Fig. 4. Hydrodynamic analysis of the dorsal fin and caudal fin in swimming bluegill sunfish, to show that these two fins can act as dual flapping foils in series, and that flow leaving the dorsal fin can affect caudal fin function. The caudal fin of ray-finned fishes does not move through undisturbed free stream flow, but rather has its flow environment highly modified by upstream fins. The left panels show the laser-imaged dorsal fin and tail of a bluegill sunfish (16.5 cm L) swimming at 17 cm s^–1; laser light illuminates from top to bottom in these images, and the dorsal fin and tail cast shadows toward the bottom. Yellow arrows in B show the left–right oscillatory motion of the dorsal fin and tail as seen from above. In the right panels these images are analyzed to show water flow velocities around the fins (vectors were not calculated in the fin shadows) and vorticity. The views shown in this figure are from above, looking down on the upper surface of the fish with the dorsal fin and tail (also see Fig. 1). In A, the dorsal fin has shed a clockwise vortex that is moving toward the tail. This vortex passes above the tail (B) while the dorsal fin sheds a new vortex of opposite sign on the return stroke. This pattern repeats as a clockwise vortex is just leaving the dorsal fin again (C). Note that flow in the gap between the dorsal fin and tail is nearly orthogonal to free-stream flow. Free-stream flow has been subtracted from the right panel images to reveal flow structure; images on the left have been contrast-enhanced.