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1 <br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br />1 <br /> <br /> <br /> <br /> <br /> <br />1 <br /> <br /> <br /> <br /> <br />interpretation of SDCR results by addressing specific questions) will facilitate determination of <br />factors affecting annual reproduction, abundance, and recruitment of Colorado squawfish in the <br />Green River basin. <br />Data from this study will be used as part of an annual assessment of reproduction and <br />recruitment of Colorado squawfish so that various hypotheses regarding the affects of modified <br />operation of Flaming Gorge Dam can be evaluated. Information will facilitate assessment of <br />relationships between reproductive success and annual flow regimes, and provide a relative <br />measure of input offish larvae into downstream nursery habitats. Specific objectives of this <br />research were to: <br />1) Document timing, duration, and intensity of reproduction by Colorado squawfish <br />as measured by capture of larvae drifting from the lower Yampa River (Yampa <br />Canyon) and the lower Green River (Grey Canyon) spawning areas, <br />2) Determine the relative abundance of Colorado squawfish larvae transported from <br />spawning areas into downstream nursery habitats in the middle Green River <br />(Jensen-Ouray reach) and the lower Green River (Stillwater and Labyrinth <br />canyons) in relation to environmental variables, and <br />3) Determine if abundance of larvae produced from spawning areas was related to <br />abundance of juveniles in the fall. <br />Colorado squawfish natural history.--Although most recruitment studies on animals with <br />ecologically and morphologically distinct life stages have been in marine systems (Thorson 1950, <br />Roughgarden et al. 1988, Cushing 1995), freshwater animals with analogous life-history <br />strategies exist. Potadromous Colorado squawfish, a piscivorous cyprinid endemic to the <br />Colorado River basin, is an example. Adult Colorado squawfish attain a maximum length of <br />about 1 m and may exceed 25 years of age (Tyus 1991, Osmundson et al. 1997). In the Green <br />River basin of Colorado and Utah, adult Colorado squawfish migrate to one of two known high- <br />gradient canyon reaches in early summer for spawning (Tyus 1990, Irving and Modde 1994). <br />Spawning begins four to six weeks after peak spring runoff, when water temperatures exceed 16 <br />to 18°C, and extends up to six weeks (Kesler et al. 1988, Tyus and Haines 1991). Embryos are <br />2 <br />