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<br />lJ(J2433 <br /> <br />SECTION I. INTRODUCTION <br /> <br />The endemic fishes of the upper Colorado River Basin (UCRB; Figure 1) are <br />declining in abundance. The problem is particularly acute for the so-called "big river" <br />fishes that occupy main channel habitat. Four of the seven large species are <br />endangered and federally listed. The other three are considered species at risk. <br />Significant anthropogenic changes to the physical habitat have undoubtedly played an <br />important role in the decline of these species, but changes in the biological <br />environment may now be equally significant. Nonnative species have been introduced <br />into the Colorado River Basin and have been so successful that they have displaced <br />native species in some river reaches. However, the problem of introduced species is <br />pervasive and by no means confined to the UCRB. Before reviewing the problems that <br />nonnatives have caused in the UCRB, it is instructive to review the issue in a broader <br />geographical and biological context. <br /> <br />Many nonnative species have been introduced into North America. A recent <br />study by the Office ofTechnology Assessment (USOTA 1993) concluded that <br />introductions of harmful species have produced cumulative impacts and "are creating a <br />growing economic and environmental burden for the country." The report states that <br />4,500 foreign species have established populations in the United States, Many of <br />these introductions were intentional, but others occurred unintentionally through human <br />activities (Taylor et al. 1984). <br /> <br />Humans have a penchant for supplementing their local biological environment <br />with imported plants or animals that are perceived to have special beauty or <br />usefulness, or are simply reminders of a pleasing biological environment in another <br />geographic location. Exotic species were introduced from Europe and other continents <br />with the intent of benefiting their "new ecosystem," but most exotic species were the <br />bane of, rather than a benefit to, the new environments (Kurdila 1988). There are <br />many examples of these harmful introductions. The melaleuca (Me/a/euca <br />quinquenervia) was imported because it grows quickly and is fire resistant, but the tree <br />is having devastating effects on the Florida Everglades. The attractive purple <br />loosestrife (Lythrum salicaria) has become a major wetland weed and the water <br />hyacinth (Eichhornia crassipes) is choking waterways. The salt cedar (Tamarisk sp.) is <br />displacing native riparian vegetation in arid regions and changing river geomorphology <br />(USOTA 1993; Graff 1978). Introduction of the zebra mussel (Dreissena polymorpha) <br />has imposed enormous economic hardships on water and power industries particularly <br />in the Great Lakes region (Nalepa and Schlosser 1993). <br /> <br />Fish species have been introduced intentionally for such diverse purposes as <br />food resources (e.g., the common carp Cyprinus carpio), for biological control (e.g., the <br />mosquitofish, Gambusia affinis), or unintentionally by release of bait fish (e.g" <br />sheepshead minnow, Cyprinodon variegatus). These examples, and many others, <br /> <br />1 <br />