Our fish require oxygen for their metabolism and, if you are using a recirculating system, the bacteria in the biofilter also need oxygen for their life processes.  The oxygen is present in the water in a dissolved form and taken up via the gills of the fish by simple osmosis.  As there is a base requirement for oxygen within the blood of the fish, the more oxygen there is dissolved in the water the greater the gradient and easier it is for the fish to absorb oxygen into their blood.  If the level of oxygen in the water decreases then the osmotic potential also falls, reducing the amount of oxygen the fish can access from the water, placing them under stress.

We generally do not aerate cages, but rather select the site carefully and stock the cages appropriately such that water movement through the cages replenishes the oxygen utilised by the fish.  In earth ponds planktonic algae produce oxygen by day through photosynthesis, but beware, at night the algae also respire, adding to the demand for oxygen within the system.  Recirculating systems have no significant external source of oxygen and water movement is heavily relied upon to supply the oxygen required by the fish and bacteria.

There are several methods that can be utilised to get oxygen into water, including water replacement, passive transfer, aeration and oxygenation.  As mentioned, water replacement is the preferred method in cage culture where high exchange rates ensure water quality inside the cage remains high at all times.  Earth ponds can be stocked at low fish densities so that oxygen dissolving from the air into the pond at the water surface (passive transfer) is adequate to supply the needs of the fish, or at far higher densities where the additional oxygen requirement is supplied by means of active aeration.  Usually such aeration is only required at night when there is no photosynthesis and the algae are also consuming oxygen.  In recirculating systems we supply aeration to each tank to stir the water and stimulate active transfer of oxygen into the water at the surface.  With sensitive species at high densities we may consider introducing oxygen directly into the water to significantly increase the levels, but this is an expensive process and substantially increases risk associated with equipment failure, so proceed with caution.

At this time of the year when the water is warm and holds less oxygen it is especially important that we ensure adequate oxygen is available to our fish and bacteria, whichever infrastructure type you are using.  Stress compromises growth and feed conversion efficiency, and is the beginning of the sickness and mortality continuum, and should be avoided at all costs.