Figure 6.

Summary schematic showing the anatomy of the flow around a D-cylinder and the positions and associated fish swimming midlines from approximately one tail-beat cycle. (a) A trout swimming in uniform flow is shown at the far left for comparison. Trout in the (b) bow wake (Liao et al. 2003b), (c) entraining in the suction region (Sutterlin & Waddy 1975; Webb 1998; Montgomery et al. 2003; Liao 2006) and (d) Kármán gaiting in the vortex street. In all cases except a, trout are holding station, defined as maintaining position in a current relative to the earth frame of reference without actively swimming (see text for discussion on Kármán gait kinematics and neuromuscular activity). By convention, fish are flow refuging whenever they maintain position (relative to the cylinder) in regions where the average flow velocity is reduced (b, d and potentially c). Flow refuging and vortex capture may therefore not be mutually exclusive, as exemplified in Kármán gaiting. A hydrodynamic reason for why the body of an entraining trout (c) is angled into the wake is currently unclear (Liao et al. 2003b; Liao 2006).