ENTRAINMENT BY RIVER CHUB NOCOMIS-MICROPOGON AND SMALLMOUTH BASS MICROPTERUS-DOLOMIEU ON CYLINDERS

Sympatric stream-dwelling river chub (Nocomis micropogon) and smallmou th bass (Micropterus dolomieu) were video-taped swimming in the presen ce of single cylinders spanning a flume. Fish were acclimated and test ed at 13, 18 and 23 degrees C swimming in an increasing-velocity test with single vertically or horizontally oriented cylinders 6.4, 13, 19 or 25 mm in diameter. Cylinder orientation had little effect on entrai nment. Both species avoided cylinders at low speeds, At intermediate s peeds, some fish entrained on the cylinders, but they were displaced a t higher speeds. Entrained fish made no regular swimming motions, but the median and paired fins and the body moved continuously and irregul arly. The periodicity of body perturbations decreased and the amplitud e increased with current speed. These also increased with cylinder dia meter, but the variability was high and differences were not significa nt, Chub entrained in larger numbers and for longer over a wider range of current speeds and temperatures than bass. Entrainment by chub inc reased with cylinder diameter up to 13-19 mm, but declined for 25 mm c ylinders, Bass did not entrain on 6.4 mm and 25 mm cylinders and showe d very little entrainment at 13 and 18 degrees C, but entrainment was similar to that of chub at 23 degrees C.