Fig. 5. Visualization of the dorsal fin wake during steady swimming at 1.0 L s^–1 by rainbow trout. The frontal-plane light sheet is at position 1 (Fig. 1A), intersecting the middle of the dorsal fin, which is erected at this speed (see Fig. 2A); all other conventions follow those of Fig. 4. Note the continuous trail of vortex centers over the body, a wake morphology that stands in contrast to the discrete, bilaterally positioned vortex pairs observed at 0.5 L s^–1 (Fig. 4). Free-stream velocity of 19.4 cm s^–1 has been subtracted from the vector field. The white arrow shows the direction of dorsal fin movement. Yellow scale arrow, 10 cm s^–1; white scale bar, 1 cm.