The importance of oxygen applied in aquaculture
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Adequate oxygen levels guarantee optimum growth of fish and strengthen their immune system. Resistance against diseases increases significantly. As a consequence, a reduced mortality rate, faster growth and a larger size of fish can be achieved. To maintain optimum growth conditions, the oxygen concentration should be controlled permanently. In aquaculture, especially in hatcheries and for rearing delicate species, ozone is used to further prevent the occurrence of pathogenic microorganisms. Ozone is preferably produced from pure oxygen as this allows the use of smaller ozone generators.

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Boosting profitability

Pure oxygen brings the following economic advantages:

– Higher stock densities

– Optimum growth rate

– Further increased production capacity due to reduced cycle times

– Feed cost savings due to improved feed utilisation

– Minimum fish mortality

– Pumping cost savings due to reduced water recirculation

– Higher sales price due to average weight gain and higher quality of fish

 

Oxygen for fish transport

Fish need oxygen at any time, at any place. In many cases, however, they have to be transported: from hatcheries to grow out facilities and from there to the processing units. In order to maintain optimal conditions, the injection of oxygen into the transport containers is of value. 



High demand

Fish is an important component of a balanced diet and its good taste is enjoyed all over the world. According to estimates, fish consumption will rise by nearly 40 percent to 180 million tonnes by the year 2030. In order to meet this increasing demand, more and more fish have to be raised in fish farms. In fact, aquaculture turns out to be the fastest growing sector in food production. Aquaculture is the future because it protects the oceans from further overfishing and produces healthy food with an extremely high feed conversion efficiency.

 

Dissolved oxygen is the most important factor in achieving good results in fish farming. The amount of oxygen needed for optimal fish growth depends on the species, its size, feeding rate, activity level, and the water temperature. As pure oxygen optimises the dissolution of oxygen in water, it plays an significant role in effective aquaculture.

 

Advantages of pure oxygen

Why should pure oxygen be applied in aquaculture if air is available free of cost? If an aeration system is applied, the oxygen transfer into the water is limited by the small difference of the oxygen saturation in the air and the oxygen concentration in the water basin. Poor aeration capacities and high energy consumption are the consequence.

 

The use of pure oxygen, on the other hand, provides considerable advantages. The fact that air contains only 21 % of

oxygen implicates that in an atmosphere consisting of pure oxygen, the oxygen saturation is about five times higher than in

air. According to the principle of diffusion, the transfer rate of oxygen is proportional to the difference between the saturation concentration (in balance with the gas phase) and the actual concentration in the water. This is why oxygenation systems are much more efficient. For example, in order to maintain an oxygen saturation of 80 % in a fresh water basin at 15 °C, a system using pure oxygen at ambient pressure is about 20 times more effective than an aeration system. Moreover, with pure oxygen, much higher oxygen concentrations can be obtained, thus allowing the use of bypass solutions, which require less pumping energy. Thereby, the risk of supersaturation can be safely avoided due to the fact that no other gas – like nitrogen in aeration – is dissolved. All these considerations clearly demonstrate that the use of pure oxygen is technically as well as economically superior to the operation with air. Its advantages more than compensate for the extra cost.

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Dissolved oxygen (DO)

One major characteristic of water quality that is important to the growth and survival of fish is dissolved oxygen. Oxygen consumption by fish and dissolved oxygen requirements increase with temperature and feed consumption. As intensification increases, dissolved oxygen can often become a limiting factor.  While tilapia is generally tolerant of low dissolved oxygen, extended periods of hypoxia may cause reduced growth and increased mortality, particularly when algal blooms occur.  Many fish have shown reduced growth or survival when dissolved oxygen condition fall below levels as high as 5mg/L, or 25 to 50% saturation. However, there appear to be no guidelines as to what level of dissolved oxygen causes reductions in growth of tilapia. Dissolved oxygen can be tested with chemical method (titration with Na2S2O3 dropper). Different species of fish have different lethal dissolved oxygen.

Table 1 Lethal D.O of different species of fish

  Species of fish

Lethal D.O. (mg/l)

Fish

Lethal D.O. (mg/l)

  Common carp

0.5

Tilapia

0.23

  Silver carp

3.0

Butter fish

0.5

  Bream grass carp

3.5

Rainbow trout

3.5

Mandarinfish

5.0



Generally, fish of bottom inhabiting requires lower dissolved oxygen; fish of surface or middle niche requires higher dissolved oxygen; fry and fingerlings require the highest dissolved oxygen. Culturist can increase dissolved oxygen by aerator or adding fresh water into pond.



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