When anglers reel in a fish from deep waters, they sometimes observe a phenomenon: the fish’s eyes appear to bulge or pop out. This has a clear scientific explanation. It is an involuntary response to rapid environmental shifts that fish cannot control.
The Role of Pressure Changes
Barotrauma, injuries from pressure changes, explains this phenomenon. Fish in deeper waters exist under significant external pressure. As they are rapidly brought to the surface, this external pressure decreases dramatically.
This rapid decrease in pressure causes gases within the fish’s body to expand. This process is analogous to a diver experiencing “the bends” if they ascend too quickly. Boyle’s Law states that if temperature is constant, gas volume increases as pressure decreases. For fish, this means gases in their bodies expand when external water pressure lessens.
Anatomical Impact on Fish
Expanding gases within a fish’s body impact its internal anatomy, leading to eye protrusion. The swim bladder, a gas-filled organ, helps fish maintain buoyancy. As a fish is brought to the surface, the gases inside this bladder expand significantly due to the reduced external pressure.
This expansion creates immense internal pressure, pushing on the surrounding organs and tissues. The eyes are particularly susceptible because they are contained within rigid sockets, and the internal pressure forces them outwards. This internal pressure can also displace other internal organs, causing them to protrude from the fish’s mouth or vent. These physical manifestations are direct consequences of the swim bladder’s rapid overexpansion.
Factors Influencing Eye Protrusion
Several variables influence eye protrusion. Fish caught from greater depths are more susceptible because they experience a larger pressure differential during their ascent. A fish brought from 100 feet deep will undergo a more significant pressure change than one caught in 20 feet of water.
Species differences also play a role, depending on how a fish regulates its swim bladder. Physoclistous fish, such as tuna and perch, have a closed swim bladder system that exchanges gas through their blood and cannot rapidly release gas. These fish are more prone to severe barotrauma. In contrast, physostomous fish, including goldfish, catfish, and trout, have a pneumatic duct connecting their swim bladder to their gut, allowing them to gulp or burp air to adjust buoyancy, making them less susceptible to eye protrusion. The speed of ascent is another factor; a rapid retrieve does not allow the fish sufficient time to adjust to the decreasing pressure, increasing the severity of barotrauma.