Fig. 3. Lateral view of particle tracks along the trailing (A, finlets present, gray particle tracks) and leading (B, finlets absent, black particle tracks) surface of the caudal peduncle and caudal fin (outlined in blue) as the peduncle decelerates during steady swimming at 1.2FLs^-1, where FL is fork length. The caudal keels (blue lines) are shown on the lateral surface of the caudal fin. These particles were digitized from four consecutive tail strokes by a mackerel 24 cm in fork length. Each track represents the movement of a single particle; each arrow represents the distance traveled by that particle in 4ms. Particle tracks along the finlets and caudal peduncle (within the dotted lines in A) were compared with the tracks of particles above and below the finlets and peduncle (within the solid lines in A) to describe flow in the region of and adjacent to the finlets and to test hypotheses about finlet function.