Fig. 2. Lateral view of the trailing surface of the caudal peduncle [A, note that the tips of the dorsal and ventral fifth finlets meet at the lateral body midline (a) on the trailing surface of the caudal peduncle] and the leading surface of the peduncle (B, note that the finlets are not visible along the peduncle) of a 24 cm fork length (FL) mackerel during the deceleration phase of the stroke at a steady swimming speed of 1.2FLs^-1. The flow is seeded with reflective glass beads and illuminated by a vertically oriented laser light sheet. Velocity vectors in yellow were calculated using digital particle image velocimetry (DPIV) (see Materials and methods). Scale bars (white, lower left) represent 1 cm; scale arrows (white, lower left) represent a flow speed of 25 cms^-1. ß, particle trajectory angle. Values are means ± S.D.