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Executive Summary Selenium is a metalloid that occurs in geologic formations and surface waters in the Colorado River basin. Human activities have increased selenium concentrations in surface waters. To investigate the potential for effects on larval razorback sucker, toxicity tests were conducted to quantify the relative importance of exposure to inorganic (dissolved) and organic (dietary) selenium. Effects of exposure were evaluated by culturing athree-trophic level food chain (algae, rotifer, razorback sucker) in a series of dissolved selenium concentrations (5.35, 8.23, 11.1, 16.5, and 27.2 µg/L) and exposing the fish to gradients of dietary and dissolved selenium. Measurement endpoints included survival and growth during the 28-day exposure period. We did not observe changes in survival or growth of larval razorback sucker in response to food-chain exposure. Other investigators have suggested that exposure to dietary selenium concentrations greater than 3µg/g dry weight will produce adverse effects in fish. In our study, the highest concentrations of selenium in rotifer were about half of this value (1.40 µg/g). This magnitude of bioaccumulation in food organisms was less than anticipated and is the most likely explanation for lack of adverse effects on razorback sucker. Because of low dietary exposure we were not able to estimate a relationship that describes adverse effects as a function of selenium concentration in diet. Results of this study do confirm that exposure to dietary selenium concentrations below 1.4 µg/g dry weight do not adversely effect survival and growth of larval razorback sucker. A separate study was conducted to compare the relative sensitivity of razorback sucker and fathead minnow using standard 96-hour acute toxicity tests. Exposure concentrations in acute toxicity tests were over 1000 times higher than in the dietary study. The 96-hour median lethal concentrations and 95% confidence limits (in parentheses) for razorback sucker and fathead minnow exposed to dissolved selenium were 40.8 (37.0, 44.9) and 33.3 mg/L (29.8, 37.1), respectively. Ongoing investigations will expand this research and allow evaluation of its predictive accuracy. The new data will be combined with findings of this report to provide a description of effects over a broad range of environmental and dietary concentrations that can be used to compare predicted and observed toxicity of water collected from potential razorback sucker nursery habitats in the Colorado River basin. vii