Why Do Dead Fish Float Upside Down?
Dead fish float in water because decomposition fills the fish’s gut with buoyant gases. The reason fish typically go “belly up” is because the spine of the fish is more dense than its belly. Oxygen remains in the bladder after a fish dies. Additional gases are released during decomposition.
As the fish decomposes, these additional gases fill the body cavity. The belly becomes a guts-filled balloon and the fish floats to the surface. Most of a fish’s mass is bone and muscle on its dorsal side, so as the belly balloon rises, fish tend to flip upside down. Fish don’t always float to the surface right away. They may sit on the bottom for a while until the gases build.
Fish are slightly more dense than the water in which they swim. They are almost neutrally buoyant, meaning the forces acting against the fish to make it sink are about equal to the forces inside the fish causing it to float. It also means fish don’t have to work too hard to keep from floating or sinking.
Pressure increases with water depth. Most species of fish counteract fluctuations by using an internal pouch called a swim bladder (also called the gas bladder or air bladder). Water enters a fish’s mouth and passes through gills, where oxygen is extracted and carried by hemoglobin through the bloodstream. Hemoglobin releases some of that oxygen into the swim bladder.
The amount of oxygen in the bladder determines the fish’s buoyancy. If a fish begins to dip, oxygen is absorbed into the bladder. If it floats too much, gas diffuses into the blood and out the gills.