Fig. 4. Simultaneous development of dorsal and anal fin jets in brook trout swimming at 0.5 L s^-1. (A) Dorsal fin at peak amplitude. Due to its location forward on the body the dorsal fin reaches maximum excursion before the anal fin. (B) Dorsal fin peak jet velocity. Peak jet formation by the dorsal fin occurs just after maximum kinematic excursion. (C) Anal fin at peak jet velocity, and (D) anal fin at peak amplitude. Peak jet formation by the anal fin occurs prior to maximum anal fin excursion resulting in a shorter maximum jet phase lag than would be expected from the kinematic phase lag seen between dorsal and anal fins. Note that x and y axes represent the scale of the video image.