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proportion to yield of humpback chubs (i.e., we presumed the fish to be <br />most abundant in habitats in which they were most consistently captured). <br />Sampling efforts were generally restricted to the period around spring <br />runoff (i.e., May through July depending on type of water year) because we <br />were primarily interested in locating ripe humpback chub and because the <br />canyon was relatively inaccessible to sampling at other times due to low <br />flows. However, two areas in Yampa Canyon which yielded humpback chub in <br />the spring (Big Joe Rapid and vicinity, about km 38.4; Warm Springs Rapid <br />and vicinity, about km 6.4) were sampled in September 1989 via helicopter <br />and foot to assess habitat availability and use during low flows. We did <br />not attempt to capture young chubs with seines because of problems with <br />their identification (R. Muth, Larval Fish Laboratory, Colorado State <br />University, personal communication) and thus, do not address habitat use <br />of age-0 humpback chub. <br />All chubs were identified to species using established morphological <br />characters (see Douglas et al. 1989 for review) and, depending on relative <br />condition, suspected humpback forms greater than 250 mm total length (TL) <br />were tagged with a uniquely-numbered Carlin-dangler tag for recapture <br />information (e.g., growth and movement data). Sex determination was based <br />only on expression of eggs or milt either voluntarily or following manual <br />pressure on the abdomen. Fish with breeding tubercles but without <br />expressible sex products were also considered in breeding condition <br />because presence of such tubercles in Colorado River chubs is considered a <br />prenuptial condition. All fish were photographed on a grid board divided <br />into 1-cm squares and released near the capture site. Field <br />identifications were later verified by the authors using the photographs. <br />5