Almost all fish have a gas bladder (except for tuna, sharks, or some flatfish living in the ocean floor).
This bladder varies according to its volume, which allows the fish to adapt its buoyancy!
The body tissues of fish are denser than water. To avoid sinking, they must compensate for the effects of gravity. Most fish have an internal organ called the swim bladder. The bladder adjusts the buoyancy of the fish by reducing or increasing the volume of gas it contains, much like the ballast tanks in submarines.
Why do dead fish float?
When a fish dies, its swim bladder fills with gas from the decomposition of its organs. This low-density gas causes the fish to rise to the surface. And as it is contained in the abdomen, the corpse turns over and floats with its stomach in the air.
A small aside: fish that live at the bottom of the water do not have a swim bladder. Sharks also have no swim bladder. To keep from sinking, they must always swim!
The swim bladder is located in the abdomen of the fish, under its center of gravity, which is located near the head. This forces the fish to keep its balance by moving its fins.
Do fish always float when they are dead?
Most of the time, when a fish floats on the surface, it is because it is dead, there are some isolated cases where the fish could have been sick, we wrote an article about this. To be sure, you just have to check if the fish is moving its fins or not. If it has not moved for many minutes, it is most likely dead.
The reason is actually quite simple, as soon as it dies, the fish starts to rot inside. The inside of its belly fills with gas, coming from the decomposition of its organs. This low density gas causes the fish to float to the surface.
Why do dead fish float upside down?
As explained just above, the belly fills with gas, and since the belly is located towards the bottom of the fish, floating turns the fish upside down. The corpse turns over and floats with its belly in the air, quite simply.