Fig. 5. Representative flow field from behind an eel at 90% of the tail beat cycle. The field is a phase average of 14 tail beats. Vorticity is shown in color in the background, and contours of the discriminant for complex eigenvalues at–200,–500 and–1000 are shown in red. The eel's tail is in blue at the bottom, with red arrows, scaled in the same way as the flow vectors, which indicate the motion of the tail. Vector heads are retained on vectors shorter than 2.5 cm s^-1 to show the direction of the flow.