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Executive Summary <br />Selenium is a metalloid that occurs in geologic formations and surface waters in the <br />Colorado River Basin. Human activities have increased selenium concentrations in surface <br />waters. Elevated environmental selenium concentrations have been shown to adversely affect <br />fish populations in other systems. The objective of this investigation was to compare the relative <br />sensitivity of razorback sucker (Xyrauchen texanus) and flannelmouth sucker (Catostomus <br />latipinnis) to selenium exposure using early life-stage (ELS) toxicity tests. Species comparisons <br />were made by exposing larval fish to gradients of selenate-contaminated water (<1, 25.4, 50.6, <br />98.9, and 190. µg/L) and food organisms (<0.702, 1.35, 2.02, 4.63, and 8.24 µg/g). Dietary <br />exposure was accomplished by culturing food chains (algae, rotifer, and 41-d-old razorback or <br />11-d-old flannelmouth sucker) in the selenium gradient. Survival, growth, and whole-body <br />selenium concentrations of larvae were measured at the end of a 28-d exposure period. <br />No negative effects on survival or growth were detected. Existing guidelines suggest that <br />exposure to dietary selenium concentrations greater than 3µg/g dry weight produce adverse <br />effects in fish. In our study, the highest dietary exposure concentration in rotifer was 8.24 µg/g. <br />Results are consistent with findings of other laboratory food-chain investigations with fathead <br />minnow. Together, results of our food-chain investigations suggest that the threshold for adverse <br />effects from dietary exposure is above 8.24 µg/g selenium for larval razorback and flannelmouth <br />suckers. <br />A separate study was conducted to compare the relative sensitivity of razorback sucker, <br />flannelmouth sucker, and fathead minnow to dissolved selenate using standard 96-hour acute <br />toxicity tests. Median lethal concentrations and 95% confidence limits for razorback sucker, <br />flannelmouth sucker, and fathead minnow exposed to dissolved selenium were 35.5 (32.3, 38.8), <br />32.5 (28.9, 35.1), and 21.8 mg/L (18.6, 25.2), respectively. <br />Results of this investigation do not support the hypothesis that larval razorback sucker are <br />more sensitive to selenium exposure than flannelmouth sucker. Both species had similar <br />responses to dietary selenium concentrations up to 8.24 µg/g, and to acutely toxic dissolved <br />selenium concentrations. It should be noted that larval razorback sucker were 30 and 5 days <br />viii <br />