Fig. 7. Hydrodynamic function of the dorsal fin in yellow perch (Perca flavescens, 18.0 cm total length, L) to illustrate wake flows during a low-speed yawing turn elicited during steady swimming at 0.5 L s^–1. A horizontal laser light sheet is illuminating both the spiny and soft portions of the dorsal fin, as well as the dorsal lobe of the caudal fin. Fin positions and wake flows are illustrated for the early, middle and late stages of the turn. The yellow arrows in the left-hand panels show the direction of fin surface motion from one panel to the next. Initial wake flows during the turn are generated by the soft dorsal fin (A). As the turn develops, both the soft dorsal and the caudal fin have generated distinct momentum jets, which result in a yawing turn of the perch body (B). At the end of the turn, each of the three fins has shed a distinct vortex ring (C). Images on the left have been contrast-enhanced to better reveal the laser-illuminated particles.