Biologists have studied the damage to several freshwater and marine fish species caused by a sudden decrease in water pressure termed “barotrauma.” Anglers who fish for deepwater gamefish have seen gas bladders expand, sometimes pushing the stomach out the fish’s mouth. Victims float helplessly at the surface, often with popped eyeballs and vents. In severe cases, anglers can feel the fish cease fighting at some point and it reaches the surface D.O.A.

In the 1980s, researchers quantified damage to internal organs, ruptured blood vessels, and bloated gas bladders that occurred when juvenile and young adult largemouth bass were depressurized, in an amount equivalent to a rapid rise to the surface from a depth of 30 feet. Greater depth changes increased damage. They also found tissue damage and a tendency for bladders to expand when small adult bass were exposed to quick changes in pressure equivalent to a shift from 20 feet to the surface. A move to the surface from just 15 feet caused minor damage, as well. Anglers have observed larger bass suffering from expanded bladders when held in livewells after capture in even shallower water.

Fish Physiology

The walls of the gas bladder as well as the body cavity and ribs resist expansion for at least a few seconds during short, natural, vertical movements. Bass placed in livewells seem to resist expansion for up to about 5 minutes, when loss of balance may accompany increased buoyancy. Gas bladders, much like new balloons, begin to expand after a minute or so of greatly increased gas pressure. Immediately released fish pulled from moderately deep water (say, 18 to 25 feet for largemouth bass) often can swim back down and repressurize, minimizing effects of barotrauma.

Bass and other species that suspend evidently have a protective instinct to avoid dramatic vertical upward movements, holding instead at depths where the pressure changes in the gas bladder don’t cause discomfort or disrupt balance. It seems that as bass grow, their bladder walls don’t increase proportionally in strength. As a result, they gradually experience a more restricted range of vertical movement.

Species like shad and trout have bladders with open ducts that allow gas pressure to escape. They can ascend far more dramatically without damage than fish with closed bladders like bass, crappie, and walleye. Fish of both types become heavier as they descend from their acclimated depth, however.

As external pressure increases with depth, bladder walls collapse; so most fish species, with or without closed ducts, lose neutral buoyancy whenever they descend more than a few inches below the depth at which they’ve acclimated their gas bladder pressure. Fish with open ducts can gulp air at the surface and hold it during descent to build gas pressure in their bladders before diving. Shad at the surface often gulp air before swimming down, a behavior anglers call “plipping.”

Physical Laws

Gas pressure in closed bladders is controlled by a basic gas pressure-volume ratio known as Boyle’s Law (p1V1 = p2V2). As pressure declines, volume increases, and vice versa. The process is automatic in fish as external pressure changes. Gas slowly diffuses into or out of bladders in response to changes in the partial pressure of gasses in the fish’s blood. With decreasing water pressure, pressure builds inside the bladder until it overpowers and stretches the bladder walls, allowing the gas to expand to the volume dictated by changes in the pressure-volume ratio of Boyle’s Law.

The weight or pressure of 33 feet of water equals one atmosphere. We have one atmosphere of pressure at the surface, two atmospheres of pressure 33 feet down, three atmospheres at 66 feet, four at 99 feet, and so on. The pressure change when fish move or are moved from 33 feet to the surface represents a 50 percent decrease from two atmospheres to one atmosphere of pressure. This yields a 50 percent increase in gas volume, which theoretically doubles the bladder size.