Why do fish float on their sides when they die? — Cierra, Franklin, Ind.
Marshall Brain Answers:
When a fish dies, it can do one of two things: it can float or sink. Dead fish tend to float, at least for awhile, as seen here:
That happens because bacteria in the intestines produce gas, and the gas trapped inside the body causes it to float (see What causes flatulence? for info on bacteria in the gut).
Given that the dead fish’s body contains an air pocket, it is going to float head-up, tail-up, top-up, bottom-up or on its side. If the air bubble were in the head and the tail were heavy, the fish would float head-up. The reverse would cause the fish to float tail-up. But the gas bubble is toward the center of the fish’s body. Therefore, like a board, the fish floats on its side.