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<br />and upland vegetation is invading the flood zone. As this vegetation becomes <br />established, it accumulates sediment and reduces the availability and quality <br />of flood zone habitats (Fisher et al. 1983). The accumulation of sediments by <br />plants such as saltcedar (Tamarix oentandra) under controlled flows is of <br />particular concern relative to changing river channel morphology (Graf 1978). <br /> <br />Backwaters used by young Colorado squawfish are also used by other fishes, <br />and the squawfish were sympatric with adults and young of 22 other fish <br />species in backwaters of the Green River, where they comprised 6% of standing <br />crops (Haines and Tyus 1990). No evidence of resource partitioning between <br />young Colorado squawfish and these fishes was detected when fish densities <br />were contrasted against backwater depth, temperature, size, or substrates; <br />however, the abundance of most species was weakly correlated with the presence <br />of Colorado squawfish (Haines and Tyus 1990). <br /> <br />Effects of competition and predation by introduced fishes on growth and <br />survival of young Colorado squawfish have yet to be adequately assessed, but <br />the common use of backwater habitats and foods by young Colorado squawfish and <br />other small introduced fish species (e.g., channel catfish, green sunfish <br />Leoomis cvanellus, red shiner Notroois lutrensis; Jacobi and Jacobi 1982; <br />McAda and Tyus 1984; Muth et al., in prep) is evidence of negative <br />interactions. Karp and Tyus (1990b) suggested that growth and survival of <br />young Colorado squawfish may be adversely affected by the aggressive behavior <br />of introduced green sunfish, red shiner, and fathead minnow, Pimeohales <br />oromelas. This may be most acute when increases or decreases in river level <br />reduces the availability of quality backwater habitat and causes resource <br />limitation. We assume that some non-native fishes prey on larval Colorado <br />squawfish (and other native forms), but the extent and significance of such <br />predation is as yet unknown. <br /> <br />There is some indication that abundance of non-native fishes may be <br />adversely affected by periods of high flow, whereas native species appear to <br />be little impacted (Haynes and Muth 1984; Minckley and Meffe 1987; T. Nesler, <br />written communication). However, the hypothesis that native Colorado River <br />fishes exhibit greater tolerance to extreme flooding has not been adequately <br />tested. In the Green River, abundance of Colorado squawfish and red shiner in <br />backwaters was lowest in years of high summer flows (Haines and Tyus 1990). <br /> <br />Backwater nursery habitats were influenced by inundation and <br />resuspension of organic material from shorelines during increased flows, and <br />this energy source was important for standing crops of fish food organisms <br />(Grabowski and Hiebert 1989). Backwaters in the Ouray area, where young <br />Colorado squawfish were most abundant, were richer in food than upstream areas <br />studied. Reduced water-level fluctuations in that area resulted in more stable <br />backwater habitats, and possibly reduced the export of nutrients and food <br />(Grabowski and Hiebert 1989). <br /> <br />Juveniles <br /> <br />Little is known about the habitat requirements of juvenile Colorado <br />squawfish, but they have been captured in a variety of shoreline habitats <br />including backwaters and flooded bottomlands. Although usually considered <br /> <br />21 <br />