Locomotion in scombrid fishes: Morphology and kinematics of the finlets ofthe chub mackerel Scomber japonicus

Finlets are small non-retractable fins located on the dorsal and ventral ma rgins of the body between the second dorsal and anal fins and the tail of s combrid fishes. The morphology of the finlets, and finlet kinematics during swimming in a flow tank at speeds of 0.8-3.0 fork lengths s(-1), were exam ined in the chub mackerel Scomber japonicus, Functionally, S, japonicus has five dorsal and anal triangular finlets (the fifth finlet is a pair of fin lets acting in concert). Slips of muscle that insert onto the base of each finlet indicate the potential for active movement. In animals of similar ma ss, finlet length and area increased posteriorly, Finlet length, height and area show positive allometry in animals from 45 to 279 g body mass. Summed finlet area was approximately 15% of caudal fin area. During steady swimming, the finlets typically oscillated symmetrically in t he horizontal and vertical planes. Finlet excursions in the x, y and z dire ctions ranged from 1 to 5 mm, increased posteriorly and were independent of speed. The timing of the maximum amplitude of oscillation was phased poste riorly; the phase lag of the maximum amplitude of oscillation was independe nt of speed. During some periods of gliding, a finlet occasionally moved in dependently of the body and the other finlets, which indicated active contr ol of finlet movement. The angle of attack of the finlets averaged approximately 0 degrees over a tailbeat, indicating no net contribution to thrust production via classical lift-based mechanisms. However, the timing of finlet movement relative to that of the tail suggests that more posterior finlets may direct some how l ongitudinally as the tail decelerates and thereby contribute how to the dev eloping caudal fin vortex.