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fish was probably dead since it did not appear to be moving, and Colorado pikeminnow do not tend to expel radio transmitters (Tyus 1988). Another fish was never relocated after being radio- tagged. It is possible that the radio failed or that this fish moved upstream beyond the uppermost area where searches for radio signals were made. However, it is more likely that this fish moved downstream beyond the lowermost limit where radio searches were made, since all of the Colorado pikeminnow that appeared to be alive at the end of the study had moved downstream before finding a localized area where they remained for extended periods of time. Some of these fish moved long distances soon after release. These patterns are similar to those noted in other studies. Cavalli (1999) also found that radio-tagged Colorado pikeminnow tend to move downstream immediately after radio implantation, while Tyus (1984) found that immature Colorado pikeminnow tend to be more sedentary than mature fish. A fish passage structure may be needed to allow native species to pass the Tusher Wash Diversion Dam during average low-flow conditions (which were not observed during this study). Native fish can pass the diversion dam at some flows, and it has been assumed by some researchers that a passage structure is not necessary as long as fish can occasionally pass the dam. However, no data exists to support or refute this assumption. Modifications to the dam would certainly be expensive to design and build, and maintenance costs could also be high. Therefore, all options for increasing fish passage and reducing mortality rates, including reducing the amount of water that enters the canal system, should be evaluated before any modifications are made. CONCLUSIONS • Some subadult fish have passed Tusher Wash Diversion Dam at a discharge of approximately 4,820 cfs (137 cubic meters per second). However, flows are almost always below this level from August through February, and flows below this level have been recorded during every month of the year; it is not known whether this dam becomes a barrier to fish passage at lower flows. • Few fish were found during visual searches of the Green River Canal, but operation of sluice gates and the activity of scavengers may have affected the number of fish found. However, during the period when a wall was in place in front of the Thayn Power Plant, relatively large numbers of subadult and adult native fish, including one razorback sucker, were impinged on the trash rack at the Thayn Power Plant. • The numbers of larval fish caught in drift nets were the lowest ever recorded in this portion of the Green River, due to higher than average discharge. Only 12 Colorado pikeminnow larvae were captured in the Green River during this study, while over 100 pikeminnow larvae were often caught each year with a similar effort from 1991 through 1996. However, native species, including Colorado pikeminnow and Gila spp. were found entrained in the Green River Canal system. 10