The trick is the swim bladder, which is basically like an air-inflated balloon that can expand and contract depending on how much gas is inside. … When the swim bladder deflates the fish’s buoyancy decreases and it will sink as it displaces less water. Divers use the same concept for their buoyancy-control devices.

How do fish get air into swim bladder?

These fish, called physostomes, fill their swim bladder with oxygen by gulping air at the water’s surface, where it quickly passes through a pneumatic (air) duct to the bladder. In physoclist fish, a specialized gas gland that pulls gases from the blood keeps the bladder filled.

How do fish deflate their swim bladder?

Physostomous swim bladders are directly connected to the gastrointestinal tract so that fish with these swim bladders, such as herrings, must “gulp” air to inflate their swim bladder and “burb” or “fart” air to deflate them.