Figure 4.

(a) Schematic illustrating the hydrodynamic benefit gained by an individual fish swimming in a two-dimensional school if a diamond formation is adopted. A reverse von Kármán street (thrust wake) is generated by the propulsive motions of two individuals (grey fill), which can be exploited by a downstream individual (black fill) if positioned between the two preceding individuals. Side-by-side thrust wakes generated by two individuals in a school (b) create a Kármán street (drag wake, highlighted in the grey box) analogous to that seen behind a bluff body in the flow such as a D-section cylinder (c; Weihs 1973). Fish can exploit the energy of discrete vortices as well as the average reduced velocity in the Kármán street. Additional mechanisms are discussed in the text. Blue represents clockwise vorticity and red represents counter-clockwise vorticity.