Locomotor function of the dorsal fin in rainbow trout: kinematic patterns and hydrodynamic forces

In this study, we examine the kinematics and hydrodynamics of the soft dorsal fin in a representative basal teleost, the rainbow trout (Oncorhynchus mykiss), during steady rectilinear locomotion at 0.5-2.0 body lengths (L) s(-1) and during maneuvering. During steady swimming, dorsal fin height and sweep amplitude decrease with increasing speed. The dorsal fin wake, as viewed within a horizontal plane, consists of paired vortices on each side of the body (0.5 L s(-1)) or nearly linearly arrayed vortex centers above the body (1.0 L s(-1)) with central jet flows directed predominately laterally (lateral:thrust force ratio=5-6). At 2.0 L s(-1), the dorsal fin is no longer recruited to add momentum to the wake. This pattern of decreasing involvement of the trout dorsal fin in thrust production with increasing speed contrasts with the results of our previous study of the soft dorsal fin of sunfish (Lepomis), which is hydrodynamically inactive at low speed and sheds a propulsive vortex wake at higher speed. Yawing maneuvers by trout involve unilateral production of a single vortex ring by the dorsal fin with a strong jet flow oriented almost directly laterally. During steady swimming, interception by the tail of the dorsal fin's vortical wake and the adipose fin's non-vortical (drag) wake is hypothesized as a mechanism for enhancing tail thrust. This study provides the first experimental evidence that the plesiomorphic soft dorsal fin of ray-finned fishes acts as an ancillary force generator during axial locomotion. We suggest that the distinction often made between median and paired fin (MPF) propulsion and body and caudal fin (BCF) propulsion in fishes obscures the important role of multiple propulsors acting in a coordinated fashion. Using a combination of anterior median fin oscillation and axial undulation, without continuous paired fin excursions, trout employ an 'M-BCF' gait during steady swimming. The primarily lateral orientation of dorsal fin force in trout induces corresponding roll and yaw moments, which must be countered by forces from the caudal, anal and paired fins. Locomotion in trout therefore involves the simultaneous active use of multiple fins, presumably to maintain body stability in the face of environmental perturbations.