NotEinstein
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We’ve all seen dead fish floating belly-up, whether it is at home in the aquarium or out on the river. They seem to do pretty well staying under the water when they are alive, so what makes them surface once they are dead? The hard part is having to explain to small children that the fish are not sleeping. In my opinion, it is much easier explaining the science behind the floating.
Objects less dense than water float. Objects denser than water sink. Live fish have no buoyancy. A live fish’s body is about the same density as the water they swim in. They neither float nor sink. Because the density and pressure of water changes with depth and composition, most fish have an organ that helps them climb and dive in the water. This organ is called a swim bladder. It extracts oxygen from the gills and stores it to create extra buoyancy. The fish has no control of its swim bladder. It works automatically, just like the other organs of the body.
When a fish dies, oxygen gets trapped in the swim bladder. Additional gases are created as the fish begins to decompose. The stomach of the fish and the entire abdominal cavity fill up with gasses. When fish die, their bodies close up, and the gasses can’t escape. This turns the fish’s stomach into a balloon. Just as any lightweight, air-filled container, the fish floats to the surface, balloon (belly) side up. Fish don’t always immediately float to the surface. It often takes several hours, or even days, before their body accumulates enough gas to allow them to float.
 Posted 5123 day ago
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