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Colorado, bioconcentration factors for zooplankton range from 267 to 5700 (B.C. Osmundson, pers. comm.). Our relatively simple food chain exposure system did not include many components of natural systems (e.g., sediment). However, the system was not intended to mimic all aspects of natural systems. It was intended to use a natural process to incorporate selenium into the diet of razorback sucker. The magnitude of bioaccumulation in diet of razorback sucker was less than anticipated. To account for this apparent difference in environmental cycling, we have modified exposure conditions in ongoing studies (see below) to prevent depletion of dissolved selenium in algae and rotifer cultures. However, the apparent sensitivity of bioaccumulation rates to cycling of selenium suggests that results of this and other investigations should be interpreted based on concentrations in tissues of organisms and not based on water concentrations. Exposure to contaminants can reduce an organisms ability to obtain food, as well as increase its metabolic demand for energy (Beyers et al. 1999). Lemly (]993b) showed that selenium exposure increases metabolic stress in fish. The growth of fish in this investigation was relatively slow (from 10.6 to 11.5 mm TL in 28 days) suggesting that the quantity of food offered was smaller than the maximum ration that the fish could have consumed. Slow growth can be considered a weakness of this study, but it also suggests that fish had limited resources to offset effects of selenium exposure. Because energy for compensating effects of stress induced by selenium exposure was limited, this investigation may provide a conservative (worst-case) estimate of effects over the concentration ranges studied. Results of this study 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. 22