Physics in Fish—Bringing fish to the surface from deep water can cause damage to their eyes and internal organs. The extent of damage depends somewhat on depth range, species of fish, and physiological state of the fish.
The causes of this damage, called barotrauma, are based on two important laws of physics, Boyle’s Law and Henry’s Law. Boyle’s Law, defined by Robert Boyle, an Irish scientist, in the 1660s, relates to the relationship of pressure and volume of a gas. It states that within a closed system at constant temperature, the volume of a gas is inversely proportional to the pressure acting on that volume. When pressure is cut in half, volume is expected to double. Related injuries include ruptured swim bladders and exopthalmia (bulging eyes), everted stomach or intestine, internal hemorrhaging, and gas bubbles in the organs.
Henry’s Law, derived by the English chemist William Henry in 1803, states that at a constant temperature, the amount of a gas that dissolves in a type and volume of liquid, such as blood, is directly proportional to the partial pressure of that gas in equilibrium with that liquid. When pressure surrounding a fish is reduced, the dissolved gas can come out of solution, resulting in gas bubble formation. This is the mechanism that can cause “the bends” in scuba divers who ascend too quickly. For fish, it most commonly occurs when they suddenly pass through the areas of low pressure created by turbines at hydropower dams, causing blood and bodily fluids to become supersaturated, allowing gas bubbles to form. Bubbles grow as pressure drops, which can cause internal hemorrhaging.
Carbonated drinks give an everyday example of Henry’s Law. In an unopened can, the gas above the drink is almost pure carbon dioxide, kept at a pressure slightly greater than atmospheric pressure. And the drink contains dissolved carbon dioxide. When the can is opened, gas escapes, producing a hiss. Because the partial pressure of the carbon dioxide above the liquid is reduced, carbon dioxide comes out of solution as bubbles. Left out, the drink becomes “flat” when the carbon dioxide in solution comes into equilibrium with the carbon dioxide in the air.
Barotrauma problems associated with Boyle’s Law most often affect fish species with a closed duct between the swim bladder and esophagus, such as bass, walleyes, perch, and crappies. Salmonids, catfish, and other species with a duct, are more often damaged during extremely fast decompression, such as young fish passing through a dam. They generally can expel gas by belching when brought to the surface by an angler.
Keep these physics principles in mind if you fish deep for bass, walleyes, perch, or crappies. Damage can occur when fish are raised as little as 20 or 25 feet. Because internal tissues are elastic, they can stretch somewhat before breaking so swift release can sometimes prevent permanent damage. It’s a common fallacy that raising ductless species slowly prevents barotrauma. It does little good, as gas release for them requires hours rather than minutes. Rather, raising them quickly and releasing them immediately might work better to reduce barotrauma effects.