Fig. 9. Dorsal–caudal fin kinematic phase relationship and hydrodynamic interaction in rainbow trout. (A–D) Dorsal-view video frames showing mediolateral excursion of the trailing edge of the soft dorsal fin and the leading edge of the tail during steady swimming at 1.0 L s^–1. (E) Corresponding plot of dorsal and caudal fin motion versus time for approximately two fin beat cycles showing that mediolateral oscillation of the tail is approximately one-quarter cycle out of phase with that of the dorsal fin. (F) Path of the caudal fin's leading edge (red dots) plotted over the course of one tail beat cycle and shown in relation to the wake of the dorsal fin within the frontal (XY) plane (position 2, Fig. 1A) at 1.0 L s^–1. At this speed, the tail passes directly through the centers of the dorsal fin vortices and experiences an incident flow whose velocity exceeds that of the free-stream by 3%. Free-stream velocity of 18.0 cm s^–1 has been subtracted from the vector field.