Fig. 3. Hydrodynamic function of the pectoral fin in bluegill sunfish swimming at 0.5 L s^–1, as seen in posterior view looking upstream. A laser-generated sheet of light illuminates a thin slice of water flow as well as the pectoral fin and body, which casts a shadow to the right. Laser light penetrates the translucent fin, allowing flow between the fin and the body to be quantified. Water flow in this figure is out of the page, toward the reader. Images were obtained from 500 Hz digital video. (A–C) Particle image velocimetry images showing the movement of the fin illuminated by the laser light sheet in relation to the body and position of the other fins. Duration of movement shown=0.48 s from panels A–C. Yellow arrows show the key fin movements: note the cupped fin shape in A and B. (D–F) Water flow patterns as a result of pectoral fin movement. This column is from a different sequence than the frames in the left column. Yellow arrows indicate water velocities (every other vector is shown), and the background color scheme is coded so that black color indicates free stream flow velocity (7 cm s^–1), red color flow accelerated by the fin to greater than free stream velocity, and blue color showing flow slowed below free stream. Note that the pectoral fin accelerates flow on both the outstroke and return stroke (red color in E and F).