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<br />002472 <br /> <br />SECTION II: SCOPE OF THE NONNATIVE PROBLEM: <br /> <br />General <br /> <br />Problems caused by introduction of nonnative species into aquatic habitats have <br />become a national and international concern. In an extensive review of fish <br />introductions in the United States, Taylor et al. (1984) stated that harmful effects to <br />native populations should be a "foregone conclusion." The evidence was sufficiently <br />compelling that these authors believed a "no effects. argument would be implausible to <br />the point of straining "one's confidence in ecological principles." The scope of the <br />problem is captured in a document prepared by the national interagency Aquatic <br />Nuisance Species Task Force (ANSTF 1994). <br /> <br />"By competing for resources, preying on native fauna, <br />transferring pathogens, or significantly altering habitat, the <br />introduction of a nonindigenous species may work synergistically <br />with other factors, such as water diversions or pollutants, to alter <br />the population and distribution of indigenous species. The <br />factors are often cumulative and/or complementary. For <br />example, habitat degradation may make a species more <br />vulnerable to the introduction of nonindigenous species." <br /> <br />The ANSTF (1994) review found that the species cited most frequently for <br />endangering native fishes nationwide was the largemouth bass (Micropterus <br />sa/moides), but other centrarchids such as green sunfish (Lepomis cyanellus), bluegill <br />(L. macrochirus), crappies (Pomoxis spp.), and small mouth bass (M. d%mieu) were <br />also contributors. The family Ictaluridae, which includes channel catfish (/cta/urus <br />punctatus) and bullheads (Ameirus spp.), was the second most cited group. Smaller <br />species, including the red shiner (Cyprinella lutrensis) and fathead minnow (Pimepha/es <br />prome/as) also were mentioned (ANSTF 1994). All of these species have been <br />introduced into the Colorado River Basin, either for recreational sport fishing or as <br />baitfish (Miller 1952, Minckley 1982, Tyus et al. 1982). <br /> <br />Endemic Colorado River Fishes <br /> <br />The scope of the nonnative fish problem in the UCRB is best understood by <br />considering the evolution of the native fauna, which originated in a system that was <br />very different than the one that exists today. Native Colorado River fishes had a long <br />evolutionary history of adaptations to a river system characterized by extreme seasonal <br />variations in flow and generally turbid water. Peak flows produced extensive seasonal <br />flooding of low-lying areas. Smaller tributaries were subject to flash flooding after <br />unpredictable storm events. In the geologic past, the river system was wetter, and large <br />lacustrine areas were prevalent (Minckley et al. 1986, Stanford and Ward 1986a). In <br /> <br />4 <br />