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DISCUSSION Water velocities generated in this study were intentionally kept minimal to avoid sweeping the young fish downstream due to shear force of water movement. Water velocities were near zero in the middle and lower tank levels even at the moderate flows generated. Thus, it must be assumed that the results obtained aze representative of an active response to the conditions provided. Movements of Colorado pikeminnow larvae in the experimental tank followed some general patterns according to the age of the fish. These patterns also apply to the razorback sucker, because behavior of the two- week old larvae was generally consistent with that of the younger pikeminnows. These small larvae were relatively active during the daytime, when they tended to move downstream with even a small flow. The smallest larvae had little tendency to move at night in the absence of flow, but even a small flow was sufficient to stimulate their activity at night, when they tended to move downstream. As larvae grew, the amount of movement tended to increase. They remained responsive to the direction of flow at night, but their responsiveness was diminished during the day. To be the most useful, behavioral patterns obtained for an organism in a laboratory setting must be related to the ecology of compazable life stages in a natural setting. We believe the fish that we studied aze especially appropriate for obtaining insight into the adaptations of natural populations. The youngest larvae used in this study were comparable in age to larvae captured in drift nets placed downstream of spawning areas in the natural riverine environment and appear to have similar movement patterns (Haynes et al. 1984, Nesler et al. 1988). Three-week old and six- week old larvae we studied are found most commonly in backwaters (Tyus and Haines 1991). The oldest fish in the study (36-week juveniles) are capable of lateral and upstream movements in the main channel (Tyus 1991) but still spend time in the backwaters. Backwaters are essential habitat for young of these two endangered fishes during most of their first year of existence. A key element of the experimental design was constructing the laboratory tank so that it would mimic flows in the natural backwater habitat. Each chamber in the tank was designed to simulate a backwater in which most of the habitat volume is dominated by quiet water. Each chamber was also connected to adjacent chambers by a shallow, narrow connection in which velocities were comparable to what the fish would encounter in connections to the river. Laboratory results indicated that larvae uniformly moved downstream at night in response to flow. This implies larvae would also move downstream in a riverine setting and thus be transported out of the system. This is not the case for these fishes, which typically reside in alongshore backwaters where they have been repetitively recaptured (Tyus 1991). Small-scale changes in river stage can have an immediate effect on such backwaters by causing ~s~ater to flow into the backwater when the river rises, and to flow out of the backwater when river stage falls. Because 532