Hydrodynamic function of dorsal and anal fins in brook trout (Salvelinus fontinalis)

Recent kinematic and hydrodynamic studies on fish median fins have shown that dorsal fins actively produce jets with large lateral forces. Because of the location of dorsal fins above the fish's rolling axis, these lateral forces, if unchecked, would cause fish to roll. In this paper we examine the hydrodynamics of trout anal fin function and hypothesize that anal fins, located below the fish's rolling axis, produce similar jets to the dorsal fin and help balance rolling torques during swimming. We simultaneously quantify the wake generated by dorsal and anal fins in brook trout by swimming fish in two horizontal light sheets filmed by two synchronized high speed cameras during steady swimming and manoeuvring. Six major conclusions emerge from these experiments. First, anal fins produce lateral jets to the same side as dorsal fins, confirming the hypothesis that anal fins produce fluid jets that balance those produced by dorsal fins. Second, in contrast to previous work on sunfish, neither dorsal nor anal fins produce significant thrust during steady swimming; flow leaves the dorsal and anal fins in the form of a shear layer that rolls up into vortices similar to those seen in steady swimming of eels. Third, dorsal and anal fin lateral jets are more coincident in time than would be predicted from simple kinematic expectations; shape, heave and pitch differences between fins, and incident flow conditions may account for the differences in timing of jet shedding. Fourth, relative force and torque magnitudes of the anal fin are larger than those of the dorsal fin; force differences may be due primarily to a larger span and a more squarely shaped trailing edge of the anal fin compared to the dorsal fin; torque differences are also strongly influenced by the location of each fin relative to the fish's centre of mass. Fifth, flow is actively modified by dorsal and anal fins resulting in complex flow patterns surrounding the caudal fin. The caudal fin does not encounter free-stream flow, but rather moves through incident flow greatly altered by the action of dorsal and anal fins. Sixth, trout anal fin function differs from dorsal fin function; although dorsal and anal fins appear to cooperate functionally, there are complex interactions between other fins and free stream perturbations that require independent dorsal and anal fin motion and torque production to maintain control of body position.