Fig. 1. Mackerel swam at a constant speed of 1.2 or 2.2 FLs^-1, where FL is fork length, in a flow tank, and a laser light sheet oriented in the vertical (parasagittal, XY) (shown) or horizontal (frontal, XZ) plane illuminated small particles in the flow. Camera 1 recorded images of the caudal fin and the area of the light sheet posterior to it; in the example shown in A, the tail is beating out of the plane of the light sheet towards the reader with the caudal fin tilted at an acute angle to the Y axis (as also shown by Gibb et al., 1999). The ventral lobe of the caudal fin, illuminated by the light sheet, is trailing the dorsal lobe, which has previously passed through the sheet. Camera 2 recorded synchronous images of the body of the mackerel that were used to determine the orientation of the body to the X axis. Images of the light sheet oriented in the XZ plane (B) were recorded from below the tank using a front-surface mirror. Note that the caudal peduncle and fin blocked the light sheet as they moved through it (the arrow in B indicates the direction of fin movement), creating a shadow (seen, for example, in B). Yellow dotted lines in A and B outline the approximate image area analyzed using digital particle image velocimetry. The white scale bars in the lower left of A and B represent 1 cm.