Fig. 6. Instances during the time course of wake generation behind a steadily swimming eel (speed U=0.121 m s^-1, sequence 1, see Table 1). The black arrows indicate the flow velocity, and the colour code indicates vorticity. Blue shades indicate clockwise vorticity, red shades indicate counterclockwise vorticity. Darker shades indicate higher levels of vorticity. The flow fields are a continuation of the swimming sequence in Fig. 3. The time t is arbitrarily set to t=0 s for the first frame shown in Fig. 3. The field of view is 108 mmx108 mm.