Fig. 4. Particle trajectory along the trailing (gray circles, finlets present) and leading (black circles, finlets absent) lateral surfaces of the caudal peduncle as a function of the vertical position of the particle on the body (A) for all four mackerel examined. Each symbol represents a single particle; mean trajectory and position (N=3 or more) for each particle are plotted here. The lines represent the linear regression models fitted to the data sets. Note that the abscissa of this panel indicates the position of each point relative to the y axis (see Fig.1), with positive and negative values indicating dorsal and ventral locations relative to the horizontal midline, respectively. The ordinate indicates the angular trajectory of each particle track. These data are summarized graphically in B, in which a lateral view of the caudal peduncle (gray dashed lines), finlets 4 and 5 (gray solid lines), the lateral body midline (black dotted line) and the caudal keels (black lines) is depicted. Average values of flow trajectory (arrows) along the caudal peduncles and finlets and above and below these structures are shown on the trailing (gray arrows) and leading (black arrows) surface of the peduncle.