Fig. 6. The position of two particles (highlighted by the green and orange open circles) are tracked through a time series of images (A–D) showing the ventral view of a 26 cm fork length (FL) mackerel swimming at 1.2FLs^-1 as the peduncle declerates to the right (direction of movement indicated by the white arrow). The ventral surface of the mackerel and reflective particles in the water are illuminated by a laser light sheet oriented in the horizontal plane. The ventral margin of the fifth finlet is highlighted by the black line. The scale bar (white line, lower left of C) represents 1 cm. (E–H) Diagrams presenting hypothetical lift (F_L, green arrows), drag (F_D, red arrows) and resultant forces (F_R, gray arrows) created by the interaction between a finlet (solid black lines) and the local flow (solid blue lines) of varying trajectories. The orientations of the finlet and flow with respect to the horizontal are indicated by dotted black and blue lines, respectively. Note that thrust, which in these cases is produced when the resultant vector is angled to the left, is created only if the angle between the finlet and the horizontal is less than that of the local flow (E and H). Data shown in A–D correspond to the theoretical diagram in F, in which no thrust is generated.