Fig. 8. Wake of the pectoral fin measured on a parasagittal (x–y) plane of the light sheet approximately parallel with the fish body and intersecting the pectoral fin. The schematic drawing (not to scale) at the top of the figure shows a lateral view of a swimming fish during forward sinking. The average swimming velocity of the fish was approximately 0.066 m s^–1; the sinking angle was approximately 27°; the body angle was approximately 20°. (A–C) Instantaneous velocity fields of the wake for the down-stroke, stroke reversal and up-stroke, respectively. The time was set to t=0 s for A; (B) t=0.064 s; (C) t=0.224 s. The black vectors represent the in-plane velocity in A–D. The color contour represents the magnitude of the z-direction velocity W in A–C. (D–F) Vorticity contours corresponding to the velocity fields in A–C, respectively. The bold black arrows indicate the fluid jets generated by the pectoral fin. White K₁, K₂ and K₃ denote vortices adjacent to the down-stroke and up-stroke jets. The white and curved arrows indicate the directions of rotation of vortices K₁, K₂ and K₃. A three-dimensional down-stroke jet (A) and an up-stroke jet (C) were observed.