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<br />---, <br />'-_.' <br /> <br />("':) <br />,,~ <br />I~:::'" <br /> <br />'.~ J <br /> <br />~ <br /> <br />spawning is limited to an area known as the "Mixer" and that prior to spawning, adults stage at <br /> <br />the confluence of the Mancos River and San Juan River located about 11.6 km downstream of <br /> <br />the Mixer (Ryden and Pfeifer 1995, Miller 1994), Adult Colorado pikeminnow use a variety of <br /> <br />habitats, including runs, riffles, eddies, shorelines, and backwaters and their habitat use is <br /> <br />dependent on season, hydrology, temperature, and availability (USFWS 1990, 1994), Adults <br /> <br />seem to prefer backwaters and flooded bottomlands during the runoff period (Tyus 1990) and are <br /> <br />commonly found in shoreline habitats during mid-to-Iate summer and winter (Tyus and Karp <br /> <br />1989), These habitats occur along the San Juan River at or below areas where tributaries or <br /> <br />irrigation flows enter the main stem river. <br /> <br />A 001 irrigation drainwater investigation of the San Juan River from the Hammond <br /> <br />Project Diversion to the mouth of the Mancos River found that concentrations of selenium and <br /> <br />other inorganic elements were elevated in water, sediment, and biota at various locations, usually <br /> <br />associated with irrigation drainage (Blanchard et al. 1993). Whole-body selenium concentrations <br /> <br />in fish collected from the San Juan River, which are potential food items for adult Colorado <br /> <br />pikeminnow, ranged from 1.3 to 8.3 flg/g dry weight (Blanchard et al. 1993). Waterborne <br /> <br />selenium concentrations in the same region of the San Juan River ranged from <I to 4 flg/L <br /> <br />(Blanchard et aI. 1993). A separate 001 investigation of the San Juan River from the Four <br /> <br />Comers to Mexican Hat, UT, and tributaries in Utah and Colorado also reported elevated <br /> <br />selenium concentrations in some samples (Butler et al. 1995). As a result of these investigations, <br /> <br />concern has been raised about the potential impact of elevated selenium concentrations in water <br /> <br />and food-chain organisms on native Colorado pikeminnow. <br /> <br />Very little is known about the sensitivity of Colorado pikeminnow, and other listed fish <br /> <br />species, to long-term waterborne and dietary selenium exposures. Previous toxicological studies <br /> <br />2 <br />