An air bladder is very much like a balloon. A long narrow balloon. Air bladders, also called swim bladders or gas bladders, change the buoyancy of fish, enabling them to maintain a certain position in the water column. A fish can add or remove air from the bladder, allowing it to move up or down in the water. Salmonids are among the generalized fishes known as Physostomi, which have a direct connection (pneumatic duct) between the air bladder and digestive tract. The duct facilitates the direct passage of air in either direction. from the surface and releasing air from the bladder by “burping.”