix A).

Lake Victoria, East Africa, provides a spectacular and recent example of an ecosystem transformed by species introduction. Introduction of Nile perch (Lates nilotica) and Nile tilapia (Oreochromis niloticus), combined with heavy fishing pressure, has depleted native cichlid stocks, and the introduced species now are the mainstay of

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