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Introduction <br />Selenium is a metalloid that occurs in geologic formations and surface waters in the <br />Colorado River basin (Stephens and Waddell 1998). It occurs in natural waters in inorganic and <br />organic forms and is an essential micronutrient for biological organisms. Because of its role as <br />an essential micronutrient, selenium is readily absorbed by biological organisms. When <br />exposure to selenium is increased due to high environmental concentrations toxic effects may <br />result. Toxicity in fish occurs because selenium replaces sulfur in amino acids which changes <br />structure and function of synthesized proteins (Maier and Knight 1994; Lemly 1998). Organic <br />forms of selenium like selenoamino acids are more bioavailable than inorganic forms, thus they <br />are more toxic and bioaccumulate rapidly. For fish, the most important exposure pathway for <br />organic selenium is through the diet. <br />Human activities have increased selenium concentrations in surface waters in the <br />Colorado River basin. Humans have increased rates at which selenium is mobilized from <br />geologic formations, and have created conditions that allow concentration of dissolved selenium <br />in evaporation basins. Several objectives of the Recovery Implementation Program for <br />Endangered Fish Species in the Upper Colorado River basin relate to restoration of nursery <br />habitats for razorback sucker. Recently it was discovered that many of these nursery habitats <br />have elevated selenium concentrations and concern exists about potential effects on larval <br />razorback sucker that may reside in these habitats. To investigate the potential for effects on <br />larval razorback sucker, toxicity tests were conducted to quantify the relative importance of <br />1 <br />