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~ ~ r.G Movements of Young Colorado Pikeminnow and Razorback Sucker in Response to Water Flow and Light Level Harold M. Tyus Center for Limnology Cooperative Institute for Research in Environmental Sciences University of Colorado at Boulder Boulder, CO 80309-0216 USA Caryl W. Brown U.S. Fish and Wildlife Service 10950 Tyler Road Red Bluff, CA 96080 USA and James F. Saunders, III Center for Limnology Cooperative Institute for Research in Environmental Sciences University of Colorado at Boulder Boulder, CO 80309-0216 USA ABSTRACT We studied the effects of three flow regimes on the activity patterns and movements of one-, three-, six-, and 36-week old Colorado pikeminnows (Ptychocheilus Lucius) and two-week old razorback suckers (Xyrauchen texanus) in an experimental tank under light and dark conditions. The seven-chambered tank simulated a system of backwater habitats, which are used by the young fishes in nature. Water flows through the tank were slow, and velocities were discernable only in the vicinity of surface ports that connected each chamber. Fish movements were related to light or dark condition and activity tended to increase with age. Larvae, relatively active in daylight, moved little at night with no flow, but flow stimulated night activity and downstream movement. Older fish also had a diminished response to flow direction during daylight, but responded at night. The tendency for night movement may be a predator avoidance mechanism, but the tendency to move downstream with flow is likely a response to life in backwater habitat. With inflow and backwater filling "downstream" larval movements would access expanding habitat. 1F'ith outflow and backwater draining, downstream movement would avoid stranding and potential death in a shrinking habitat. INTRODUCTION Colorado pikeminnow (Ptychocheilus Lucius), formerly Colorado squawfish, and razorback sucker (Xyrauchen texanus) are two endangered fish species of the big river fish community of the Colorado River system. Extant populations of the Colorado pikeminnow inhabit main channels of the Colorado River and major tributaries where spawning typically occurs in rocky canyons shortly after the peak of spring runoff (Tyus 1990). Eggs are deposited in gravel or cobble substrate in the main channel and hatch in about four-six days at 20~C ;newly hatched larvae spend a few days within the substrate matrix before entering the flow of the river (Hamman 525 Journal of Freshwater Ecology, Volume 15, Number 4 -December 2000