A dead fish can swim upstream. It has been measured in a lab. In 2006, researchers from MIT and Harvard (Beal, Hover, Triantafyllou, Liao, Lauder) placed a dead trout behind a cylinder in a flow tank. The cylinder shed a Karman vortex street: vortices peeling off alternately from each side, producing a sinusoidal pattern of rotating flow downstream. The lifeless body resonated with those vortices, flexed passively, and moved forward against the current. How it works: - Each vortex creates a local low-pressure region and a sideways velocity component - A flexible body tilts into that pressure gradient, much like a sail tacking upwind - When the body’s natural bending frequency matches the shedding frequency, it extracts energy from the flow - Net thrust can exceed drag, so the body moves upstream with zero metabolic input Live trout use the same physics. Liao’s 2003 work showed fish slaloming between vortices, a behavior called Karman gaiting, with sharply reduced muscle activity. W...