A hydrodynamic analysis of fish swimming speed: Wake structure and locomotor force in slow and fast labriform swimmers

Past study of interspecific variation in the swimming speed of fishes has f ocused on internal physiological mechanisms that may limit the ability of l ocomotor muscle to generate power. In this paper, we approach the question of why some fishes are able to swim faster than others from a hydrodynamic perspective, using the technique of digital particle image velocimetry whic h allows measurement of fluid velocity and estimation of wake momentum and mechanical forces for locomotion. We investigate the structure and strength of the wake in three dimensions to determine how hydrodynamic force varies in two species that differ markedly in maximum swimming speed. Black surfp erch (Embiotoca jacksoni) and bluegill sunfish (Lepomis macrochirus) swim a t low speeds using their pectoral fins exclusively, and at higher speeds sw itch to combined pectoral and caudal fin locomotion. E, jacksoni can swim t wice as fast as similarly sized L, macrochirus using the pectoral fins alon e. The pectoral fin wake of black surfperch at all speeds consists of two d istinct vortex rings linked ventrally. As speed increases from 1.0 to 3.0 L s(-1), where L is total body length, the vortex ring formed on the fin dow nstroke reorients to direct force increasingly downstream, parallel to the direction of locomotion. The ratio of laterally to downstream-directed forc e declines from 0.93 to 0.07 as speed increases. In contrast, the sunfish p ectoral fin generates a single vortex ring per fin beat at low swimming spe eds and a pair of linked vortex rings (with one ring only partially complet e and attached to the body) at maximal labriform speeds. Across a biologica lly relevant range of swimming speeds, bluegill sunfish generate relatively large lateral forces with the paired fins: the ratio of lateral to downstr eam force remains at or above 1.0 at all speeds. By increasing wake momentu m and by orienting this momentum in a direction more favorable for thrust t han for lateral. force, black surfperch are able to swim at twice the speed of bluegill sunfish using the pectoral fins. In sunfish, without a reorien tation of shed vortices, increases in power output of pectoral fin muscle w ould have little effect on maximum locomotor speed. We present two hypothes es relating locomotor stability, maneuverability and the structure of the v ortex wake. First, at low speeds, the large lateral forces exhibited by bot h species may be necessary for stability. Second, we propose a potential hy drodynamic trade-off between speed and maneuverability that arises as a geo metric consequence of the orientation of vortex rings shed by the pectoral fins. Bluegill sunfish may be more maneuverable because of their ability to generate large mediolateral force asymmetries between the left- and right- side fins.