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EXECUTIVE SUNIlVIARY This five year study to determine the effect of Colorado River flows on humpback chub (Gila cypha) reproduction and recruitment is a component of the Recovery Implementation Program (RIP) Aspinall Studies. The goal of this project was to develop relationships between observed flows and humpback chub life history responses, concentrating on the earlier life stages. To meet this goal, the following study objectives were identified: determine spawning and nursery requirements, describe the relationship between geomorphic processes of sediment transport and nursery habitat formation, identify and describe reproductive isolating mechanisms, and assess recruitment. Similar research is being conducted on the Green River in Desolation Canyon as part of the Flaming Gorge Studies. ' Three components of the Westwater Canyon fish community were studied: the young of year (YOY), juveniles (AGE 1+ and 2+) and late juvenile /adults. Both humpback chub (Gila cypha) and roundtail chub (G. robusta) are common in Westwater Canyon. Unlike some Upper Basin chub populations, the adults of these two Gila spp. are morphologically distinct in Westwater Canyon. However, young chubs are difficult to identify to species wherever more than one of these species is found. YOY sampling was therefore broken into sub-reaches: above Westwater Canyon (where only roundtail chubs are common), within Westwater Canyon (roundtail and humpback are equally represented in the adult fish community) and below the canyon (where roundtail chubs are present in low numbers and humpback chub are absent). YOY were sampled with seines. Juvenile chubs were monitored with shoreline electrofishing in the canyon, and late juvenile and adults were sampled with hoop and trammel nets as well as with electrofishing. The design of the study focused on YOY densities (fish ! m2 of seined habitat) and habitat use to establish relationships with observed flows. Juvenile chubs (still not readily identified to species) were monitored in the canyon to take the flow /reproduction relationship to the next step. Only three of the five cohorts produced during this study (1992, 1993, and 1994) could be tracked past their first summer (Age 0); the 1995 cohort was virtually non-existent and further monitoring of the 1996 cohort was beyond the scope of this study. Late juvenile and adult chubs were monitored to determine population trend and stability in light of the recent recruitment. Pre-project data were incorporated into these analyses. Hatching time and growth of young Gila spp. were correlated with flow parameters (peak flow at State Line gage and pre-peak flow in excess of 6000 cfs (a cumulative flow metric similar to degree days)) and water temperature (degree days > 10°C prior to June 15}. In general, YOY chubs hatched earlier above the canyon indicating roundtails likely spawned earlier than humpbacks. Catch rates of YOY Gila spp. were greatest in the above-canyon sub-reach with a project high density, .679 ,recorded there during July and August of 1993. The greatest density of YOY chubs within Westwater Canyon was nearly as high, .673, recorded during the summer of 1996. The catch rates of chubs were significantly higher in the above-canyon and canyon sub-reaches than in the sub-reach below the canyon. Reproductive success as measured by densities of YOY chubs was positively correlated with the previous year's peak flow, and negatively correlated with the amount the June monthly mean flow deviated from the historic monthly mean. The greatest canyon catch rates occurred when the river peaked near 30,000 cfs. A multinomial analysis indicated YOY chubs used backwater habitats as they were available, but did not select for them. Similar use was recorded in embayments and shoreline habitats within Westwater Canyon. Habitat availability within Westwater Canyon was not dependent on the spring peak as much as instantaneous flow. Nursery habitats were basically any low velocity area, whether that be a typical secondary channel backwater or merely a sculpted area in the shoreline bedrock (referred to as embayments). Achi-square analysis of presence /absence YOY chub in Westwater Canyon by habitat depth indicated no selection -supporting the finding of opportunistic habitat use. Habitat depth was positively correlated with peak flows and more strongly correlated with flows at the time of sampling. Shallow habitats were defined as those having a maximum depth < 0.7 m. Much of the available habitat within Westwater Canyon was not formed by sediment deposition as the classic xi