Kinematics and steady swimming performance were recorded for steelhead trout (approximately 12.2 cm in total length) swimming in channels 4.5, 3 and 1.6 cm wide in the centre of a flume 15 cm wide. Channel walls were solid or porous. Tail-beat depth and the length of the propulsive wave were not affected by spacing of either solid or porous walls. The product of tail-beat frequency, F, and amplitude, H, was related to swimming speed, u, and to harmonic mean distance of the tail from the wall, z. For solid walls: FH = 1.01(+/−0.31)u0.67(+/−0.09)z(0.12+/−0.02) and for grid walls: FH = 0.873(+/−0.302)u0.74(+/−0.08)z0.064(+/−0.024), where +/−2 s.e. are shown for regression coefficients. Thus, rates of working were smaller for fish swimming between solid walls, but the reduction due to wall effects decreased with increasing swimming speed. Porous grid walls had less effect on kinematics, except at low swimming speeds. Spacing of solid walls did not affect maximum tail-beat frequency, but maximum tail-beat amplitude decreased with smaller wall widths. Maximum tail-beat amplitude similarly decreased with spacing between grid walls, but maximum tail-beat frequency increased. Walls also reduced maximum swimming speed. Wall effects have not been adequately taken into account in most studies of fish swimming in flumes and fish wheels.

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