like all living creature. gases build up inside a fish's body when it dies, and this keep it afloat in the water. These gases collect mainly in the abdominal cavity, which is on the underside of the fish. When this happens, the lighter, gas-filled abdomen becomes buoyant and the more compact back forms the centre of gravity, with the result that the fish is turned upside down. It is only when the decomposition is further advanced that the gases are released, and the fish sinks.