Fig. 3. (A,B) Fish wake of Lepomiss gibbosus in six layers as velocity vector fields. The different layers are indicated by the laser number in the upper left corner of each vector plot, where laser 1 illuminated the uppermost and laser 6 the lowermost layer. The illuminated layers were spaced equidistantly (12 mm apart). Water velocities and divergence are shown at t10 s (A) and t60 s (B) (t=0 s is the time when the fish entered the field of view; times of vector fields are slightly different because of the scanning procedure). Divergence is defined as (d/dx) v_x + (d/dy) v_y (x and y are the cartesian coordinates, v_x and v_y are the x and y components of the velocity) and indicates the flow out of or into a small section of the plane, which is caused by out-of-plane movements.