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We do not know to what degree roundtail chub abundance estimate assumptions were met. We do not expect recruitment or tag loss during the period. Some fish were moving within the study in the relatively short time period encompassed by the three-pass sampling, but we do not know how this might have affected estimates. Broader spatial sampling was needed to determine if movement of tagged fish out of the study reach and movement of untagged fish into the study reach biased abundance estimates. Most fish we released were in relatively good condition, and the high recapture rate for the few humpback chub captured (3 of 8, see below) suggested high survival of marked animals during our sampling. Longer-term mortality from tagging effects may merit more attention, given other investigators' concerns over handling mortality (pers. comm., C. McAda, U. S. Fish and Wildlife Service, Grand Junction). Nevertheless, estimated abundance of roundtail chubs in Whirlpool Canyon may offer some benchmark for future comparisons. The wider range of roundtail chub size classes present in Whirlpool Canyon compared to Lodore indicated a more stable population with some recruitment. The Whirlpool Canyon reach doubtless gets some addition of young from upstream reaches because young chubs are present in drift samples collected in summer in the lower Yampa River (Bestgen et al. 1998). Near absence of certain predators such as abundant brown trout may contribute to the relative success of roundtail chubs in Whirlpool Canyon. Salmonids are significant predators on humpback chubs in the Colorado River, Grand Canyon (Marsh and Douglas 1997). Smallmouth bass and channel catfish are also potential predators in both Lodore and Whirlpool canyons. Continued population monitoring may be needed to reveal whether the Whirlpool Canyon population of roundtail chub is stable. Humpback chub.-A total of eight humpback chub were collected during sampling conducted from 2002 to 2004. Humpback chub in Whirlpool Canyon were not the classic deep- bodied form found in Colorado River in places such as Black Rocks, Westwater, and Grand canyons. Rather, they conformed to the humpback chub morphotype found in the rest of the Green River Basin in Desolation-Gray canyons of the Green River, Yampa Canyon of the Yampa River, and the Little Snake River (Fig. 45). They were typified by a relatively fine- 53