In order to remain suspended in the water without constantly swimming to keep from sinking to the bottom or floating to the surface, many fishes have bodies with nearly the same density as the water surrounding them.
Flesh and bone are heavier than water, of course, but, to compensate for this weight, many fishes, the striped bass included, have a swim bladder, or air bladder, which is a long sac, filled with gas, between the stomach and the backbone. In some fishes this bladder opens into the gullet; in others, like the striped bass, it is sealed off completely. In either case, however, the volume of gas in. side can be changed to suit different conditions.
Air bladders are also sometimes used as breathing organs, as aids in hearing, and in the production of sound. A good number of fishes get along without them, mostly bottom-inhabiting forms, but even some that live in mid-water, such as certain sharks and the Atlantic mackerel, lack an air bladder.