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Results <br />Distribution, Abundance, and Habitat Use <br />A total of 1,619 Gila were captured in Yampa and Whirlpool canyons, <br />1986-1989. These were identified as roundtail chub (N=1,482), humpback <br />chub (N=133), suspected roundtail chub X humpback chub hybrids (N=3), and <br />suspected bonytail (N=1). <br />Humpback chub were collected only in whitewater canyon reaches of <br />DNM, including Yampa (N=130) and Whirlpool (N=3) canyons (Figure 1). No <br />humpback chub were captured in the Green River downstream of km 545.6 in <br />Whirlpool Canyon. Humpback chub comprised 7.2% (N=51) of the angling and <br /><1% (N=59) of the electrofishing catch, and were most abundant (85%) in <br />the upper 44.8 km of Yampa Canyon, a moderately steep-gradient river <br />section (3.2 m/km) dominated by rocky runs, riffles, and rapids. Lower <br />Yampa Canyon (km 0-28.8), a lower-gradient system (1.0 m/km) consisting <br />mostly of long, deep runs and incised meanders, yielded few juvenile or <br />adult humpback chubs (N=12). <br />During spring runoff, humpback chub were predominantly captured in <br />large shoreline eddies downstream of large boulders and upstream of <br />rapids, in smaller eddies within shoreline runs, and in pockets adjacent <br />to sheer canyon walls. Many of the larger fish were consistently taken <br />from a few large shoreline eddies that formed each spring with snow-melt <br />and runoff. A few fish were collected in the interface microhabitat <br />between the shoreline eddy and adjacent run. Eddies used by humpback chub <br />were typically low or negative velocity habitats that were influenced by <br />river surges (i.e., water velocities at any particular point continually <br />varied in magnitude of upstream and downstream currents). Substrate <br />7