DRUCKER, E.G.*; LAUDER, G.V.: Function of the teleost dorsal fin: experimental analysis of wake forces in sunfish

A major structural transformation characterizing the evolution of teleost fishes is the elaboration of the dorsal fin into anterior spiny and posterior soft-rayed portions. As yet, however, there are no hydrodynamic data on the functions served by these distinct fin surfaces. In order to initiate study of the locomotor repertoire of the teleost dorsal fin, we examined the wake of bluegill sunfish (Lepomis macrochirus) during steady swimming at low and high speeds, and while turning in response to a visual stimulus. Patterns of water flow within frontal-plane sections of the dorsal fin wake were measured using digital particle image velocimetry (5 W laser, 250 Hz video framing rate) and locomotor forces were estimated from rates of momentum transfer. For fish 21 cm in length (L), neither the spiny nor soft dorsal fins shed detectable vorticity at 0.5 L/s, indicating that they do not play an active role in propulsion at this low speed. At higher speeds, however, the dorsal fin is recruited to supplement locomotor forces exerted by the pectoral and caudal fins. At 1.1 L/s, continuous oscillation of the soft dorsal fin introduces paired counterrotating vortices into the wake. The thrust associated with the dorsal fin’s momentum jet is on average 24% of the total thrust developed by all fins. During turning maneuvers, the soft dorsal fin exerts laterally oriented forces that exceed thrust forces by a factor of 1.7 on average. The dorsal fin, therefore, plays propulsive roles both in translating the body during steady swimming and in exerting torque around the body’s center of mass during unsteady locomotion. This study documents the ability of teleost fish to use simultaneous, independent action of multiple fins for the control of vorticity and for the modulation of locomotor force.