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(Archer et al. 1985) in the Black Rocks area of the Colorado River; and, 2) There appears to be a difference in habitat use of the YOY. Roundtail chub YOY were found in greatest concentrations in backwaters above the canyon. YOY chubs in Westwater Canyon did not select for the backwaters there. The relationship between observed flows and nursery habitat availability supports the supposition that periods of high flow would benefit humpback over roundtail chub in Westwater Canyon. There are likely differences in the specif c spawning habitats utilized by each of these native chubs, but gathering that type of information (radio-telemetry) was beyond the scope of this study. Project catch rates were lower than those recorded when UDWR started monitoring the canyon chubs in 1988. However, sampling protocols have changed over time. Considerably more sampling effort is expended in the canyon on an annual basis in recent years. Length-weight relationships of both the humpback and roundtai] chubs developed in the current study were compared with pre-project data (Appendix Figure 1). The numerical exponents for humpback chub length weight regression equations were in excess of 3 during the years prior to the J current study indicating isometric growth, similar to the relationship reported for the Grand Canyon population (Valdez and Ryel 1995). During the current study, the slope of that relationship decreased (exponent = 2.75) indicating a shift towards slightly smaller individuals and perhaps poorer condition. Mean length of humpbacks collected between 1987 and 1989 was 283.9 mm, with the largest individuals collected in 1987 (mean TL = 319.7mm), which greatly influenced the length-weight relationship. The current study mean total length was 277.7 mm. A similar trend was found with the roundtail length- weight relationship. Sympatric Species Various age classes of channel catfish were encountered in Westwater Canyon and large individuals (TL > 400 mm) were collected regularly. Their numbers fluctuated throughout the study. They were most abundant at the upper site indicating they maybe entering Westwater Canyon from upstream. However, as discussed above, channel catfish never comprised a very sizeable portion of the catch, particularly when compared with catch rates in Desolation Canyon. In Desolation, catfish dominate main channel and shoreline catches with juveniles often collected at rates in excess of 200 fish/hr electrofishing (Chart and Lentsch 1998). It is our feeling that competition between Gila spp. and catfish at the juvenile life stages is limiting the chub population in Desolation. In Westwater Canyon, adult catfish are collected in relatively low numbers, juvenile catfish are scarce. Catfish likely prey on chubs in Westwater, but they are not competing for resources at the juvenile life stages. Native chubs appear to thrive under those conditions. The habitats available in Westwater Canyon, particularly during high flows, appear to keep the resident channel catfish population in check. This is likely key to the success of the native chubs in Westwater Canyon as well as Black Rocks. The high flows recommended earlier for successful reproduction are likely integral to controlling nonnatives in Westwater Canyon as well. PIT tagging Movement Archer et al. (1985) found roundtail chub to be more mobile than humpback chub in their study of movements in the Black Rocks area. Humpback chub move substantially less than other Colorado River fishes (Valdez and Ryel 1995) and exhibit a strong fidelity for restricted reaches of river. Nine Gila spp. recaptured in Desolation Canyon displayed no net movement (Chart and Lentsch 1998). One roundtail chub, one bonytail, and seven humpback chub captured by H. Tyus were recaptured at their original capture locations in Desolation /Gray Canyon (Valdez and Clemmer 1982). Humpback chub in the Grand Canyon exhibit somewhat greater movements primarily due to local migrations from the cold mainstem Colorado River to the warmer Little Colorado river to spawn. Of 60 humpback chubs that were tagged in the Colorado, spawned in the Little Colorado and were recaptured in the mainstem; 90% 26