Three-dimensional analysis of finlet kinematics in the chub mackerel (Scomber japonicus)

Finlets, which are small non-retractable fins located on the body margins b etween the second dorsal and anal fins and the caudal fin of scombrid fishe s, have been hypothesized to improve swimming performance. The kinematics o f three posterior finlets of the chub mackerel, Scomber japonicus, were exa mined using three-dimensional measurement techniques to test hypotheses on finlet rigidity and function during steady swimming. Finlet bending and fin let planar orientation to the xz, yz, and xq planes were measured during st eady swimming at 1.2 lengths s(-1) in a flow tank. Despite very similar morphology among the individual finlets, there was con siderable variability in finlet flexure during a stroke. Several of the fin lets were relatively rigid and Rat (with intrafinlet angles close to 180 de grees during the stroke), although intrafinlet angle of the proximal portio n of the most posterior finlet varied considerably over the stroke and was as low as 140 degrees midstroke. Finlets showed complex orientations in thr ee-dimensional space over a stroke, and these orientations differed among t he finlets. For example, during tail deceleration the proximal portion of t he fifth finlet achieves a mean angle of approximately 75 degrees with the xz plane, while the distal portion of this finlet is oriented at 110 degree s. Our data suggest that the trajectory of local water flow varies among fi nlets and that the most posterior finlet is oriented to redirect flow into the developing tail vortex, which may increase thrust produced by the tail of swimming mackerel.