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must negatively affect one or both of the species (Odum 1971; Lawton and <br />Hassell 1981). Using standard notation for species interactions (Odum 1971), <br />these alternative outcomes indicate competition (-/-) or a strongly <br />asymmetrical form of competition known as amensalism (-/0). <br />Historically, Colorado squawfish were relatively abundant in large <br />rivers of the Colorado River Basin, but distribution and abundance have <br />declined. Colorado squawfish are currently restricted to warm-water reaches <br />of the Green, Colorado, and San Juan rivers and their larger tributaries <br />(Behnke and Benson 1983; Tyus 1991; Platania et al. 1992). Factors <br />responsible for decline of Colorado squawfish have been associated with <br />(1) modification and loss of habitat, and (2) introduction of non-native <br />fishes (Stanford and Ward 1986; Carlson and Muth 1989; Minckley 1991; <br />Tyus 1991). Studies of effects of non-native fishes on Colorado squawfish <br />have emphasized predator-prey interactions and interspecific competition; but <br />studies of interspecific competition have not provided convincing evidence of <br />either negative or positive effects. Previous studies of interspecific <br />competition for food were conducted in the field, and the potential for <br />competition was inferred based on diet overlap (Jacobi and Jacobi 1982; McAda <br />and Tyus 1984; S. J. Grabowski and S. D. Hiebert, unpublished report). The <br />weakness of using diet overlap to infer interspecific competition has been <br />discussed (Schoener 1982, 1983; Wiens 1992). However, study of diet overlap <br />can contribute to a convincing case for the occurrence of interspecific <br />competition if it is part of a larger study which demonstrates that <br />exploitation of a limited resource by one species has negative effects on <br />another (Wiens 1992}. <br />5 <br />