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2.43 to 2.89, compared to 2.42 fish/h, 95% CI = 2.24 to 2.61 with 2002 data included). Also, of nine species most often captured by electrofishing, five increased in mean abundance when 2002 data were excluded and four decreased. Since inclusion of that data appeared to have little effect on overall abundance values, we decided to retain 2002 data in all analyses. Trammel net sampling was mostly confined to Whirlpool Canyon, because that reach contained deep eddy habitat most likely to support humpback chub. A few trammel net samples were collected in Mitten Park and Lodore Canyon. Multi-filament trammel nets (23 in long x 1.8 in deep; 25-cm outer mesh, 2.5-cm inner mesh) were set in late summer or autumn when water temperatures were relatively low so that fish mortality would be minimized. In autumn 2003 we conducted three consecutive sampling trips (passes) in an attempt to obtain an abundance estimate for chubs in Whirlpool Canyon. The main sampling sites for trammel nets were in upper Whirlpool Canyon just upstream and downstream of Rain Camp (river kilometer (RK) 550.8), at a large eddy pool about 1 km upstream of Jones Hole Creek (RK 546.4), and at Compromise Camp (RK 544). Trammel nets were usually fished during crepuscular and nighttime hours, and lesser amounts in day time, and were set in low velocity habitats and in the separation zone of the main channel current and eddy pools (eddy lines). The number of nets set (up to six) was contingent on habitat availability and accessibility. Nets were cleared of fish every 2 h. Fish captured by electrofishing and trammel nets were measured (total length [TL]) and sex and reproductive condition was noted, if discernable. Large samples of small-bodied fish captured by electrofishing were counted. In addition, endangered fishes and roundtail chubs were individually weighed and scanned for a passive interrogated transponder (PIT) tag. If none was detected, a PIT tag was implanted and the fish was released. Special care was taken to identify potential catostomid hybrids. We identified these based on morphological characteristics of the mouth and body that were intermediate between putative parental types and on intermediate squamation patterns and scale counts in the lateral series (Hubbs and Miller 1953). Vouchers of some specimens, including catostomid hybrids, were preserved in 10% formalin and are housed at the Larval Fish Laboratory, Colorado State University. 10