Movements of young Colorado pikeminnow and razorback sucker in response towater flow and light level

We studied the effects of three flow regimes on the activity patterns and m ovements of one-, three-, six-, and 36-week old Colorado pikeminnows (Ptych ocheilus lucius) and two-week old razorback suckers (Xyrauchen texanus) in an experimental tank under light and dark conditions. The seven-chambered t ank simulated a system of backwater habitats, which are used by the young f ishes in nature. Water flows through the tank were slow, and velocities wer e discernable only in the vicinity of surface ports that connected each cha mber. Fish movements were related to light or dark condition and activity t ended to increase with age. Larvae, relatively active in daylight, moved li ttle at night with no flow, but flow stimulated night activity and downstre am movement. Older fish also had a diminished response to flow direction du ring 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 ba ckwater filling "downstream" larval movements would access expanding habita t. With outflow and backwater draining, downstream movement would avoid str anding and potential death in a shrinking habitat.