Question

When a fish expands its air bladder, the density of the fish

Answer 1

Answer: decreases

Every substance, body or material has mass and volume, however the mass of different substances occupy different volumes.

This is where density  $D$  appears as a  physical characteristic property of matter that establishes a relationship between the mass  $m$ of a body or substance and the volume  $V$ it occupies.

$D=\frac{m}{V}$

So, according to this equation, the density is inversely ptoportional to the volume:

If the volume increases, the density decreases.

This is what a fish does to have buoyancy, since the density of a body is related to its buoyancy:

A body will float on another fluid if its density is lower.

This is what the fish does when it expands its air bladder, incrementing its volume, hence decreasing its density.