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DISCUSSION Assessment of the effects of Tusher Wash Diversion Dam, Thayn Power Plant and pump station, and the Green River Canal on larval fish was hampered by low catch rates in the larval drift samples. The total catch of larval Colorado pikeminnow in the Green River was an order of magnitude lower in 1998 than the catch recorded during most years. This low catch rate was probably due to the high flows that existed late into the summer (i.e. the nets sampled a smaller proportion of the total flow than during years with lower flow) rather than due to low numbers of larvae in the river (Kevin Bestgen, Larval Fish Lab, personal communication). No razorback sucker larvae were captured in the Green River or the canal system during this study, but other native-fish species, including Colorado pikeminnow and Gila spp, were captured in both locations. Drift nets were set during the periods when water temperatures were suitable for spawning by razorback suckers and Colorado pikeminnow. Ripe razorback suckers have been captured on suspected spawning areas in the Green River from late April through May in water temperatures ranging from 9°C to 17°C (Tyus 1987; Tyus and Karp 1990). Colorado pikeminnow have been found spawning in the Green River from late June through mid August when water temperatures are between 19.5°C and 27.5°C (Tyus 1990). ,The two periods that were sampled during this study included water temperatures that fell within these ranges. The numbers of larvae of most fish species caught in the canal were similar to those found in the Green River. Many of the flow meters used to calculate the volume of water sampled did not function properly, but this problem occurred in nets set in both the Green River and the canal system. Comparisons of catch rates between locations are probably valid, even though the actual volumes of water sampled are not accurate, due to the fact that some of the meters were functional in both locations. The similarity in catch rates between the Green River and the canal system suggests that both native and non-native fish larvae are unable to avoid the canal system. During typical years, up to 2.2% of the Green River is diverted for irrigation during the period when larval razorback suckers are drifting, and up to 4.9% of the river is diverted for irrigation during the period when larval Colorado pikeminnow are drifting. Assuming that larval fish are randomly distributed in the water column, and nearly total mortality of larvae that are entrained in irrigation water, then the facilities at Tusher Wash are responsible for the loss of about 2% of razorback sucker larvae, and nearly 5% of Colorado pikeminnow larvae. Studies conducted on other species in the Pacific Northwest show that mortality of fish through powerplants can be as low as 4% to 10%, depending on the design of the runner, the head through the power plant, the operation of the power plant, and the clearance between the runner and the hub (Office of Technology Assessment 1995). The mortality rate of endangered fish going through the power plant at Tusher Wash is unknown, but it is safe to assume that some larvae are killed in this structure. Therefore, the structures at Tusher Wash are probably responsible for the loss of more than 2% of razorback sucker larvae and over 5% of Colorado pikeminnow larvae that pass through the area. Entrainment (and probable mortality) of native species into the irrigation canals and the power plant is an issue that needs to be addressed. The Green River carries massive amounts of debris during the period when larval fish are drifting (especially during the period when razorback sucker larvae are drifting), so deployment of a screening system that would be small enough to protect larval fish would probably be impractical. Use of rotating drums designed to move larval fish back to the river is another option, but the cost of such a system would probably be extremely high, and may not totally eliminate mortality. Another option for reducing 8