Hydrodynamics of caudal fin locomotion by chub mackerel, Scomber japonicus (Scombridae)

As members of the derived teleost fish clade Scombridae, mackerel exhibit high-performance aquatic locomotion via oscillation of the homocercal forked caudal fin. We present the first quantitative flow visualization of the wake of a scombrid fish, chub mackerel Scomber japonicus (20-26 cm fork length, FL), swimming steadily in a recirculating flow tank at cruising speeds of 1.2 and 2.2FL s(-1). Thrust was calculated from wake measurements made separately in the horizontal (frontal) plane and vertical (parasagittal) planes using digital particle image velocimetry (DPIV) and compared with drag measurements obtained by towing the same specimens of S. japonicus post mortem. Patterns of flow indicated that the wake consisted of a series of linked elliptical vortex rings, each with central jet flow. The length of the minor axis (height) of the vortex rings was approximately equal to caudal fin span; the length of the major ring axis was dependent on swimming speed and was up to twice the magnitude of ring height. Profiles of wake velocity components were similar to theoretical profiles of vortex rings. Lift, thrust and lateral forces were calculated from DPIV measurements. At 1.2FL s(-1), lift forces measured relative to the X axis were low in magnitude (-1+/-1 mN, mean +/- S.D., N=20) but oriented at a mean angle of 6 degrees to the body axis. Reaction forces tend to rotate the fish about its center of mass, tipping the head down. Thus, the homocercal caudal fin of S. japonicus functions asymmetrically in the vertical plane. Pitching moments may be balanced anteriorly via lift generation by the pectoral fins. Thrust estimates for the two smallest fish based on DPIV analysis were not significantly different from drag measurements made by towing those same animals. At a speed of 1.2FL s(-1), thrust magnitude was 11+/-6 mN (mean +/- S.D, N=40). Lateral force magnitudes were approximately double thrust magnitudes (22+/-6 mN, mean +/- S.D., N=20), resulting in a mean mechanical performance ratio (thrust/total force) of 0.32 at 1.2FL s(-1). An increase in speed by a factor of 1.8 resulted in a mean increase in thrust by a factor of 4.4, a mean increase in lateral forces by a factor of 3, no change in the magnitude of lift produced and an increase in mean mechanical performance to 0.42. The relatively high lateral forces generated during swimming may be a necessary consequence of force production via propagated waves of bending.