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Discussion <br />Symmetry of competition <br />Patterns of distribution and abundance of organisms may depend on the <br />relative importance and intensity of intraspecific and interspecific <br />competition. The symmetry of these interactions is of interest because <br />effects of one may outweigh those of the other under certain <br />circumstances (Underwood 1986). In this experiment, effects of intraspecific <br />exploitative competition were accounted for by using regression to describe a <br />density-dependent relation between growth and feeding regime in single-species <br />assemblages, and then subtracting these effects from the response of relative <br />growth in mixed-species assemblages. Thus, interpretation of interspecific <br />effects was simplified because d and the index of competitive ability only <br />reflect effects of interspecific competition. Several authors have argued <br />that competition is usually not symmetrical and that one species is typically <br />more affected than the other (Lawton and Hassell 1981; Connell 1983; <br />Schoener 1983; but, see Underwood 1986). This experiment showed that <br />competitive effects between Colorado squawfish and fathead minnow were <br />asymmetrical, and that negative effects were greater and more frequent for <br />Colorado squawfish than for fathead minnow. <br />It is uncertain why the outcome of competition varied and fathead minnow <br />were superior in only seven of nine mixed-species experimental treatments. <br />The most likely explanation we have presented, attributes variable outcome of <br />the experiment to our inability to compensate for mortality of study fish. <br />Alternatively, statistical sensitivity to number of replicates, variability of <br />data, and selected level of significance may have contributed to low <br />statistical power and an inability to detect experimental treatment effects. <br />17 <br />