The result would be a drag-inducing backwash. However, the researchers have shown that fish have evolved a clever trick to avoid this. As the vortices trail off the body at the tail, they roll up into bundles. The fish seems to sense the direction in which each is spinning. With flicks of its tail, it sweeps each bundle of vortices spinning clockwise towards the left and anticlockwise ones to the right.
Fish exhibit a similar level of control when it comes to changing direction.
Just as some jet aircraft can use thrust vectoring to manoeuvre quickly, the body and tail of a fish combine to create a sudden, directed spurt of water from a pair of counter-rotating vortices which spins the fish around on the spot. Remarkably, the researchers found that the fish sheds no uncontrolled vortices as it turns. Triantafyllou believes that a fish does a similar trick with smaller vortices formed along its sides by its forward motion. As a fish swims, the water molecules that it touches are dragged along with it. This sets up microscopic vortices in a thin layer of water along the fish's body.