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DW (English)
  • 11/8/2024
Why don't fish sink in water? This Tomorrow Today viewer question comes from Sante K. in Zambia.
Transcript
00:00Why don't fish sink in water?
00:09Many fish can float in one place without moving their fins much and the ability is all down
00:14to their swim bladders. It's an organ located in their trunks, a kind of sack filled with
00:21gas. Without one, a fish would sink to the bottom like a stone. That's because their
00:29bodies are denser, hence heavier, than the surrounding water. Without a swim bladder,
00:35they'd have to constantly exert themselves to remain in the same place in the water column.
00:40That'd be strenuous. The swim bladder allows a fish to equalize its weight with that of
00:46the surrounding water. Every 10 meters, water pressure increases by 1 kilogram per square
00:53centimeter. If a fish swims up, its swim bladder expands as the pressure of the water column
00:59above it falls, so the fish grows more buoyant. To keep from rising even higher, it has to
01:05release some gas to compensate. If, on the other hand, a fish swims deeper, the water
01:11pressure acting on it rises, compressing the swim bladder. It then must force more gas
01:16into the organ in order to achieve neutral buoyancy with its surroundings. The principle
01:23works in the same way as a buoyancy compensator. By pumping air into a vest or releasing it,
01:30divers can also float effortlessly at different depths. The swim bladder also serves to stabilize
01:37fish in the water. Those that have problems with their swim bladders often seem unbalanced,
01:43swim chaotically, or hang upside down. Many cartilaginous fish like rays and sharks have
01:50to remain in constant motion to avoid sinking to the bottom. Instead of a swim bladder,
01:56they have light skeletons and very large livers that store lots of fat. Because fat is also
02:02lighter than water, that helps keep them buoyant. Bottom-dwelling fish like flatfish and moray
02:11eels also get by without swim bladders. That's because they don't have to adjust to changes
02:18in water pressure.

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