How to Float,
How the Air Bladder Works

Buoyed Up The gases in the air bladder, mostly oxygen and nitrogen, prevent the salmon from sinking by making it more buoyant. The salmon can stabilize itself in a hovering position by adjusting the volume and pressure of gas to a neutral position. (The neutral gas level in freshwater is slightly higher than in saltwater.)

When the salmon needs to move deeper into the water, it releases some of the gas to become less buoyant. When its ready to move upward, gas in the bladder expands to make the fish lighter. Fish that change depth frequently, such as salmon, tend to have high oxygen levels in their bladders because it fills in faster and leaves faster than other gases.

That Sinking Feeling Unlike bottom-dwelling fish, salmon must be able to move easily from one depth to another, swimming close to the surface of a freshwater stream, or sinking deep into the depths of the ocean. Without an air bladder, and with body tissue that is denser than water, the salmon would have to use an enormous amount of swimming energy to keep from sinking.