What basic principle is involved in the up and down motion of a fish in water and how is it achieved?

Explanation

An object can sink or float on water if its density is more or less than that of water. Therefore, principle of adjustment of density is involved in the up and down motion of a fish in water. A fish can regulate its density by expanding or cntracting an inbuilt air sac taht changes its volume. The fish can move upward by increaseing its volume (which decreases its density) and downward by contraciting its volume (which increases its denstity).