The Perils of Pressure: Understanding Barotrauma in Fish and Divers

The underwater world, while captivating, operates under a different set of physical rules than we experience on land. One of the most significant factors is pressure, which increases dramatically with depth. This pressure can have profound effects on living organisms, leading to a condition known as barotrauma. Let's explore how pressure changes impact both fish and scuba divers, and what can be done to mitigate these effects.

The Inflated Fish: Boyle's Law in Action

Have you ever seen a fish reeled up from the depths with bulging eyes and its stomach protruding from its mouth? This startling sight is a direct result of rapid pressure changes. As you descend into the sea, pressure increases by 14.7 pounds per square inch for every 33 feet of depth. Consider the yelloweye rockfish, which can live at depths of up to 1800 feet, enduring over 800 pounds of pressure per square inch!

Boyle's Law explains this phenomenon: the volume of a gas is inversely related to pressure. This means that as pressure increases, the volume of a gas decreases, and vice versa. Fish have air-filled spaces, most notably a swim bladder, which helps them control their buoyancy. When a fish is quickly brought to the surface, the pressure decreases rapidly, causing the air in its swim bladder to expand dramatically. This expansion can force the fish's stomach out of its mouth and its eyes out of their sockets, a condition called exophthalmia. In some cases, gas bubbles can even build up inside the cornea, leading to a crystallized appearance.

The Diver's Dilemma: Henry's Law and Decompression Sickness

Scuba divers don't have swim bladders, so they avoid the same rapid expansion issues as fish. However, they face their own set of pressure-related challenges. Divers regulate the pressure in their lungs by breathing out as they ascend, but they must also be aware of Henry's Law, which states that the amount of gas that dissolves in a liquid is proportional to its partial pressure.

The air divers breathe is about 78% nitrogen. At higher pressures underwater, nitrogen from the air in a scuba tank diffuses into the diver's tissues in greater concentrations than it would on land. If a diver ascends too quickly, this built-up nitrogen can come out of solution and form microbubbles in their tissues, blood, and joints. This causes decompression sickness, also known as the bends.

Avoiding the Bends

Decompression sickness can cause severe pain and even death. Divers avoid this by:

• Ascending slowly
• Taking breaks along the way, called decompression stops

These measures allow the gas to diffuse back out of their tissues and be released through their breath.

Returning Fish to the Deep: A Proper Recompression

Just as divers need decompression, fish need recompression to recover from barotrauma. This means returning them to the depth where the pressure will reduce the gas volume in their bodies. However, simply tossing an inflated fish overboard is not the answer. The inflated body will float, making it easy prey for predators.

There's a common myth that piercing the fish's stomach with a needle will release the air and allow it to swim back down. However, this can harm the fish and is not recommended. Instead, fishermen can use a descending device to lower the fish on a fishing line and release it at the appropriate depth. As the fish heads home and recompression reduces gas volume, its eyes can return to their sockets, its stomach can move back into place, and it can live to swim, eat, reproduce, and replenish the population.

Understanding the effects of pressure on living organisms is crucial for both divers and fishermen. By following safe diving practices and using proper recompression techniques, we can minimize the risks of barotrauma and ensure the health of our underwater ecosystems.