STANDEN, E.M.*; LAUDER, G.V.; Harvard University, Cambridge, MA: Kinematic and hydrodynamic function of dorsal and anal fins in teleost fishes.

Dorsal fins in trout are known to produce primarily lateral forces during locomotion. These forces have been hypothesized to cause torque imbalances and possible instabilities in body position when swimming. We hypothesize that the anal fin produces a similar lateral force, acting below the centre of mass, to compensate for dorsal fin forces and maintain stability during locomotion. In order to clarify patterns of median fin function we conducted both a kinematic and hydrodynamic analysis of three teleosts: perch, sunfish, and trout. We simultaneously filmed with three high-speed (500 fps), high resolution cameras to examine 3D kinematics and curvature of dorsal and anal fins with respect to each other and the fish body. In all species the height from the top of the dorsal fin to the bottom of the anal fin decreased as steady swimming velocity increased. Dorsal and anal fin undulation differed among species and depended upon steady swimming velocity. In sunfish, lateral excursion of dorsal and anal fins during steady swimming was maximal at 2 BL/s and less at both higher and lower speeds. During maneuvering, all species showed a similar pattern where both dorsal and anal fins move to the same side of the animal with larger excursions than seen in steady swimming. Dual light sheet PIV was used to simultaneously image flow around dorsal and anal fins during steady and non-steady swimming. During steady swimming momentum jets are visible moving laterally from both the dorsal and anal fin toward the same side of the fish. Lateral moving jets of larger magnitude are seen during maneuvers, but again both the dorsal and anal fins produce jets to the same side of the animal. Median fins thus function in concert as force producers and stabilizers during fish locomotion.