Fig. 6. Visualization of the dorsal fin wake during steady swimming at 2.0 L s^–1 by rainbow trout. The frontal-plane light sheet is at position 2 (Fig. 1A), intersecting the middle of the dorsal fin (at left), which is depressed at this speed (see Fig. 2C). The caudal fin is visible at the far right; all other conventions follow those of Fig. 4. Note the absence of vortex centers and fluid jets; background turbulence dominates the dorsal fin wake at this speed. This wake morphology contrasts sharply with the strong dorsal fin vortices and propulsive jets observed at lower swimming speeds (Figs 4, 5). Free-stream velocity of 38.4 cm s^–1 has been subtracted from the vector field. Yellow scale arrow, 10 cm s^–1; white scale bar, 1 cm.