Fig. 5. Flow velocity fields of a selected swimming sequence measured on a transverse plane. The schematic drawings (not to scale) at the top of the figure illustrate lateral and dorsal views of a fish swimming (nearly steadily) across the light sheet during forward sinking. For the fish, the sinking angle was approximately 21°; the body angle was approximately 19°. The average swimming velocity of the fish was approximately 0.05 m s^–1; the beat frequency of the caudal fin was 1.3 Hz. The angle between the swimming direction and the y–z plane was approximately 28°. (A–D) The color contour represents the magnitude of z-direction velocity W, i.e. the out-of-plane velocity; the in-plane velocities are represented by black vectors. The bold white curves indicate the projection of the trailing edge of the caudal fin on the light-sheet plane. (A) The dorsal part of the caudal fin was beating toward the left side, and the ventral part toward the right side (at approximately 29% of a tail beat cycle that was initiated when the dorsal part of the caudal fin was at the last right excursion). (B) Both the dorsal and ventral parts of the caudal fin were decelerating and approaching their lateral beat excursions (at approximately 50% of the tail beat cycle). At approximately t=0.2 s, both the dorsal and ventral parts of the caudal fin attained their lateral beat excursions and began to beat toward the opposite side. (C) The accelerating dorsal and ventral parts of the caudal fin were beating toward the right and left sides respectively (at approximately 60% of the tail beat cycle). (D) Both the dorsal and ventral parts of the caudal fin were still accelerating (at approximately 65% of the tail beat cycle). The bold white arrows indicate the dorsal and ventral CFP jets that were generated by the caudal fin executing a caudal fin-wave propagation (CFP). White numbers (1–4) in A and C indicate vortices adjacent to CFP jets.