Fig. 3. Stereo-DPIV measurements of the wake of a 16.5 cm BL fish, where BL is total body length, swimming at 1.2 BL s^-1 with free-stream flow subtracted to reveal wake structure. The caudal fin of the fish was positioned immediately upstream of the wake flow (i.e. to the left of the vector plot). Vector length indicates flow magnitude; vector color indicates the magnitude of W (m s^-1). The alternating areas of blue, red and blue vectors visible in the parasagittal plane (A) are clearly visible as regions of jet flow to the right and left of the fish, respectively, when viewed from above (B). Note that the lateral component of jet flow, emphasized here by the small scale of the z-axis for ease of viewing, is 10-60% greater in magnitude (on average) than that of U (see Fig. 4A). Rotating A counterclockwise also shows the alternating pattern of strong lateral jet flows. Ten vectors from the jet flow of 6-8 strokes for each individual were sampled to determine U, V and W magnitudes, where U, V and W are flow velocities in the downstream (x), vertical (y) and lateral (z) directions, respectively.