Fig. 10. Constructive wake interaction between the soft dorsal fin (D) and the dorsal lobe of the tail (T) during steady swimming at 1.1Ls^-1, where L is total body length. Silhouettes of the two fins within a frontal-plane laser sheet (Fig.2D, position 2) are shown moving over the course of one stroke cycle. The direction of fin movement is indicated by solid-line arrows. Vortices observed in the raw DPIV video recording are indicated by dashed lines. Vortex a is shed as the soft dorsal fin sweeps laterally (A,B) and migrates downstream during the development of an analogous tail vortex b (C). As the tail completes its stroke, the two counterclockwise-rotating vortices coalesce, forming a single larger downstream vortex c (D). The process illustrated in A–D is repeated on both sides of the body to yield the tail’s reverse von Kármán street wake (cf. Fig.8). Reinforcement of developing circulation around the tail through interception of the dorsal fin’s vortices is proposed as a mechanism for enhancing thrust.