A swim bladder is an oxygen-filled organ found in most fish, although it is not found in sharks and lampreys. It helps the fish stay buoyant. In some species it's used to produce sounds.
A swim bladder is often found in the dorsal or upper part of the fish, right above the digestive organs. It's usually made up of two sacs that can expand or contract in response to pressure. Some types of fish can adjust the pressure by gulping air. In other kinds of fish, the gas in the swim bladder is adjusted by the levels of carbon dioxide and acidity in the fish's blood.