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nonnative predators find prey visually. In the evolutionary history of these native fishes, there was little or no experience with visual predators. Given the susceptibility to predation and their daytime activity, their behavior would place them at risk. Young Colorado pikeminnow have been stocked in streams and rivers with less than satisfactory results, In one case, juvenile pikeminnow were stocked in large numbers in Kenney Reservoir, near Rangely, Colorado, in an effort to establish a sport fishery. In a very short time, fish abundance decreased in the reservoir to the point where few could be found. Sampling below the dam revealed that many of the stocked fish had moved downstream and over the dam (Trammel et al. 1993). This fish movement can be understood in the context of our study results. The young fish would be likely to move downstream in response to any detectable water flow, which would lead them out of the reservoir. If we are correct about this mechanism, it has important implications for future stocking efforts. Laboratory studies can lead to an improved understanding of the behavior and ecology of these endangered fishes that would be virtually impossible to obtain in the Colorado River. In addition, controlled studies can suggest a need for improved field studies. As an example, our study suggests that antecedent changes in river stage could influence estimates of larval abundance in backwaters, and should be considered as part of an ongoing fish monitoring program. This study also suggests that a better understanding is needed about the downstream effect of flow regulation from reservoirs; short-term oscillations in river stage may be very disruptive and energetically costly if larvae respond by frequently moving in and out of backwaters. ACKNOWLEDGMENTS This study was conducted at the Colorado River Fishes Project station at Vernal, Utah with funding and support provided by the U.S. Bureau of Reclamation and the U. S. Fish and Wildlife Service, which also issued permits under provisions of the Endangered Species Act. Personnel from the Dexter National Fish Hatchery provided larval fish. Technicians involved in the construction of the test tank and collection of data from trials also included J. Bigelow, J. Johnson, K. Paulin and L. Trinca. LITERATURE CITED Balon, E.K. 1984. Reflections on some decisive events in the early life of fishes. Trans. Amer. Fish. Soc. 113:178-185. Bestgen, K.R. 1996. Growth, survival, and starvation resistance of Colorado squawfish larvae. Env. Biol. Fishes 46:197-209. Fausch, K..D. and R.J. White. 19$3. Sinuous stream aquaria to simulate lotic fish habitat. Prog. Fish Cult. 5:113-116. Hamman, R.L. 1981. Spawning and culture of Colorado squawfish in raceways. Prog. Fish Cult. 43:173-177 Haynes, C.M., T. A. Lytle, E.J. Wick and R.T. Muth. 1984. Larval Colorado 534