The swim bladder is one of the fish body organs. The swim bladder, commonly known as the fish bladder, is generally sac-shaped, located in front of the body cavity, under the spine, and is filled with air. The swim bladder is a depth regulator in the water when fish swims. It can adjust the specific gravity of the fish body by inflating and deflating. In this way, the fish only needs minimal muscle activity when swimming to maintain a stable state of neither sinking nor floating in the water.

Most have swim bladders. The volume of the swim bladder accounts for about 5% of the body. Its shapes include oval, conical, heart-shaped, horseshoe-shaped, etc. The gases filled in the swim bladder are mainly oxygen, ammonia and carbon dioxide, with the highest content of oxygen. Therefore, in an oxygen-deficient environment, the swim bladder can serve as an auxiliary respiratory organ to provide oxygen to the fish. The most important function of the swim bladder is to adjust the specific gravity of the fish body through inflation and deflation, thereby adjusting the water pressure balance inside and outside the bony fish's body and controlling the ups and downs of the body.

When the fish sinks to a certain water depth, the huge external pressure will make it unable to adjust the volume of its swim bladder. At this time, the buoyancy force it received was less than its own gravity, so it sank to the bottom of the water involuntarily, could no longer float, and eventually drowned due to being unable to breathe.

The fish maw is cooked into dishes, which are soft, glutinous and delicious. Folks often stew fish maw with longan, red dates, walnut kernels and rice wine before and after the winter solstice.