Why Dead Fish Float Upside Down

The Science Behind Dead Fish Floating Belly Up

Dead fish float upside down because they fill with light gases. The muscles and bones of the spine are heavier, so fish float belly up.
Dead fish float upside down because they fill with light gases. The muscles and bones of the spine are heavier, so fish float belly up. Mike Kemp / Getty Images

If you've seen dead fish in a pond or your aquarium, you've noticed they tend to float on the water. More often than not, they'll be "belly up", which is a dead giveaway (pun intended) you're not dealing with a healthy, living fish. Have you ever wondered why dead fish float and live fish don't? It has to do with fish biology and the scientific principle of buoyancy.

Why Living Fish Don't Float

To understand why a dead fish floats, it helps to understand why a live fish is in the water and not on top of it.

Fish consist of water, bones, protein, fat, and a smaller amount of carbohydrates and nucleic acids. While fat is less dense than water, your average fish contains a higher amount of bones and protein, which makes the animal neutrally buoyant in water (neither sinks nor floats) or slightly more dense than water (slowly sinks until it gets deep enough).

It doesn't require much effort for a fish to maintain its preferred depth in the water, but when they do swim deeper or seek shallow water they rely on an organ called a swim bladder or air bladder to regulate their density. How this works is that water passes into a fish's mouth and across its gills, which is where oxygen passes from the water into the bloodstream. So far, it's a lot like human lungs, except on the outside of the fish. In both fish and humans, the red pigment hemoglobin carries oxygen to cells. In a fish, some of the oxygen is released as oxygen gas into the swim bladder.

The pressure acting on the fish determines how full the bladder is at any given time. As the fish rises toward the surface, the surrounding water pressure decreases and oxygen from the bladder returns to the bloodstream and back out through the gills. As a fish descends, water pressure increases, causing hemoglobin to release oxygen from the bloodstream to fill the bladder.

It allows a fish to change depth and is a built-in mechanism to prevent the bends, where gas bubbles form in the bloodstream if pressure decreases too rapidly.

Why Dead Fish Float

When a fish dies, its heart stops beating and blood circulation ceases. The oxygen that is in the swim bladder remains there, plus decomposition of the tissue adds more gas, particularly in the gastrointestinal tract. There's no way for the gas to escape, but it presses against the fish's belly and expands it, turning the dead fish into a sort of fish-balloon, rising toward the surface. Because the spine and muscles on the dorsal side (top) of the fish are more dense, the belly rises up. Depending on how deep a fish was when it died, it might not rise to the surface, at least not until decomposition really sets in. Some fish never gain sufficient buoyancy to float and decay under the water.

In case you were wondering, other dead animals (including people) also float after they start to decay. You don't need a swim bladder for that to happen.

Dead Fish Key Takeaways

  • Dead fish float in water because decomposition fills the fish's gut with buoyant gases.
  • The reason fish typically go "belly up" is because the spine of the fish is more dense than its belly.
  • To some extent, you can gauge how long a fish has been dead by how well it floats. This is because it takes time for bacterial decomposition to produce gases. However, it the fish is fed upon by another animal and the gas is allowed to escape, the fish may not float.
  • Healthy living fish don't float because they have an organ called a swim bladder that regulates the amount of gas present in the body of a fish and thus its buoyancy.


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