In fish farming, large fluctuations in the oxygen content of water have a negative impact on the well-being of the fish and consequently on its growth. High water temperatures or rich vegetation in the water are important alarming factors to observe, especially in summer periods.
Oxygen saturation for economic growth
The oxygen content of water is affected by many different parameters, in particular temperature, day-night variations, air pressure, feeding, size of fish and stress levels. Economic growth rates are only possible when these variations are taken into consideration.
Trout for instance generally require a relatively high level of oxygen supply as they consume between 0.3 and 0.5 kg of oxygen for every 1 kg of feed to be digested. In traditional trout farming, for the needs of the trout to be adequately met, the recommendation is to keep the oxygen saturation level at 100 % as it enters the fishpond and that it should still be at least 70 % at the outlet.
100 % oxygen saturation means that the water contains approx. 14 mg of oxygen per litre at 2 °C and only approx. 9 mg at 20 °C. In practice, the saturation level at the inlet is often different, especially if the farm is run on surface water, for instance due to high summer temperatures or to rich vegetation.
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Oxygen levels for economic growth in rainbow trout farming (fresh water). In order to achieve optimal feed utilisation, it is important to regulate the oxygen content of the water depending on the water temperature. At 15 °C, theory suggests an oxygen saturation level for economic growth rates of 70 %, while oxygen levels at water temperatures over 15 °C should be higher than 70 %.
Day-night variation caused by rich vegetation
In plant-rich watercourses, the oxygen content is influenced by the day-night rhythm. In summer periods, water bodies with strong plant or algae growth are often supersaturated with oxygen during daytime. This is a result of the photosynthesis of the plants and algae - they produce oxygen throughout the day in such quantities that the oxygen content of the water increases significantly. When it gets dark, photosynthesis stops, and during the dark hours, the plants consume oxygen. The oxygen level is often at its lowest shortly before the break of day.
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Day-night variation. Photosynthesis influences on the oxygen content in water and its impact vary depending on if the water is rich or low in vegetation.
Impact of oxygen on FCR and growth
The negative consequences of fluctuating oxygen levels on feed conversion call for oxygen control. A low oxygen content without oxygen enrichment leads to an unsatisfactory FCR, while oxygenation results in a good FCR and faster growth. In a trial run in summer, a traditional trout farm taking its inlet water from a stream with ample vegetation was divided into one section with and one without oxygenation. Using a 24-hour oxygen-measuring device with one probe in the inflow and another probe in the outflow, readings of the oxygen content were taken every 10 minutes.
Oxygen levels in the trial
The oxygen saturation level in the inlet to the farm fluctuated very much. At night, it showed out to be even lower than the minimum of what it should be in the outflow for the needs of the trout to be adequately met. In the section with no oxygenation the outlet water was very unsatisfactory throughout most of the trial period, especially at nighttime. In the section with oxygenation, the outflow values were balanced out by enriching the water with oxygen. It was clearly demonstrated that the outflow values could be kept at an optimal level by oxygenating the water from 4pm to 8am (16:00-08:00).
FCR and growth – trial results
In the section of the fish farm with no oxygenation, the FCR in the trial period was 1.04; the daily growth was 0.80 %. In the section with oxygenation, in contrast, the FCR was 0.86 and the daily growth was 1.30 %. The fish in the section with oxygen regulation needed 19 % more oxygen. This meant that 30 % more feed could be given, which in turn led to increased growth. This is a clear indication of how important the oxygen supply is in terms of commercial production.
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Oxygenation can widely improve FCR and fish growth, at 13.5 °C. Source: Danish Institute for Fish Technology and Aquaculture.
Influence of oxygen content on growth
The growth of the fish is influenced by the oxygen content of the water. Comparing the optimum growth to the reduced growth, the loss in growth caused by a lack of oxygen becomes obvious. If there is a lack of oxygen, the fish eat less. It was demonstrated that the feed intake can fall to a level below 50 % in the feeding guide.
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A lack of oxygen causes reduced feed intake and subsequently loss in growth. Trial result: On 6th December, the difference per fish amounts to 100 grams.
BioFarm – BioMar’s Expert Advisory Team
Many years of experience, a enormous number of measurements, and reports from farmers who have introduced oxygenation equipment show that the endeavours for keeping good oxygen conditions are soon covered by the increased revenues from higher growth/better feed conversion rates and an increased vitality and health status of the fish. BioMar’s technical advisory team, BioFarm, offers support in the management of oxygen and other fish farm management topics. Feel free to contact the BioFarm team member responsible for your area for further information.
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The oxygen content of the water is often the most prominent limiting factor in intensive fish farming. It is a well-known fact that poor oxygen content means that it is not possible to utilise the full potential of both the fish and the feed. A jet platform is an example of an efficient and yet simple mechanical method for oxygen enrichment of water.
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Just like humans, fish need certain levels oxygen to thrive. Sub-optimal oxygen levels may have consequences on the well-being of the fish and consequently on the economic outcome of a fish farm. Active observation and assessment are required to take the necessary actions in good time.
In fish farming, being attentive to oxygen levels and implementing oxygenation are important prerequisites for maintaining economic growth rates.
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