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<br />002<173 <br /> <br />more recent times, the climate has been characterized by extreme aridity, Stanford and <br />Ward (1986a) consider the Colorado River basin one of the driest in the world, <br />Because of its geographic isolation, Molles (1980) described the Colorado River as an <br />"aquatic island in a terrestrial sea." The fishes have adapted to a system that <br />historically exhibited a wide range conditions ranging from lacustrine to riverine, and <br />they are considered extreme generalists (Smith 1981), They exploit every available <br />habitat to their advantage, and developed some complex life histories in the process <br />(e.g., see Minckley and Deacon 1991). The geographical isolation of the Colorado <br />River fish fauna suggests that the concepts of island biogeography (insular ecology) <br />provide an appropriate model for understanding the process of endangerment and <br />possible extinction (Smith 1978, Molles 1980). <br /> <br />The native fish fauna of the Colorado River is characterized by a high level of <br />endemism. Of the 467 native fishes (species and subspecies) present in recent times, <br />38 of these are classified as endemic (Miller 1958, Stanford and Ward 1986b). This <br />high level of endemism was heavily influenced by the Quaternary history of the <br />intermountain area of western North America. Populations were isolated by <br />desertification and faunal composition was changed by local extinctions during the <br />Pleistocene (Smith 1978, Stanford and Ward 1986b). At one point, native Colorado <br />River fishes consisted of only 32 to 36 species, depending on taxonomic interpretation <br />(Stanford and Ward 1986b, Carlson and Muth 1989), and they lived in three main <br />habitat types. Native salmonids and sculpins live in cooler headwater or low order <br />streams at high and intermediate elevations. These species also occur, or have close <br />relatives in similar habitats of adjacent basins. A second group consists of daces and <br />minnows in small warmwater streams at low and intermediate elevations. The third <br />group of fishes, which are the focus of this document, inhabit the mainstream river . <br />channels and are called the "big river" fishes. These include seven large fishes of the <br />mainstream channels, and two smaller forms that are restricted primarily to shallower <br />habitat (Minckley et al. 1986). <br /> <br />Decline of Native Fishes <br /> <br />Fishes of the Colorado River basin have not fared well since the time of human <br />settlement. Several fishes, including the Las Vegas dace (Rhinichthyes deacont), <br />Pahranagat spinedace (Lepidomeda a/tive/is), and the Monkey Springs pupfish <br />(Cyprinodon sp.) are recently extinct, and bonytail chub (Gi/a e/egans) may only survive <br />due to hatchery stocks. Of the extant native species, 19 (40%) are federally listed or <br />proposed for listing as threatened or endangered (Carlson and Muth 1989). Declining <br />fish populations of the mainstream ecosystem are not restricted to listed species. The <br />f1annelmouth sucker (Catostomus /atipinnis) and roundtail chub (Gi/a robusta) have <br />been considered as candidates for listing as threatened or endangered species <br />(USFWS 1994). The flannelmouth sucker, which was previously reported in various <br />locations in the lower basin, has been extirpated south of Lake Mead (Minckley 1973). <br /> <br />5 <br />