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incur reduced growth rates and increased adult mortality (Stearns and <br />Crandall 1984; Wootton 1991; Roff 1992). Three other species of Ptychocheilus <br />(P. oreg_onensis, P. grandis, and P. umpQUae) achieve smaller adult sizes than <br />Colorado squawfish (Lee et al. 1980). If modern Colorado squawfish have <br />retained the genetic basis, or phenotypic plasticity for this alternative <br />life-history strategy, a decrease in size of adult Colorado squawfish is a <br />predictable consequence of the introduction of non-native competitors. <br />Extrapolation of conclusions of this laboratory experiment to field <br />localities is, as always, questionable. We demonstrated that, if larvae of <br />Colorado squawfish and fathead minnow share an environment where food <br />resources are limiting, growth and survival of Colorado squawfish is reduced; <br />but the experiment lacked the physical and biological complexity of the <br />natural environment. Larval fathead minnow are not the only potential <br />competitors with young Colorado squawfish. Other life stages of fathead <br />minnow, and other non-native fishes (e.g., red shiner, Cyarinella lutrensis), <br />are widely distributed within the Colorado River Basin. These non-native <br />species warrant further study because they are abundant and well adapted to <br />habitats that Colorado squawfish use during early life. Results of this <br />experiment can be used to make predictions about the outcome of future <br />studies. Semi-controlled field experiments could provide a framework for <br />testing predictions, while yielding new information about the interaction of <br />biotic and abiotic variables on the potential for competition between Colorado <br />squawfish and non-native fishes. <br />23 <br />