NAUEN, J.C.*; LAUDER, G.V.: Small-scale flow around the finlets and caudal peduncle of free-swimming chub mackerel (Scomber japonicus)

The trajectory and speed of flow in the vicinity of the finlets (small fins on the dorsal and ventral body margin anterior to the caudal fin) and caudal peduncle of chub mackerel (Scomber japonicus) swimming steadily in a flow tunnel at 1.2 lengths/s were determined by particle tracking. Small, reflective particles in the water were illuminated by a vertical (xy) or horizontal (xz) light sheet produced by a 5W argon-ion laser; images of the flow were recorded at 250 Hz using a two camera NAC HSV 500 video system. A total of over 800 particles were tracked for at least 12 ms for four tail beats from each of four fish 20 – 24 cm in length. Particles visible in the xz light sheet approximately 3 mm below the ventral body surface were tracked for up to 40 ms, indicating strong horizontal flow. As the tail decelerates, the most posterior finlet achieves an angle of attack of at least 20 degrees to incident flow indicating lift production and vortex formation. In the xy light sheet the trajectory of particles on the left side of the fish above and below the body midline were significantly different when the finlets were present and the tail was decelerating to the right than when the finlets were absent and the tail was decelerating to the left. On average, when finlets were present and the tail was decelerating to the right the trajectory angle of particles above the body midline decreased from �3 to �10 degrees relative to the horizontal, and the trajectory of particles below the body midline increased from �2 to 16 degrees, indicating the creation of convergent flow. These data are consistent with the hypothesis that finlets increase the flow directed into the caudal fin vortex, and thus increase the thrust produced by the tail of the swimming mackerel.