Locomotion in scombrid fishes: Visualization of flow around the caudal peduncle and finlets of the chub mackerel Scomber japonicus

Scombrid fishes are known for high-performance locomotion; however, few dat a are available on scombrid locomotor hydrodynamics, In this paper, we pres ent flow visualization data on patterns of water movement over the caudal p eduncle and finlets (small fins on the dorsal and ventral body margin anter ior to the caudal fin), Chub mackerel, Scomber japonicus, ranging in fork l ength from 20 to 26cm, swam steadily at 1.2 fork lengths s(-1) in a recircu lating flow tank. Small, reflective particles in the flow tank were illumin ated by a vertical try) or horizontal (xz) laser light sheet. Patterns of f low in the region near the caudal peduncle were measured using digital part icle image velocimetry, Patterns of flow along the peduncle and finlets wer e quantified using manual particle tracking; more than 800 particles were t racked for at least 12 ms over a series of tailbeats from each of four fish , In the vertical plane, flow trajectory and flow speed were independent of t he position of the finlets, indicating that the finlets did not redirect fl ow or affect flow speed. Along, above and below the trailing surface of the peduncle, where the finlets were oriented along the peduncular surface, ho w was convergent. Along, above and below the leading surface of the peduncl e, where the finlets were absent, the flow trajectory was effectively horiz ontal, The lack of divergent flow on the leading surface of the peduncle is consistent with cross-peduncular flow formed by the lateral motion of the peduncle interacting with convergent flow resulting from forward movement o f the body. In the horizontal plane, particles illuminated by the xz light sheet situat ed approximately 3 mm below the ventral body surface were tracked within th e laser light sheet for up to 40 ms, indicating strong planar flow. As the peduncle decelerates, the most posterior finlet is frequently at an angle o f attack of at least 20 degrees to the incident flow, but this orientation does not result in thrust production from lift generation, Finlet 5 does re direct cross-peduncular flow and probably generates small vortices undetect able in this study, These data are the first direct demonstration that the finlets have a hydrodynamic effect on local flow during steady swimming.