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<br />EXECUTIVE SUMMARY <br />~ The Nursery Habitat study on the Colorado River was part of the Aspinall Unit Five Year <br />Research Program. This report covers spring, summer and fall sampling on two 16 KM reaches <br />on the Colorado River from 1992 to 1996. The rationale for this work was to determine the <br />influence of high spring flows on the formation and maintenance of nursery habitat and the <br />subsequent biological response by Young-of--Year (YOY) Colorado pikeminnow (Ptychocheilus <br />~ lucius) in the Colorado River. The study assisted in refinement of the summer/fall <br />recommendations for operation of the Aspinall Unit, and the definition of quality nursery habitat. <br />Colorado pikeminnow YOY were collected by seining all backwaters and other low velocity <br />habitats within the two reaches. Habitat types were classified using a formation process-based <br />system. Temperature data were obtained from remote thermographs in backwaters and from a <br />~ nearby USGS gage. Native fishes comprised a small percentage of the total catch for all <br />sampling trips. Pikeminnow were collected from several habitat types, but appeared to prefer <br />relatively persistent habitats, particularly in the fall. These habitats tended to be deep backwaters <br />associated with scour channels (SC). These habitats are shared with several other native and <br />non-native species. All factors affecting the formation and maintenance of nursery habitat are not <br />~ understood. Increasing the relative quantity of deep, persistent habitats may increase overwinter <br />survival of YOY pikeminnow, but the procedure for increasing quality habitat is still unclear. <br />We saw no clear correlation between total habitat, or habitat quality, and flow. Moderate flow <br />years appear to be beneficial for Colorado pikeminnow. We recommend a moderately high peak <br />flow, which quickly declines to a stable summer/falUwinter flow, in order to maximize the <br />~ growing season for Colorado pikeminnow YOY, and enhance recruitment to age 1. <br />LIST OF KEY WORDS <br />~ Colorado pikeminnow, Young-of--Year, Colorado River, Aspinall Unit, habitat quantity, nursery <br />habitat <br /> <br /> <br /> <br />. viii <br />