December 11, 2010
Why does fish turn their belly up in the air when they die or become paralyzed?
Many fish in our waters have an air-filled swim bladder that helps them to keep themselves floating in the water. The more air that is in this bladder, the higher the fish swims i the water. The bladder is a kind of balloon that automatically fills with air from the fish's digestive and metabolic system. To keep a certain level in the water, the fish must actively pump air out of this bladder.
But when the fish dies, it loses control of the swim bladder and the air builds up inside this bladder, and then the fish rises. If you throw a bomb into a lake, the fish with a biological swim bladders will immediately ascend to the water surface while the fish that do not have a bladder, will sink to the bottom. In time, they also grow up and lie with belly up, because decomposition gases accumulated in the fish's internal organs.
Link Science.dk (PS. it's written in danish)