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common carp), as a biological control (e.g., the mosquitofish, Gambusia affinis), as bait <br />fish (e.g., sheepshead minnow, Cyprinodon variegatus), and as "forage" for other <br />introduced sport fishes (e.g. gizzard shad, Dorosoma cepedianum) and rainbow smelt. <br />All of these introduced fishes, and many others, have become abundant in river systems <br />throughout the United States. Although these introduced fishes have some practical or <br />aesthetic value, all have caused problems in natural ecosystems and resulted in <br />unanticipated economic and environmental costs ( USOTA 1993, Taylor et al. 1984, <br />Courtenay and Robins 1989). Even seemingly innocuous species like grass carp <br />(Ctenopharymogodon 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 al. <br />1995). <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. In complex native fish communities, it is <br />difficult to predict the outcome of introducing a new species (Li and Moyle 1981), but <br />most introductions prove harmful and have unanticipated (usually adverse) effects on <br />native communities. In general, the biological system receiving the invader will be <br />altered, typically by displacement of a native species (Li and Moyle 1981, Courtenay and <br />Robins 1989, Courtenay 1993). The problems caused by many introduced plants and <br />animals seem to worsen with time (Leopold 1949, Laycock 1966). In the extreme, <br />introduced species can cause the local extirpation or extinction of native species with <br />resultant loss of biodiversity (reviewed by USOTA 1993). <br />Threats to the Native Missouri River Fishes <br />Data and other information needed for a direct evaluation of adverse interactions <br />between native Missouri River fishes and nonnative introductions is generally lacking. <br />However, some comparisons can be made with other areas where problems due to <br />competition and predation between introduced and native species have been <br />documented. In general, aquatic introductions:"... have been harmful; in some cases <br />they have been catastrophic." (Fuller et al. 1999). <br />The fish species most likely to cause problems are well known. According to the <br />ANSTF (1994) nonnative fish introductions related to sport fishing were most frequently <br />cited as a reason for listing fishes under the Endangered Species Act (73% of all fish <br />listings) : <br />"Members of the bass and sunfish family (Centrarchidae) were the most <br />frequently cited sport fish group contributing to ESA listings. The largemouth bass <br />(Micropterus salmoides) was the most frequently cited individual species. Other <br />centrarchids included green sunfish, bluegill, crappie, "other sunfish," and <br />smallmouth bass. Members of the freshwater catfish and bullhead family <br />(Ictaluridae) were the second most commonly cited group. Channel catfish <br />14 <br />