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Sympatric Species Other studies have taken a critical look at the relationship between flows and non-native species year class strength (Day and Crosby 1997; Trammell and Chart 1998a; 1998b). As with these other studies, the results of this study indicate a negative correlation between non-native cyprinid densities and spring flows above as well as within Westwater Canyon. Nonnative cyprinds were present throughout the study area. Densities of red shiners and fathead minnows were lowest in the Westwater Canyon sub-reach. Sand shiner densities were low in the canyon as well, but not as low as the sub-reach above the canyon. The general decrease in density ofnon-native cyprinids within Westwater Canyon is due to the lack of their preferred habitat. Concurrent nursery habitat investigations on the Green and Colorado Rivers (Trammel and Chart 1998a; 1998b) indicate nonnative cyprinids used a variety of backwater habitat types with densities greatest in scour channel backwaters. The lower densities of small bodied nonnative species presumably contribute to the success of native chubs in Westwater Canyon. By comparison, densities of red shiners in Desolation Canyon f were 2-3 times greater than those observed during the same time frame in Westwater Canyon (Chart and Lentsch 1998). An index of recruitment -Age 1 and Age 2 Gila spp Catch rates. Three YOY cohorts (1992, 1993, and 1994) were tracked into their second and third growing season. Chubs did not successfully reproduce in 1995. Although the 1996 cohort of YOY was the strongest year class sampled, assessing recruitment of that cohort was beyond the scope of the study. In Figure 12, survival to the first year appears to be best for the 1994 cohort and worst for the 1993 cohort. The 1994 cohort was well represented as Age 1+ fish in the electrofishing sample (21 fish / of hour) despite relatively poor representation as YOY the previous year. A matrix of contributing factors has been constructed to summarize the various environmental conditions (Table 18) Table 18. Matrix of environmental conditions contributing to Age 0 to Age 1 Gila spp. survival in Westwater Canyon, Colorado River, Utah; 1992 - 1996. COHORTS 1992 1993 1994 avg TL in Fall 44,6 39.5 45 rel. Density orYOY CH• med med low rel. Density or NNC" med low very hi avg winter now*• 2967 4029 3006 first nooa••~ 44300 13600 49300 * Fall densities of non-native cyprinids relative to other years of study ** Grand average of monthly mean flows for the winters (Nov. -Feb.) of l 992-93, ] 993-94, and ] 994- 95. Flows recorded by the USGS at the Near CO-UT State Line gage. *** first spring peak as recorded the State Line gage subsequent to spawn The 1992 and 1994 cohorts were spawned in low flow years, while the 1993 cohort was spawned in above average flows. As a result, the densities of the respective cohorts and the size of individuals varied greatly by late summer (Figure 12). YOY produced in 1992 and 1994 averaged 45 mm TL by mid to late August and recruited relatively well to the Age 1 cohort the following summer. In contrast, chubs 24