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I I I I I I I I I I I I I I I I I I I necessary actions taken to minimize or remove these threats before delisting. nother known cause of degraded water quality is the Atlas Mills tailing pile located on t bank of the Colorado River near Moab, Utah. The Service has determined in a biological opinion that this pile "... is likely to jeopardize the continued existence of.. " the Colorado pikeminnow and razorback sucker. A recent court decision has identified the need to remove this pile before the jeopardy determination can be relieved. This threat must be removed before deli sting of the endangered fishes can occur. A conside ooot of study has been directed toward the possible effects of the non- metallic element se . Selenium occurs in trace amounts in many natural waters, especially rams selenaceous soils, and it tends to concentrate in irrigation return flows. Soils derived from Mancos Shale in northeastern Utah (Green River drainage) contain about 1.1 mg/kg selenium (Stephens et al. 1992, Stephens and Waddell 1998), and concentrations in water from Ashley Creek (tributary of the Green River), which drains these soils ranges from 59 ugIL to 78 ugIL"selenium (Peltz and Waddell 1991). Flow depletion increases concentrations of selenium be e of lower water volume for dilution. Selenium can be highly toxic t and animals. It bioaccumulates through the food chain, and toxicity in fishes is usually associated with food stuffs, although fish are able to uptake and accumulate some selenium in direct proportion to dissolved concentrations in water (Hodson et al. 1982) rather than directly from water. Selenium can also be passed from the female through the eggs to the embryos. The USEP A (1987) recommends a criterion of 5 ugIL (4-day average concentration that should not be exceeded m once every 3 years) for acid- soluble selenium. This criterion rests on the assumption that and other predators in natural environments are exposed to selenium by ingesting forage organisms that have concentrated this element from water. The estimated threshold concentration for selenium toxicity in fish tissue that can elicit reproductive failure in sensitive species, such as centrarchids (sunfish), is 12 ug/g dry weight (3.17 ug/g wet weight) (Lemly and Smith 1987). The current USEPA (1980) water quality criterion for selenium to protect freshwater life is 35 ugIL. However, some researchers report that concentrations as high as 52 ugIL may not endanger aquatic species (A Johnson 1981). Salmonids, and perhaps other fish species, have well-developed osm 23