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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. <br />Human activities have increased rates at which selenium is dissolved and mobilized from <br />soil and geologic formations. When selenium-contaminated water collects in aquatic habitats the <br />potential exists for elevated concentrations to produce toxic effects in resident organisms. <br />Several objectives of the Recovery Implementation Program for Endangered Fish Species in the <br />Upper Colorado River Basin relate to restoration of nursery habitats for razorback sucker <br />(Xyrauchen texanus). Razorback sucker populations are presumed to have declined in the Upper <br />Colorado River Basin from cumulative effects of loss of physical habitat and historical <br />temperature regime, interactions with non-native fishes, and degraded water quality <br />(Bestgen 1990; Minckley et al. 1991; Muth et al. 1998). Recently, the impacts of changing water <br />quality have been a special concern because it was discovered that selenium concentrations in <br />some razorback sucker nursery habitats are above the USEPA criterion for protection of <br />freshwater aquatic life (5 µg/L; USEPA 1987) and elevated levels of selenium were measured in <br />1 <br />