Fig. 3. Frontal-plane water velocity vector fields in the vicinity of the dorsal fin during swimming by sunfish at 0.5Ls-1 (10.5cms-1), where L is total body length. (A) Laser plane slightly above position 1 (see Fig.2D) to illuminate the leading edge of the spiny dorsal fin (several fin spines are visible, each of which casts a discrete shadow) and the anterior portion of the soft dorsal fin (at right). (B) Laser plane at position 1 intersecting the trailing edge of the soft dorsal fin. At this low swimming speed, propulsion is achieved solely by oscillation of the paired pectoral fins (anterior to the field of view); the dorsal fin is not observed to oscillate. At a gross level, incident flow encountering the spiny dorsal fin (A) remains unchanged in orientation and velocity when measured downstream of the soft dorsal fin (B). Scales: arrow, 20cms-1; bar, 1cm.
