Fig. 7. Flow velocity fields and vorticity contours of the near-fin wake measured on a parasagittal (x–y) plane of the light sheet approximately parallel to, and overlapping, the middle plane of the fish body. The schematic drawings (not to scale) at the top of the figure illustrate the lateral and dorsal views of the swimming fish during forward sinking. For the fish, the sinking angle was approximately 35°; the body angle was approximately 25°. The average swimming velocity of the fish was approximately 0.053 m s^–1; the frequency of tail beating was 1.3 Hz. The interval between A and B was 0.376 s. (A–D) Black vectors represent the in-plane velocity; gray areas indicate zones of the light sheet that were shaded by the maneuvering caudal fin and were unobservable with the SDPIV cameras. (A,B) The color contour represents the magnitude of the z-direction velocity W. (C,D) Vorticity contours corresponding to velocity fields in A and B, respectively. The bold white curves indicate the projection of the trailing edge of the caudal fin on the plane of the light sheet. The bold black arrows indicate the dorsal and ventral caudal fin-wave propagation (CFP) jets generated by the caudal fin executing a CFP. White P₁, P₂ and P₃ denote vortices adjacent to the CFP jets; the white curved arrows indicate the directions of rotation of vortices P₁, P₂ and P₃.