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<br />,.,... 2' 7'1 <br />uU '1 'J <br /> <br />have become very abundant in river systems throughout the US. These introduced <br />fishes have some practical or aesthetic value, but all have caused problems in natural <br />ecosystems and resulted in unanticipated costs (USOTA 1993, Taylor et al. 1984, <br />Courtenay and Robins 1989). Even seemingly innocuous species like grass carp <br />(Ctenopharyrodon idella), which was introduced in 1963 to control aquatic vegetation <br />(Stanley et al. 1978), is now suspected of altering native fish communities (Raibley et <br />al. 1995). <br /> <br />If the impact of introduced species were simply to add some individuals to the <br />existing biological community, it would be less of a cause for concern. Introductions do <br />not happen in a biological vacuum, however. The complexity of fish communities, for <br />example, makes it difficult to predict the outcome when introducing a new species in a <br />native community (Li and Moyle 1981). Most introductions prove harmful and have <br />unanticipated, and usually adverse, effects on native communities. The biological <br />system receiving the invader will be altered, typically by displacement of a native <br />species (Li and Moyle 1981, Courtenay and Robins 1989, Courtenay 1993). The <br />problems caused by many introduced plants and animals seem to worsen with time <br />(Leopold 1949, Laycock 1966). In the extreme, introduced species can cause the local <br />extirpation or extinction of native species with the result that biodiversity is decreased <br />(reviewed by USOTA 1993). <br /> <br />Native freshwater fish communities have been affected severely by human <br />actions (Miller 1972, Williams et al. 1989). During the last 100 years, 27 of the native <br />North American fishes (N= 1,003) have become extinct and 265 species are threatened <br />with extinction (reviewed by Wilson 1992). The demise of species has been linked with <br />more than one factor, but displacement by an introduced species was cited an agent in <br />the decline of a native species in 68% of cases reported; introductions were about as <br />important as destruction of physical habitat (73%; Wilson 1992). In a review of 31 case <br />studies in which fishes had been introduced into stream communities, Robs (1991) <br />found that 23 (74%) caused declines in native fish populations. Native populations <br />also may experience reduced growth and survival rates as a result of their introduction <br />(Moyle et al. 1986). <br /> <br />The adverse effect of introductions on the native fish fauna has been recognized <br />by the American Fisheries Society (AFS), which is the premier fisheries organization in <br />North America. While acknowledging the benefits that stocking has brought to <br />recreational and commercial fisheries, the AFS states that stocking has had <br />"undesirable effects on native species" and that stocking policy should be tempered by <br />the need for preserving biological diversity (Starnes et al. 1996). <br /> <br />Biological diversity is at risk when nonnative species are introduced. Biological <br />diversity is not simply the number of species present, but also encompasses the <br />"ecological roles they perform, and the genetic diversity they contain" (Wilcox 1984), <br /> <br />2 <br />