When fish are alive, they can maintain a certain position in the water, neither floating to the surface nor sinking to the bottom. This is because most fish have an organ called a swim bladder, which regulates their body density. By releasing or taking in gas at different depths, the swim bladder adjusts the fish’s body density to match the surrounding water. In this way, fish can effortlessly remain at various levels in the water.
When a fish swims from deep water to shallow water, the water pressure decreases, causing the gas in the swim bladder to expand and the body density to decrease, resulting in an upward buoyancy. To counteract this upward movement, the swim bladder releases some gas. Conversely, when a fish swims from shallow water to deep water, the body density increases, requiring the swim bladder to take in some gas to help the fish stay at that depth. After a fish dies, it loses this regulatory ability, and the swim bladder fills with gas, reducing the body density. Most fish have heavier dorsal areas due to more vertebrae and muscles, while the abdominal area, which contains various internal organs (such as the digestive and reproductive systems), is more hollow and thus less dense. Therefore, when fish die, their less dense abdomen usually faces upward.
Of course, some fish do not have swim bladders, such as sharks, rays, and flatfish. Their bodies are denser than water, so to remain at a certain depth, they must continuously move their fins to counteract their tendency to sink. When these fish die, they sink to the bottom of the water.