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impoundments in the lower Colorado River, albeit in the absence of nonnative fishes (Mueller et al. 2003). However, if bonytail adults remain in floodplain depressions, temperatures exceed 20°C in early June and spawning would likely take place before or at the same time as nonnative fishes. Percent survival estimates for larval razorback sucker sympatric with nonnative fish were low. The results and behavioral observations during the study support the conclusion of Johnson ~ et al. (1993) that larval razorback sucker are predator naive and unlikely to survive with high numbers of nonnative fish. Razorback sucker larvae could frequently be observed swimming in large groups in open water in the same vicinity as large predators. They did not appear to use the ~ available cover or avoid predators (Birchell and Christopherson 2004). This was also captured with underwater video. Survival of over 3,000 razorback suckers is encouraging. This is the first time larval ~ razorback sucker survival has been detected in the middle Green River since 1996 when Levee Removal and associated studies began (Birchell et al. 2002; and Birchell and Christopherson 2004). If one applies the 56-72 percent survival reported in the 1999 study for 90 mm ~ razorback sucker (Birchell and Christopherson 2004), The Stirrup could have contributed 1,000 to 2,000 sub-adult razorback suckers to the river population. However, caution should be used projecting survival this way. ~ Small numbers of larval fish escaped from the enclosures, but it is impossible to determine from which enclosure fish escaped. Escapement of larval fish from enclosures reduced survival estimates. However, the population estimate for the razorback suckers outside ~ of the enclosures was relatively low (1,113). Because there were fewer predators outside the enclosures it is assumed that survival for fish that escaped was better than for fish inside the enclosures. Had those fish remained inside the enclosures survival estimates would have only ~ been slightly higher and the conclusions would have remained the same. ~ 17