Fig. 2. The time course of vortex production over approximately one tail beat of a Scomber japonicus 24 cm in fork length (FL) swimming at 1.2 FLs^-1. The distal tips of the caudal fin were positioned approximately 1 cm to the left of the flow fields shown (see Fig. 1B for an example of caudal fin position and image region analyzed). The caudal fin was moving through the horizontal (XZ) light sheet at the position of the fin's lateral midline (as confirmed by camera 2, see Fig. 1, and shown in the diagram of the caudal fin and light sheet). Fin position relative to the Z axis was determined at 4 ms time intervals. Arrows in the time series indicate times at which a vortex could first be seen distinct from the caudal fin; gray columns indicate the time of the wake images presented below. Flow velocity is represented by the black vectors plotted over a color background indicating flow vorticity. Vortices in the wake are numbered consecutively in the order in which they are shed from the caudal fin. V₂, V₃, vortices 2 and 3. Note that the vortex number is placed approximately in the center of the vortex (as defined by vector orientation), which does not necessarily correspond with the area of highest vorticity (i.e. the region of most intense red or blue color).

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