Fig. 5. Ventral view of a 26 cm fork length (FL) mackerel swimming at 1.2FLs^-1 illuminated by a laser light sheet oriented in the horizontal (xz) plane. The peduncle and caudal fin are beginning to decelerate to the right (direction indicated by white arrow). The ventral margin of finlet 5 is outlined by the black line; its anterior attachment point is indicated by the black circle. Flow vectors were calculated using digital particle image velocimetry (DPIV); mean values of flow trajectory (ß) indicate the formation of cross-peduncular flow on the right side of the mackerel and longitudinal flow on the left side of the mackerel. Values are means ± S.D. The scale bar (white, lower left) represents 1 cm (note that only the posterior fifth of the body is seen in the field of view); the scale arrow (white, lower left) represents a flow of 25 cms^-1.