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V <br />Keywords: Colorado squawfish, Young-of--Year, Colorado River, Aspinall Unit, Peak flow, <br />nursery habitat <br />Executive Summary <br />The Nursery Habitat study on the Colorado RiygLis part of the ongoing Aspinall Unit Five <br />Year Research Program. This report covers spring, summer and fall sampling on two 16 KM <br />reaches on the Colorado River from 1992 to 1996. The rationale for this work is to determine the <br />influence of high spring test flows on the formation and maintenance of nursery habitat and the <br />subsequent biological response by Young-of--Year (YOY) Colorado squawfish (Ptychocheilus <br />Lucius) in the Colorado River. The study also will assist in refinement of the"summer/fall <br />recommendations for operation of the Aspinall Unit, and the definition of quality nursery habitat. <br />Colorado squawfish YOY were collected by seining all backwaters and other low velocity <br />habitats. Habitat types were classified using aprocess-based system developed by UDWR and <br />USU (Geography Department). Temperature _data were obtained from remote thermographs in <br />backwaters. Native fishes comprised a small percentage of the total catch for all sampling trips. <br />Squawfish were collected from several habitat types, but appeared to prefer relatively persistent <br />habitats, particularly in the fall. These habitats tended to be deep backwaters associated with <br />scour channels (SC). These habitats are shared with several other native and ion-native species. <br />All factors'affecting the formation and maintenance of nursery habitat are not understood. <br />Increasing the relative quantity of deep, persistent habitats may increase overwinter survival of <br />YOY squawfish, but the procedure for" increasing quality habitat is still unclear. We saw no clear <br />correlation of total, or qu~, habitat availability to..fl°w. Moderate flow years appear to be <br />beneficial for Colorado squawfish. A moderately high peak flow, which quickly declines to a <br />stable summer/falUwinter flow, in order to maximize the growing season, would be the ideal <br />hydrograph for Colorado squawfish YOY recruitment to age 1. <br />