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linked to events observed in the natural setting. We hope that the results of the study will improve the understanding of habitat use in this critical life stage of these endangered fishes. METHODS Fish activity and movement was studied in a test tank that simulated conditions in riverine backwaters (Figure 1). The tank (2m long x 0.45m wide x 0.37m deep) was constructed of exterior-grade plywood and partitioned into seven equal-size chambers, each representing a backwater. The chambers were arrayed in a linear sequence to accommodate studies of movement. Small ports (6 cm x 5 cm) were cut into the top of each partition, and connected the chambers in a manner similar to the connection between backwaters and the main channel. These ports were placed on the outer wall of each chamber and arranged in an alternating fashion to give a better approximation of natural conditions than a straight or circular channel (e.g., Fausch and White 1983). / ~• .. .....• .., t....• ......... ; ; ; ; , w ' ... ... l:l~ ~ .~ :1... Figure 1. Illustration of the experimental tank used to conduct trials. The interior of the tank was sealed with epoxy resin and painted with non-toxic, white latex paint. Dechlorinated tap water was circulated through the tank by a pump (AMXCOTM model A70) that was situated in a separate container. Water from the last chamber of the experimental tank was delivered to the pump by an inverted siphon tube. A screen of fine cloth prevented the larvae from becoming entrained in flow leaving the final chamber. Output from the pump was regulated with a valve to yield flow rates of 27 and 237 ml/s. These flows resulted in velocities of 2-3 or 3-9 cm/sin the ports connecting the chambers, but no water movement was detectable near the center of a chamber (velocities measured with a Marsh-McBirney model 201 meter). Because velocities were too low to measure except in the ports between chambers, water movements were also examined with fluorescein dye. Movement patterns of the dye demonstrated quiet water in the middle and especially at the bottom of each chamber. The fish larvae (5,150 Colorado pikeminnow and 8,780 razorback suckers) were obtained from Dexter National Fish Hatchery, Dexter, New Mexico on 30 March and 6 June 1987, respectively. The larvae were received less than one week after hatching and were held in circular nylon tubs until they were used in an experiment. The fish were fed ad iibitum four times daily, with a mixture of live brine shrimp and dry pellets. Total lengths of larvae were determined by measuring random samples of the 527