Fig. 8. Representative flow fields in the wake of the oscillating caudal fin of sunfish during steady swimming at 1.1Ls^-1, where L is total body length. Flow patterns within the frontal-plane laser sheet intersecting the tail at mid-fork (Fig.2D, position 3) are shown for the early stages of two consecutive fin half-strokes. The direction of tail fin motion is indicated by large white arrows. Free-stream velocity (24.7cms^-1) from left to right has been subtracted from each velocity vector to reveal vortical wake structures. In general, the wake generated by the tail closely resembles that produced separately by the soft dorsal fin upstream (cf. Fig.4A,C). Each half-stroke generates vorticity at the trailing edge of the fin that is ultimately released into the wake (counterclockwise flow in A and B). Opposite-sign vorticity attached to the fin (in the upper left-hand corner of A) migrates to the trailing edge over the course of the stroke period (visible at the tip of the fin in B). Repeated cycles of caudal fin oscillation generate a reverse von Kármán vortex street. Scales: arrow, 10cms^-1; bar, 1cm.