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larval and adult fish (Hamilton and Waddell 1994; Waddell and May 1995; Hamilton <br />et al. 2000). The biological significance of these findings is uncertain because other co-occurring <br />native fishes remain relatively abundant in the Upper Colorado River Basin. The apparent <br />paradox about why razorback sucker populations have declined while co-occurring native fishes <br />remain abundant has led to the hypothesis that razorback sucker may be relatively more sensitive <br />to selenium exposure. The objective of this investigation was to compare the relative sensitivity <br />of razorback sucker and flannelmouth sucker (Catostomus latipinnis) to selenium exposure using <br />early life-stage (ELS) and acute toxicity tests. Flannelmouth sucker was selected for the <br />comparison because it is a closely related, co-occurring, native fish that has life-history <br />characteristics in common with razorback sucker. Species comparisons were made by exposing <br />larval fish to gradients of selenium-contaminated water and food organisms. Dietary exposure <br />was accomplished by culturing three-trophic level food chains (algae, rotifer, and razorback or <br />flannelmouth sucker) in selenium-contaminated water. Survival, growth, and whole-body <br />selenium concentrations of larvae were measured at the end of a 28-d exposure period. <br />A separate study was conducted to compare the relative sensitivity of razorback sucker <br />and flannelmouth sucker to dissolved selenium. Fathead minnow (Pimephales promelas) were <br />also tested to provide a basis for comparing results to other investigations. All species were <br />studied under similar exposure conditions using 96-hour renewal-acute toxicity tests. Data from <br />the ELS and acute studies were analyzed using regression analysis to describe the responses of <br />survival and growth as functions of selenium concentration. <br />2 <br />