How does a swim bladder help a fish maintain neutral buoyancy?

The swim bladder acts as a hydrostatic or ballast organ that enables fish to maintain neutral buoyancy, allowing them to remain at a specific depth without sinking or floating upward.12

In many fish, including protacanthopterygians, this organ functions by regulating the amount of gas—typically oxygen—contained within it. By adjusting the volume of gas in the bladder, the fish changes its overall density relative to the surrounding water.1234

• Increasing buoyancy: When gas is secreted into the bladder by special glands, the bladder expands. This increases the fish's total volume without changing its mass, thereby lowering its specific gravity and causing the fish to move upward.24
• Decreasing buoyancy: When the fish needs to descend, gas is absorbed from the bladder into the bloodstream, reducing the bladder's volume. This increases the fish's specific gravity, allowing it to sink.24

While some primitive fish, such as certain trouts, retain a duct connecting the bladder to the esophagus to manage gas levels, more advanced teleosts have lost this connection and rely entirely on specialized glands to control gas exchange.23