Fig. 4. The blue line (A) shows schematically and in exaggerated form the slight elevation of the water surface over a large area (which requires total work=MgL_b per body length travelled) that is centered over the anterior of a swimming body. (B,C) Reproductions of summary diagrams from two studies (Nauen and Lauder, 2002; Muller et al., 1997) of hydrodynamics of fish swimming. (B) The downward angle (–3°) of the central jet through the wake vortices formed by the tail of a mackerel, and the downward force (shown as a torque by the green arrow in this diagram) on the head. Both of these features are consistent with water lifted at the anterior end that falls at the posterior end. (C) A two-dimensional representation of water pressure and motion around a swimming mullet, with high pressure (P) at the anterior end and low pressure (S) on alternating sides of the vortices formed along the posterior. (D) Photos of a trout swimming slowly forward into a gentle current while held near the surface by a fine fishing line (not visible). Elevation of the water surface occurs over an arc at the anterior of the fish, with apparent patterns of flow and pressure quite similar to those depicted in C. Vertical displacement of the water surface during swimming at greater depth is spread over a larger area and harder to detect, but is fundamentally the same.