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The smallest documented fish that moved from the lower to the upper reach was no longer than 449 mm when it made its move. Movement of young adult Colorado squawfish was likely a response to their changing food requirements as they grew and matured. A net shift in abundance to upstream reaches was probably because upstream movements were rewarded with greater food availability. Two lines of evidence supported this hypothesis: 1) catch rates of roundtail chub (Gila robusta) and bluehead (Catostomus discobolus) and flannelmouth sucker (C. latipinnis), likely prey of adult Colorado squawfish, were significantly higher in the upper reach than in the lower reach, and 2) body condition of Colorado squaw-fish decreased with fish size in the lower reach but increased with fish size in the upper reach. Results suggested that young Colorado squawfish move more than older ones and that upon arrival to the upper reach they move less. Evidently, older fish establish home feeding ranges in areas they have found to be most productive and have little need for further exploratory movements. The reach upstream of Westwater Canyon evidently provides the best adult feeding opportunities in the upper Colorado River. Population status and trends To determine status, we estimated the size of the subadult and adult population. To determine short term trends, we examined the extent and frequency of reproduction and recruitment by monitoring larval and young-of-the-year catch rates and looked for signif cant changes in age or size structure of the population over the past 20 years. Past long-term trends could not be quantified because of a lack of historical data. Population abundance was estimated with both open- and closed-population models using capture-recapture data. Size frequency analysis from lengths of captured fish was used to assess trends in age structure of the population both among reaches and among time periods. Upper-reach, size-structure data were compared with those from earlier periods for which data sets were sufficiently large; these included 1974-1976, 1979 and 1982. Also, size frequency data from the lower reach, coupled with age information from scale analyses, was used to determine the frequency of strong year classes during recent times. Reproductive success in the upper reach over a nine-year period (1986-1994) was evaluated by systematically sampling for Colorado squawfish larvae with hand seines and young-of-the-year (YOY) with beach seines each summer and fall, respectively. Summary data from the Interagency Standardized Monitoring Program was also examined to compare upper reach data with lower reach YOY capture rates. Size of the adult subpopulation in the upper reach, averaged over the four-year study period, was estimated to be 263 individuals (95% C. I. = 161-440) using program CAPTURE and 253 (95% C. I. = 186-333) using program RECAP, or an average of 4.0-4.2 fish per mile of river. In the lower reach, estimated size of the subpopulation (adults and subadults), averaged over a three-year period, was 344 individuals (95% C. I_ = 196-604) using program CAPTURE and 385 (95% C. 1. = 202-733) using program RECAP, or an average of 3.1-3.4 IV