Underwater observations were employed to map, census, and measure water velocities and vertical spatial orientations of fishes at fish locations in Deer Creek, California. Several fish species were found to spatially segregate, including rainbow trout (Salmo gairdneri), Sacramento suckers (Catastomas occidentalis), Sacramento squawfish (Ptychocheilus grandis) and Sacramento hardhead minnows (Mylopharodon conocephalus). No behavioral interactions were observed between rainbow trout and two cyprinids, Sacramento squawfish and Sacramento hardhead minnows, in stream sections where all four were present. Trout densities fluctuated independent of other species densities from early summer to fall. Swimming endurance and metabolic rates were measured in the laboratory for hardhead minnows at various swimming velocities with the use of Brett-type respirometers. It was found that hardhead had lower scopes for activity than trout. Also, hardhead had lower metabolic rates than trout at different swimming velocities. These results imply that physiological differences between trout and hardhead can best explain their spatial segregation in the stream.
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