Fig. 4. Velocity and vorticity fields of the pectoral fin wake in the transverse plane. (A) Early downstroke; (B) late downstroke and stroke reverse; (C) early upstroke; (D) late upstroke. Red colors represent negative or clockwise fluid rotation, while blue colors indicate positive vorticity or counterclockwise fluid rotation. The camera recorded a posterior view of the left pectoral fin and the fish body. Every other vector is shown.