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TABLE 2. Lengths of Colo°~do squawfish and sympatric fishes <br />captured by 3-mm-mesh seine from the upper Colordo River, <br />October, 1985. <br />Species Mean(Range) N <br />(mm, TL) <br />------------------------ ---------- --- <br />Colo~~ado squawfish 26(21-39) 61 <br />Fathead minnow <br />(Pimephales promelas) 27(12-54) 172 <br />Sand shiner <br />(Notropis stramineus) 24(14-60) 155 <br />Red shiner <br />(N. lutrensis) 25(11-52) 349 <br />chubs <br />(Gila spp.) 38(27-50) 125 <br />suckers <br />(Catostomus spp.) 32(19-44) 160 <br />Our simulations provide insight into both the probable causes <br />of the population decline and the factors limiting the Colorado <br />squawfish population of the upper basin. They show the slow growth <br />of Colorado squawfish there makes the population especially <br />vulnerable to the effects of increased early-life mortality. <br />Although the causes of ea,~ly-life mortality are not well <br />understood, such mortality doubtless has increased with the advent <br />of technologic man. Colorado squawfish may have been common in the <br />upper basin under former, pristine conditions largely because their <br />slow growth then was not an important impediment to population <br />maintenance. <br />Our simulations suggest ways to increase the growth potential <br />of the Colorado squawfish population in the upper basin. Dams and <br />reservoirs might be operated to enhance growing-season length and <br />to stimulate earlier spawning. However, such growing-season <br />enhancement needs to be cautiously considered because it could also <br />benefit undesirable species, perhaps to the ultimate detriment of <br />Colorado squawfish. <br />Causes of early-life mortality need to be identified and their <br />effects reduced where possible. Introduced fishes undoubtedly have <br />-115- <br />