Why most fish never sink to the bottom of the ocean

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Why most fish never sink to the bottom of the ocean

Did you ever wonder how fish rise and sink in water?
To understand this first you need to know what forces work underwater. Water is a fluid. Two forces mainly work in an object(in this case the fish) immersed in a fluid. 

  • Downward pull by the gravity
  • Upward thrust by the buoyancy

Force of bouncy is equal to weight of fluid an object displaces when it’s immersed in water. If the buoyancy force is higher than the object’s weight, it will float on water’s surface. However if the object’s weight exceeds the buoyancy force , it will sink to the bottom. When the buoyancy force is exactly the same as the object’s weight, the object has neutral buoyancy and remains at its level.

This means that the density of the object is also important. Objects with higher densities have a higher mass and displaces a low volume of water. In this case the mass to volume ratio is high. if the same volume is displaced through a low density object the mass to volume ratio is low. This way the objects denser than water will sink while objects less dense than water will float.

So in the case of fish, it has to increase the volume without changing the mass to float, or decrease the volume to sink. For this purpose fish have a special organ called the swim bladder.

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Swim bladder from a bony (teleost) fish

The swim bladder is an inflatable organ like the human lung. Fish fills the bladder with oxygen which is gathered from the surrounding water, by using its gills. This way the bladder inflates and the volume is increased, without changing its mass. So the fish floats. when the swim bladder is filled in full capacity the fish floats to the surface. To sink in to the water, fish deflates the bladder and reduces its volume accordingly. To float in an exact level it inflates its swim bladder to a level that displaces an amount of water which weighs the same as its body.

However there are species that do not have a swim bladder. Some fish such as sharks propel using their fins in the same way that an airplane flies.

Various diseases in swim bladder may cause irregularities  such as swimming upside down.

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Swim bladder disease has resulted in this female ryukin goldfish floating upside down

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