Swim bladder/Air bladder

The density or unit mass of fish is 1.06 - 1.09 and they are therefore a little bit heavier/denser than water, which has the density of 1.000 - 1.028 (saltwater is denser than freshwater). The fish would sink without the countermeasure of the swim bladder. The air in the swim bladder is at the same pressure as the fish itself. It is 1 atmosphere at the surface, but increases additionally of 1 atmosphere for every 10 metres down into the water. Fish at 100 m depth is therefore at 11 atm pressure, as the air in the swim bladder. Its volume at that depth would be only 1/10 of its volume at the surface. The fish therefore must add air into it to keep its balance of floating at greater depth.

Fish without swim bladder must keep their balance in a different way, e.g. by swimming. Fish without swim bladder are considered to use about 20% of the energy they use for swimming to keep afloat in the water.

The swim bladder matures as an outgrowth from the oesophagus (gullet) or the digestive tract. Some fish (e.g. eels and salmonids) keep a connection between the swim bladder and the oesophagus (a so-called ductus pneumaticus) all their life. In other fish this duct disappears at the laerval stage and the swim bladder becomes totally closed (e.g. cod). In flatfish the swim bladder disappears totally, and most demersal fish types are without one. Cartilaginous fish are also without swim bladders.

The swim bladder keeps the fish at the correct depth, so it does not have to waste energy for that purpose. But the drawback is that the balance applies to the depth where the fish is. If the fish swims deeper, or if the pressure in the swim bladder is reduced, the swim bladder contracts and the fish begins to sink. The fish would have to swim to keep himself at the correct depth. Even if the swim bladder totally collapses, the fish can easily swim upwards as it is never much heavier than the water. But it is a much more difficult situation if the pressure in the bladder increases and the fish floats upwards or is suddenly brought up from some depth to the surface. In such a situation control of the pressure in the swim bladder is imperative. If the fish goes quickly from a depth of 300 m to the surface, the volume of the swim bladder would increase 30 times and it would either burst or be pressed out of the fish. In fish with closed swim bladder, the air pressure inside it is controlled by gas exchange between blood and swim bladder. Fish with open swim bladder can press air out of it through the mouth, or, into it by fetching air at the surface. They are therefore in less trouble when moving quickly from a great depth. Usually though they add air from their blood into the swim bladder if that is needed. Fish that stay mostly at the surface usually have open swim bladder. For the proper function of the swim bladder it has to be filled with air in the beginning. If yolk sac fry cannot for some reason fetch air for their swim bladder it will never function normally.