Fig. 1. Body regions tracked using Digital Particle Image Velocimetry (DPIV) cross-correlation (see text for details). Dorsal view of an adult fish (FL=3.24 cm). Yellow arrows are velocity vectors (in m s^–1) describing the change in position of local pixel intensity patterns between two consecutive frames (time between frames=1 ms). Arrowheads are constant size but arrow shafts are proportional to vector magnitude. Boxes: 1, tip of upper caudal fin lobe; 2, base of caudal fin; 3, left pectoral fin; 4, right pectoral fin; 5, base of pectoral fins; 6, head. Body velocities nearest to fins (boxes 2 and 5) were subtracted from fin velocities (boxes 1, 3 and 4) to obtain the velocity of the fins relative to the body.