When fish die, their eyes may appear to bulge or “pop out.” This occurrence is not a sign of disease or injury sustained while living, but rather a natural process resulting from several physical and biological changes that take place after death.
Internal Gas Buildup
Upon a fish’s death, internal biological processes, particularly decomposition, continue. Bacteria naturally present in the fish’s gut and tissues begin to break down organic matter. This decomposition releases various gases within the body cavity.
These accumulating gases increase internal pressure within the fish’s body. The eye sockets, being weaker points in the skull structure, can then allow this internal pressure to push the eyes outwards. This gas-driven protrusion is a common post-mortem change.
Rapid Changes in Water Pressure
Another significant cause of eye protrusion, especially in fish caught from deep waters, is barotrauma. This condition occurs due to rapid changes in environmental pressure. As a fish is quickly brought to the surface from deep water, the surrounding pressure dramatically decreases.
Gases within the fish’s body, particularly in the swim bladder, rapidly expand. This expansion generates internal pressure, which can force organs, including the eyes, to bulge outwards. Such effects are often immediate upon ascent for deep-sea species.
Additional Factors Affecting Fish Eyes
Other factors can also contribute to eye protrusion in dead fish. Rigor mortis, the stiffening of muscles that occurs after death, can exert pressure on surrounding tissues, including those around the eye sockets. Physical handling or external pressure applied to the fish after death can mechanically force the eyes from their sockets. If internal pressure is already elevated due to gas buildup from decomposition or barotrauma, even slight external forces can exacerbate eye protrusion.
What Popping Eyes Indicate
Protruding eyes in a dead fish are generally a normal physical response to the processes that occur after death. It primarily reflects the physical changes related to internal gas expansion or rapid pressure changes during capture. While the appearance might be unappetizing, bulging eyes usually just signify the natural post-mortem physical processes.
It can sometimes be an indicator of how the fish was handled or caught, particularly if it was brought up quickly from deep water. Protruding eyes are not a direct indicator of freshness or spoilage, which are better assessed by other signs like smell, gill color, and flesh texture.