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Catch Rate versus Flow One objective (no. 4) was to determine if there was a "threshold low flow" that would trigger Colorado squawfish and other large native fishes to leave the reach and move downstream into the Colorado River where more water was available and habitat conditions appear more suitable. This would be important because if a strong correlation between flow and number of fish collected in the plunge pool and reach existed, this would provide pivotal information for recommending a minimum flow for passage. If detecting biological responses during planned low test flows became difficult because sufficient numbers of Colorado squawfish could not be collected, determining if there was a relationship between individual and collective catch rates of native fishes versus flow might provide useful information for when conditions in the reach became unsuitable for sustaining fish life. A low catch rate associated with flows below the threshold would indicate unsuitable conditions. Three different ranges of flows were analyzed for fish collected in the plunge pool (89-1,000, 89-2,000, and 89-13,900 cfs) and 2.2-mile reach (217- 1,000, 217-2,000, and 217-13,900 cfs). There was no apparent relationship between total CPUE versus flow for the four native fishes or three nonnative fishes collected in the plunge pool or reach (Figures 3, 4, and 5). Catch rate was highest for Colorado squawfish in the plunge pool during July 1994 at 650 cfs. Although the correlation coefficient ("r") was highest at flows less than 2,000 cfs during 1994 and 1995 for Colorado squawfish collected in the plunge pool (-0.62; Appendix; Table E.1.), the relationship indicated that the Colorado squawfish catch rate generally decreased with an increase in flow. Relationship between CPUE and flow was poor for Colorado squawfish collected in the reach at all flows analyzed (Appendix; Table E.2. and Figure E.3.). The highest relationship for flannelmouth sucker collected from the plunge pool (r=-0.74) and reach (r=-0.54) was between 217-1,000 cfs. However, in both instances, catch rate decreased as flow increased (Appendix; Tables E.1. and E.2.). For nonnative fishes, there was no distinguishable pattern of a positive or negative relationship between catch rate and flow. The relationship of total CPUE for native and nonnative fishes versus the different range of flows analyzed by sampling date in 1994 and 1995 is provided (Appendix; Tables E.1. and E.2.; Figures E.1.-E.5.). These analyses did not offer any clear associations or identify threshold low flows when fish might vacate the reach and move downstream where more suitable habitat conditions existed. These results indicate that comparability of CPUE data can be highly variable even at similar flows. While high Colorado squawfish catch rates during low flows may be due to their being more vulnerable to capture, the high number of adult Colorado squawfish found in the reach during late-July and early-August 1994 does indicate that this reach provides temporary, seasonal habitat for "residency" at 300 cfs. There were only four sampling dates in 1995 when flows were less than 1,000 cfs. Flows between 100 and 600 cfs are needed to determine when endangered and other native use this reach and when fish might leave this reach because habitat conditions are unsuitable. 16