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The fish fauna was first described in the middle to late 1800s in reports by naturalists assigned to military or exploratory expeditions (summarized by Evermann and Rutter 1895). Fishes native to the basin include 36 species, 20 genera, and 9 families (Table 2). Many species are poly- typic within the basin. Machete, striped mullet, and spotted sleeper are marine or brackish-water fishes that enter the Colorado's delta and ascend into the Lower Mainstem Colorado. Occurrence of spotted sleeper in the lower basin was documented by a sin- gle specimen (Hubbs 1953), and Minckley (1979) listed this species as "hypothetically" present. Ten of the remaining freshwater species are known from adjacent river basins. Of these, cutthroat trout, speckled dace, and Sonoran topmin- now have subspecific representatives that are endemic to the basin. One isolated population of desert pupfish occurs natu- rally outside the basin (Miller 1943; Hubbs and Miller 1948; Miller 1981). The other 23 freshwater species are endemic and form an unique assemblage of highly special- ized and unusual fishes. Endemic fishes account for 64 % of all native species (35 % of all native genera), constituting one of the highest levels of endemism known in North America (Miller 1959). Most other native species are represented by endemic subspecies. Native fishes were never ubiquitously distributed throughout the basin and were associated with specific sub- basins and habitat types. Many species had extremely nar- row distributions within the basin (Table 2). An average of about 10 species probably occurred per major river drain- age, with a range of 5 (Bill Williams drainage) to 18 or 19 (Gila River drainage). Except for mainstream species, there have always been distinct differences between upper and lower basin fish faunas. Five species (cutthroat trout, moun- tain whitefish, mountain sucker, mottled sculpin, and Paiute sculpin) occurred only in the upper basin and were essen- tially restricted to headwaters. These fishes are very similar TABLE 2. Native fishes of the Colorado River Basin, including their current federal legal status and historic distributionllocation in the basin (Evermann and Rutter 1895; Ellis 1914; Beckman 1952; Koster 1957; Miller 1959, 1961, 1972; Sigler and Miller 1963; Miller and Lowe 1964; Rinne 1976; Bailey et al. 1970; Baxter and Simon 1970; Rinne and Minckley 1970; Deacon and Bradley 1972; Minckley 1973, 1979; Holden and Stalnaker 1975; Cross 1976; Moyle 1976; Joseph et al. 1977; Behnke 1979; Deacon et al. 1979; Hubbard 1980; Lee et al. 1980; Molles 1980; Robins et al. 1980; Carothers and Minckley 1981; Parenti 1981; Behnke et aI. 1982; Tyus et al. 1982; U. S. Department of the Interior 1982, 1983, 1985a-e, 1986a-e, 1987; Behnke and Benson 1983; Williams et al. 1985; Minckley et al. 1986; Stanford and Ward 1986c; Johnson 1987). X - indicates species has occurred;? - indicates presence questionable; * - indicates endemic genera, species, or subspecies. Species Federal legal statusC Historic distribution by major river drainage" Upper basinb Lower basinb CO GR SJ CO VR LC BW GI Historic location ELOPIDAE Machete Elops affinis N SALMONIDAE Mountain whitefish Pros opium williamsoni N X Apache trout Salmo *apache T Colorado River cutthroat trout S. clarki *pleuriticus U X X Gila trout S. *gilae E CYPRINIDAE Longfin dace Agosia chrysogaster N Humpback chub Gila *cypha E X X ? X X ? sporadic in lower section of Lower Main- stem Colorado River, AZ-CA-Mexico; mouth of Gila River, AZ headwaters of Green River drainage, UT- WY -CO X X restricted to headwaters of Little Colorado and Salt rivers, AZ-NM headwaters of Upper Mainstem Colorado and Green river drainages, WY -CO X restricted to headwaters of Verde River, AZ and Gila River, NM X X Bill Williams and Gila river drainages, AZ- NM, south to Rio Sonora, Mexico X larger river channels; primarily in canyon- bound segments 223