It is called flanking.
It's actually called fish-tailing - because the side-to-side action mimics the movement of a fish's tail as it swims through the water.
the movement of the fish is because of its bones and/or spine.
That is called "movement".That is called "movement".That is called "movement".That is called "movement".
a collection of fish is called a school of fish
the movement of fish is moving side to side up and down
The movement of a donkey is called trot!
swim
He had a fish market.
No, merely for breathing.
Circulation
The movement of rock fragments is called erosion.