A6 Barotrauma due to rapid decompression

Topic

Decompression barotrauma occurs when a fish ascends too rapidly from depth, causing the swim bladder—the central element of this model—to expand faster than the fish can absorb the excess gas via the *rete mirabile*, an organ specialized for this task. This expansion follows the same physical principle governing any compressed gas: the volume occupied by a gas is inversely proportional to the surrounding absolute pressure; thus, as the fish ascends and ambient pressure drops, the gas inside the swim bladder expands. Ascending from a depth of thirty meters—where the pressure is approximately four atmospheres—to the surface, where pressure drops to one atmosphere, causes the volume of the swim bladder to quadruple.

When the fish's ascent rate exceeds the reabsorption capacity of the *rete mirabile*, the swim bladder becomes over-distended because the gas expands faster than the organ can remove it. If this over-distension continues, the tension on the bladder wall eventually exceeds its tensile strength, resulting in a hernia, a rupture, or even eversion of the bladder into the fish's mouth—a condition known as swim bladder prolapse. The tension the bladder wall must withstand increases with higher internal gas pressure and a larger bladder radius, while decreasing with greater wall thickness; rupture occurs precisely at the point where this tension exceeds the tissue's strength. Consequently, there is a critical safe ascent rate: the speed at which the rate of gas expansion due to the pressure drop matches the rate at which the *rete mirabile* can reabsorb it. Below this critical rate, the *rete* can handle the load, whereas above it, the swim bladder is at risk of over-distension. Barotrauma damage is not limited to the swim bladder: the same drop in pressure that causes it to expand also saturates the fish's blood with gas. This supersaturated gas can nucleate to form bubbles within the blood vessels—a gas embolism phenomenon analogous to the decompression sickness experienced by divers—ultimately obstructing the fish's blood circulation. The amount of gas dissolved in the blood at a given pressure follows the same relationship with ambient pressure that governs the dissolution of any gas in a liquid. Nucleation of these bubbles in the blood occurs when the total pressure of the dissolved gas exceeds the local pressure of the surrounding tissue beyond a critical threshold; as long as the dissolved gas pressure remains below that threshold, the gas stays dissolved without forming bubbles.

The barotrauma affecting both the swim bladder and the circulatory system creates a particularly deceptive effect in catch-and-release fishing: the actual mortality rate is far higher than the fish's appearance at the surface would suggest. A fish that appears to recover and swims normally may die hours later—either from the previously described swim bladder rupture or prolapse, or from a gas embolism that progressively obstructed its circulation from the moment of ascent.

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