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produced in 1993, although moderately abundant, were only 39.5 mm TL by September 7 and apparently overwintered poorly. Winter flows (November -February) were significantly higher for the 1993 cohort than the other two (Table 17). High spring floods subsequent to a cohort's first overwinter period did not prove detrimental and may have been beneficial to their survival. Based on this limited information, it appears beneficial for the YOY chubs to get as much growth as possible in their first growing season (as was discussed earlier). These preliminary data also suggest that lower winter flows (< 3000 cfs) may prove beneficial. Relative survival of Age 1 to Age 2 is presented for each of the cohorts (Figure 12). The 1992 and 1994 cohorts experienced moderate to high winter flows during their second winter period; second winter flows were low for the 1993 cohort. The second spring peak was extremely low for the 1992 cohort, moderate for the 1994 cohort and high for the 1993 cohort. Electrofishing catch rates declined very little for this life history stage suggesting high survival rates once chubs reach 100 mm TL. Valdez _ and Ryel (1995) report an Age 1 to Age 2 survival rates of 0.1 for humpback chub in the Grand Canyon, and an adult survival rate of 0.9. Fall length frequencies of the chubs by species indicate the structure of the adult (identifiable) portion of the humpback and roundtail chub populations remained relatively stable throughout the study (Figures 22 and 23). The roundtails were more numerous throughout these fall samples. A Kolmogorv- Smirnov analysis of these frequency distributions (all years combined) indicated the adult roundtail are significantly smaller animals than the adult humpback adult population (KS stat = 0.23; p=.00). The average length of roundtails collected in Westwater was least in 1992 (236.8 mm) apparently the year of strongest roundtail chub recruitment to the adult population in this five year study. Mean length of roundtails remained remarkably consistent for the remainder of the study hovering near 260 mm TL. Average size of adult humpback chub fluctuated more throughout the study ranging between a low of 253.2 mm in 1992 to a high of 287.3 mm in 1995. The annual length frequencies analysis for humpback chub indicate individuals (> 200 mm TL) recruited to the adult population consistently and perhaps best in 1996. Fish Community Gila spp. Native chubs comprise the majority of the main channel and shoreline catch in Westwater Canyon. Humpback chub trammel net catch often exceeded 0.2 fish f net hour, which rivals that reported in the Grand Canyon near the confluence with the Little Colorado River. Combining the adult humpback chub with the coincident roundtail catch reveals densities in Westwater Canyon in excess of 0.4 chubs / net hour. Whereas catch rates (Gila spp. combined) in Desolation Canyon have reached 0.4 fish /net hour, in recent years they have averaged approximately 0.1 (Chart and Lentsch 1998). Early in the study there was concern that there was a shift in Gila spp. composition toward roundtails chubs, particularly in light ofpre-project sampling efforts (Figure 24). In this analysis, shifts in the Gila spp. dynamics become apparent. In the late 1980's, humpback chub outnumbered roundtails at all three sites often by a margin of 2:1 or more. The Miner's Cabin (RK 198.6) catch rates were much higher than the lower two sites with as many as 2.5 humpback collected per hour in 1989, almost four times this project's highest CPE. In 1990, the shift from humpbacks to roundtails was first detected at Miner's Cabin, at the upper terminus of Westwater Canyon. A similar shift was detected two years later at the two lower sites. Several years of low flows preceded 1990 and continued until 1993. It was assumed the shift in Gila spp. abundance was related to that dry cycle. The high flows of 1993 and 1995 appear to have reestablished a balance of these two native species. Earlier in this report two pieces of information were presented that in part explain the mechanism for reproductive isolation of these two native chubs: 1) It appears that during most years roundtail chubs spawned earlier, which supports the findings of USFWS 25