Why Do Fish Float to the Top When They Die?

Fish float to the top when they die because they stop swimming and gas builds up inside them. When a fish is alive, it moves water through its body with its gills and swallows air or takes in oxygen from the water. But when it stops moving, like after death, the gas that was trapped inside it starts to expand. It’s kind of like a balloon: if you stop blowing air into it, it will float up because the gas pushes the fish upward.