Fig. 7. A time series of the angle of finlet 5 (f5; black symbols) and the angles of various particles (each particle in a stroke is represented by a different colored symbol) with respect to the horizontal (left y axis) over a series of five strokes. The oscillation of the body of the 24 cm mackerel swimming at 1.2FLs^-1, where FL is fork length, is represented by the position of the body at the insertion of finlet 5 on the z axis (gray symbols, right y axis). Particles were digitized over the second half of each stroke (at times highlighted by the gray boxes), during which time the peduncle and caudal fin decelerate (Nauen and Lauder, 2000). Note that the particle trajectory angles are not equal to the orientation angle of finlet 5 over a series of strokes, indicating that finlet 5 is at an angle of attack to the local flow and that the finlet is typically oriented at a steeper angle to the horizontal than is the local flow, indicating that no thrust is produced (see Fig.6F,G).