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EXECUTIVE SUMMARY <br />This study was conducted on the Colorado River in Utah, under the Aspinall Unit Study <br />Five-Year Plan of the Colorado River Recovery Implementation Program. From 1992 to 1996, <br />drifting larval fishes were collected in the upper mainstem Colorado River to determine the <br />current status and characteristics of reproductive success of Colorado pikeminnow <br />(Ptychocheilus Lucius) and other fish species. Larvae were collected using passive drift nets at <br />two sites; above Westwater Canyon, and near Moab. Larval production was positively, but not <br />significantly, correlated with high peak flows and high average June flows, but maximized at <br />Moab in the moderate flow year of 1996. The timing of reproduction was positively correlated <br />with high peak flows, average June flows and the date the water temperature reached 18°C, with <br />reproduction occurring later in years with high peak discharge and correspondingly low <br />temperatures. Larval transport abundance was indicative of fall year class strength in each year <br />except 1995, when high larval production did not result in high fall abundance. Fall year class <br />strength was affected by flows, water temperatures, summer/fall habitat availability and <br />abundance of competitive fish species. Abundance of sympatric and competitive non-native <br />cyprinids was negatively correlated with high peak, and monthly flows. Fall year class strength <br />was maximized in the moderate flow year of 1996. A moderate flow scenario appears to be most <br />beneficial to Colorado pikeminnow for reproduction and recruitment into the fall. <br />LIST OF KEY WORDS <br />Colorado pikeminnow, larval fish, reproduction, recruitment, Colorado River, peak discharge <br />viii <br />