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^ YOY chubs grew best (0.8mm/day) in 1994, during low summer flows (< 3000 cfs) and warm <br />water temperatures (> 25°C). This cohort appeared to recruit to Age 1+ better than the 1992 and <br />1993 cohorts. The 1994 cohort also experienced low stable overwinter flows (< 3000 cfs). We <br />recommend two approaches to managing summer flow. During dry years (when the April -July <br />flow volume as measured at the Cisco, Utah, gage is exceeded >_ 70% of the time manage for <br />flows of < 3000 cfs through August to maximize growth of YOY chubs and improve overwinter <br />survival. During wet years (when April -July flow volumes as measured at the Cisco, Utah, gage <br />are exceeded < 30% of the time) maintain flows at or above 6000 cfs through September -which <br />should maximize embayment habitat within the canyon to the relative benefit of humpback chub <br />over roundtail chub /nonnative cyprinids. During moderate flow years (April-July flow <br />volumes are exceeded between 30% and 70% of the time), manage for stable summer flows < <br />6000 cfs. <br />^ Initiate aradio-telemetry study of humpback and roundtail chub in Westwater Canyon to <br />detenmine spawning habitat requirements and better describe general life history /behavior (the <br />relatively high CPEs for adult humpback chub in July 1994 and 1996) -data needed to bolster <br />the existing information on reproductive isolation. <br />^ Increase the standardized monitoring schedule to annual sampling in Westwater, Black Rocks, <br />and the other known humpback chub populations and incorporate a YOY monitoring component. <br />The flow relationships that were made as part of this study are based on five data points. There <br />was very little YOY information collected in this area of the Colorado river prior to this study <br />and therefore no data to expand this analysis. More data will better define the flow window that <br />is optimum for chub reproduction and duration of target flows needed to optimize recruitment. <br />NOTE: In average to low flow years, September is late for sampling low velocity habitats with <br />seines -young chubs are using shoreline areas with low to moderate flow by that time. In <br />addition, by September of a low flow year, Age 2+ cohort has likely grown into the adult (mega- <br />cohort) and can no longer be readily monitored without the use of ageing body scales. <br />^ Validate the YOY growth rate information presented in this report with daily growth ring <br />analysis of otoliths. <br />^ Initiate a specific study to generate population estimates of chubs in Westwater Canyon. It is our <br />feeling that with a much more aggressive sampling program and more rigid study design more <br />reliable population estimates can be achieved (similar approach to the Grand Canyon Study). <br />Enlisting the expertise of a statistician / biometrician would be crucial to the success of such a <br />project. If designed properly, this type of study could yield: population estimates, measures of <br />recruitment and survival of four age classes (Age 0, Age 1, Age2, and the adult contingent). The <br />recapture information collected in this study indicates there are long lived, slow growing chubs <br />in the system, but there is considerable mortality as well. Abetter understanding of chub <br />demographics is needed to determine how dynamic /stable these populations are. <br />^ Age body scales collected throughout the present study to validate the Age class determinations <br />made by length frequency analysis. <br />31 <br />