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u ., .~ ........ ~, ,~ .~ ...... .. ,. u ~ ~ ... a a ~. 1 ~ y v a y V u 1 ~ 5 ~, v 1 v a u ..... ....1 ....... ~ .... , .. ... _ ... <br />Colorado River coincided with reduced spring flows caused by Wayne <br />s <br />., <br />M. Aspinall (Curecanti) Unit Dams on the Gunnison River. Colorado <br />squawfish larval scarcity in the Green River coincided with low <br />' flows in 1977 (Holden 1980) and in the Colorado River in 1981 <br />(Haynes et al. 1985). <br />Colorado squawfish decline has coincided with the spread of <br />nonnative fish species (Miller 1961, Minckley and Deacon 1968), but <br />cause-and-effect relationships concerning predation and competition <br />are not clear {Behnke and Benson 1983). Colorado squawfish <br />evolution was probably influenced by isolation among few other fish <br />species (Miller 1958). It may thus be poorly adapted to compete <br />with introduced fish species (Molles 1980}. Largemouth bass <br />(Mlcropterus salmoides) and green sunfish (Lepomis <br />cyanellus}, common to backwaters and gravel pits in the Colorado <br />River, prey on young Colorado squawfish {Archer et al. 1985). <br />Gravel pits with year-round connections to the river are refugia and <br />production areas for nonnative fish species (Valdez and Wick 1983}. <br />Age-0 Colorado squawfish are probably most susceptible to <br />impacts by exotic. fish species (Valdez et al. 1982b). Shallow zero <br />velocity habitats used by age-0 Colorado squawfish are often <br />dominated by red shiners (Notropis lutrensis) and fathead <br />minnows (Pimephales promelas). These fish species may compete <br />With or prey on larval Colorado squawfish. Dietary and spatial <br />overlaps exist between young Colorado squawfish and red shiners in <br />backwaters (MeAda and Tyus 1984). <br />13 <br />