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approximately additive, particularly when the nutritional status the of the birds was compromised. (Hoffinan et al. 1991). This finding suggests that the variable reports from the field. may be the result of multiple stressors. 3.3 Discussion Much research is needed to explain the apparent contradictions in the technical literature. In particular, the mechanism by which selenium concentrates in adult fish tissues and then affects reproduction is not well established. Moreover, the effects of other ions on this mechanism and species-to-species sensitivity are poorly defined. Carefully controlled laboratory studies also are needed to determine the effects of other stressors (other ions, temperature, etc.) on selenium cycling and fish reproduction. 4. HABITAT, NONNATIVE FISH, AND LIFESTYLE 4.1 Habitat Habitat change has contributed to the loss of the squawfish and razorback sucker. As listed by McAda and Kaeding (1991), changes to the rivers include: A significant decrease in the spring runoff has occurred. Peak discharges have decreased by 45% in the Colorado and Gunnison Rivers and by 20% in the Green River (Irving and Burdick 1995). The duration of flooding has also been reduced, all by transmountain diversions and reservoir storage. A related problem is that zooplankton, the primary food of razorback suckers, do not produce well in turbid rivers. The backwaters are more productive. Although the overall mass of sediment conducted by the rivers is now less, the rivers remain turbid at the same time the lower flow limits the number of backwaters that were formerly available. These facts suggest that starvation may sometimes be a problem for the pikeminnow and razorback suckers (Irving and Burdick 1995). Finally, spawning locations have become silted over and are rarely washed free because the peak flows are insufficient to move the sediment. • Channels that formerly were braided have been lost. The decrease in habitat and habitat complexity has eliminated many locations favored by the endangered species. Because water is taken from the bottom of the reservoirs, lower temperatures at the spawning sites have been noted. The lower temperatures eliminate habitat because the fish need certain temperatures as spawning cues and for their most efficient metabolism. Moreover, there are indications that lower temperatures keep the young-of-the-year (YOY) too small. The fish are not able to grow adequately, rendering them more vulnerable to predation and other problems, such as disease and parasites (Kaeding and Osmundson 1988). The lower temperature has also eliminated habitat. For example, approximately 40 miles of additional habitat could be obtained if the Aspinall unit discharged slightly warmer water into the Gunnison. Similarly, 14 miles of additional habitat could be obtained in the Colorado River if a diversion structure, such as the one now in place at the Redlands Dam, were constructed. The habitat is suitable as far as DeBeque, Colorado, but this reach of river is inaccessible to