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<br /> <br />Executive Summary <br />Population estimates for age-O and age-l Colorado squawfish in two 32-km reaches of <br />the Green River were made using capture-recapture procedures. Comparisons of autumn and <br />spring population estimates were used to assess overwinter survival and evaluate factors that <br />affect it. Previous research using traditional methodology has demonstrated that estimates <br />obtained using catch per unit effort (CPUE) are unreliable for assessing overwinter survival. <br />Capture-recapture provides a more rigorous basis for estimating abundance of young Colorado <br />squawfish because it can account for variable capture probabilities. <br />Objectives of this study were to employ capture-recapture methods to (1) compare <br />estimates of abundance from capture-recapture and CPUE methods; (2) estimate overwinter <br />survival of age-O Colorado squawfish; (3) estimate seining catchability coefficients for autumn <br />and spring sampling; (4) determine extent of downstream movement of marked Colorado <br />squawfish between 1 November and 1 April; and (5) determine the effect of timing and <br />magnitude of spring flows on dispersal of age-l Colorado squawfish. <br />We found little evidence that abundance estimates from CPUE accurately reflect the <br />number of young Colorado squawfish in study reaches in the Green River. There was only a <br />weak correlation (r = 0.50, P = 0.14) between estimates of abundance from CPUE and <br />capture-recapture. On several occasions, CPUE estimates had precision that was comparable to <br />that achieved with capture-recapture, but the estimates differed by as much as 217%. Inaccuracy <br />and greater variability of CPUE was attributed to effects of water temperature on capture <br />probability. Evidence suggests that young Colorado squawfish are less likely to be caught when <br />water temperatures are cool, regardless of their abundance. <br />Overwinter survival probabilities of age-O fish ranged from 0.06 to 0.62. Three of four <br />estimates were similar and ranged from 0.56 to 0.62. Low overwinter survival (0.06) during <br />1995-1996 may have been due to small size of age-O fish in autumn or relatively high winter <br />discharge. <br />Recaptures of marked age-O and age-l Colorado squawfish showed that they moved less <br />than 16 km downstream during sampling periods that ranged from 2 to 21 days. Similarly, age-O <br />fish that were marked in autumn and recaptured the following spring moved less than 16 km <br /> <br />viii <br />