Fig. 4. Views of the wake in the vertical (XY) plane of an individual of Scomber japonicus 26 cm in fork length swimming steadily at speeds of 1.2 FLs^-1 (A) and 2.2 FLs^-1, where FL is fork length (B). The orientation of the caudal fin to the light sheet is shown in the diagram. Flow velocity is represented by black vectors plotted over a color background indicating vorticity. At both swimming speeds, the caudal fin sheds one pair of counter-rotating vortices with a central jet of high-velocity flow per stroke. The white arrows and labels in B illustrate the axes of velocity profiles presented in Fig. 5. X' is the longitudinal axis of the ring; Y' is perpendicular to X'.