Fig. 13. PIV and laser cross-sectional end view of sunfish pectoral fin station keeping in a stream of speed 8.5 cm s^–1 (from G. V. Lauder and P. G. A. Madden, personal communication, 2005); total fish length is 17 cm. The fin is in abduction phase. Observe the formation of contrarotating vortices at the fin tips. We hypothesize that they are cross-sections of two different stall vortices as shown in Fig. 14.