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.. ~ 1 . <br />Abstract <br />Quantitative study of resource competition has been frustrated by an <br />inability to separate effects of intraspecific and interspecific competition. <br />Two types of experimental design are commonly used to study competition in <br />two-species assemblages (1) replacement designs, and (2) additive designs. We <br />used an experimental design and analysis that incorporated the positive <br />attributes of replacement and additive designs to study resource competition <br />between larvae of federally endangered Colorado squawfish, Ptychocheilus <br />Lucius, and a widely distributed non-native species, the fathead minnow, <br />Pimephales ~romelas. Effects of competition were inferred by feeding fish <br />known quantities of zooplankton and comparing relative growth in single- and <br />mixed-species assemblages. Effects of intraspecific exploitative competition <br />were accounted for by using regression to describe the density-dependent <br />relation between relative 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. Relative growth of Colorado squawfish <br />and fathead minnow in single- and mixed-species assemblages was compared using <br />a one-sample t-statistic, regression analysis, and an index of competitive <br />ability. Conclusions of statistical analyses were confirmed by study of diet <br />overlap. <br />The response of each species to competition was consistent with that <br />predicted by ecological theory: relative growth of both fishes was reduced by <br />competition (i.e., -/-). Negative competitive effects were asymmetrical, and <br />quantitatively greater and more frequent for Colorado squawfish than for <br />fathead minnow. Study of diet overlap confirmed conclusions of relative <br />growth analysis. Diet overlap was reduced in the lowest feeding regime where <br />2 <br />