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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=MgLb 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.
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