The fish in the image is dead but it is still swimming upstream due to passive propulsion. It is propelled when its body resonates with oncoming vortices. This is how fish swim upstream without spending too much of its own energy.

The fish in the image is dead but it is still swimming upstream due to passive propulsion. It is propelled when its body resonates with oncoming vortices. This is how fish swim upstream without spending too much of its own energy.