the lake's fishery (Ogutu-Ohwayo, 1990). Although the fishery is economically successful, local processing techniques (frying and smoking) consume large amounts of wood, which is a scarce resource in the region (Bruton, 1990). The initial high productivity of the fishery was due in part to the high biomass of the native cichlid forage, but after depletion of the forage, growth rates and condition factors of the introduced predators declined (Ogutu-Ohwayo, 1990). The productivity of the fishery is likely to decline. Bruton (1990) is pessimistic about prospects for restoring the native fish community. Ogutu-Ohwayo (1990) noted that restoration of ancestral stocks would deprive an impoverished region of an important local industry and source of protein.

Introductions of exotic species sometimes lead to economic benefits (as occurred initially in Lake Victoria). However, economic benefits may be short-lived because of instabilities in the population density of the invader. In general, economic benefits deriving from the invasion must be balanced against the long-term costs of stabilizing the ecosystem. Lake Michigan (see case study, Appendix A) is an excellent example of an ecosystem with a profitable fishery for exotic species, sustained at the cost of perpetual management. Lakes dominated by exotic species tend to be more variable and less predictable than lakes that lack exotics. Unpredictability adds to the cost of management (Walters, 1986).

Acidification

The long-term solution to lake acidification, of course, is to decrease emissions of sulfur and nitrogen oxides to the atmosphere, because these are precursors of the sulfuric and nitric acids that cause acidic deposition. Control actions in the United States over the past 15 years already have had significant effects; emissions of SO2 and NOx peaked in the late 1970s and declined nationally by 10 to 20 percent from 1975 to 1985. Much higher percentage decreases were achieved in some states during the 1980s. The recently enacted Clean Air Act of 1990 (P.L. 101–549) mandates a further reduction in SO2 emissions of 10 million metric tons over the next 10 years (from current rates of nearly 21 million metric tons). These reductions will decrease the amount of acid deposition sufficiently to reverse the acidification process in some impacted lakes but will not be sufficient to restore all acid-sensitive systems (NAPAP, 1990b).

Liming is by far the most common in-lake restoration technique for acidified lakes, and a large amount of experience with this method has accumulated over the past decade. Liming is common in Scandinavia



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