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Ensuring optimal oxygen levels

    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. 

    Fish use their gills to get oxygen 

    Fish need oxygen delivered to cells throughout the body to maintain normal function. Fish use their gills to get oxygen from the water. Haemoglobin, the oxygen-transport protein in the red blood cells, absorbs oxygen from the water via the gills and directs it into the blood. From here, the oxygen is diffused to the cells in the organs / tissues.  

    Oxygen transport from the water to the blood. Oxygen in the water is being absorbed via the gills and directed it into the blood. From here, it is diffused to the cells in the organs / tissues.

    Oxygen requirement of fish 

    The most important factors which have an influence on the oxygen requirements of fish are temperature, muscle activity and digestion. The oxygen consumption of the fish reduces the content of oxygen in the water. In a flowing stream there is generally sufficient oxygen for the fish living in it to thrive. Moreover, the fish can flee from waters with oxygen deficiency. In fish farms, however, there is usually only a limited amount of water available in comparison to the fish stocks, and it is therefore of high significance to monitor its oxygen content and any fluctuations and to take the necessary actions.  

    Adaptability to poor oxygen conditions 

    By nature, fish can to some extend adapt to sub-optimal oxygen levels without this resulting in any stress. They can compensate for a reduced oxygen content in the water using various mechanisms: 

    • Increased gill ventilation: more water is pumped through the gills  

    • Increased blood circulation: more oxygenated blood reaches the organs / tissues  

    • Reduced level of activity and therefore reduced oxygen requirement 

    Critical low point of oxygen 

    A fish which is not under stress can regulate its oxygen requirement until the oxygen content in the water reaches a critical low point. If the oxygen content continues to drop, the fish must save more energy or convert in part to an oxygen-free (anaerobic) metabolism. As a consequence, the further metabolic processes cause additional stress to the fish. The residual products of this process, if in large amounts, are poisonous to the organism. It is important to note that the toxicity of most poisonous water components increases at lower oxygen levels.  

    In the event of low oxygen content in the water, the fish must pump more water through the gills in order to be able to absorb sufficient amounts of oxygen. This also increases the amount of toxins which come in contact with the gills and thereby enter the fish. This requires special attention when it comes to water parameters which even at small amounts can limit production quite drastically, for example ammonia (NH3) and last but not least, nitrite (NO2-). In order to escape this situation, the water must be enriched with oxygen during the shortage, allowing to return to an oxygen-based metabolism and to 'pay back' the oxygen deficit. 

    Oxygen saturation for optimal feed utilisation 

    The oxygen content in the water affects the oxygen content in the blood of the fish, at given water temperatures, and this can have an influence on optimal feed utilisation. At unsatisfactory oxygen saturation levels, feed utilisation decreases and thus, feed conversion ratio increases. For optimal feed utilisation, it is required that the blood is almost 100 % saturated with oxygen when it leaves the gills.  

    Oxygen saturation vs. feed utilisation. At 5 ˚C, the fish can utilise the feed optimally even if the oxygen saturation of the water is only 42 %. At 22 ˚C, the fish can only utilise the feed at a maximum if the oxygen saturation of the water is almost 100 %.

    Active observation and assessment 

    Taking the necessary actions in good time requires an active observation and assessment of oxygen levels and other water parameters.  It is important to take the fish population and its oxygen requirement into account and to determine the amount of oxygen that needs to be delivered to the water.  The oxygen content should not fall below critical low levels at any time of day.  Based on the amount of feed used, it is relatively easy to make a sufficiently correct calculation of the oxygen requirement. A portion-size fish has a requirement of approx. 300 g oxygen per kg feed administered. The requirement may vary depending on a number of factors, especially the size of the fish. For example, the oxygen consumption in fry and young fish is relatively high, because their metabolic rate is quite active. In contrast, the value may be adjusted downwards a little in case of larger fish. 

    Cleaning and maintenance of any oxygenation equipment are important routine tasks as they ensure that the devices function correctly. 

    The costs of buying measuring devices and the time spent on supervision as well as an immediate response to water values are economically worthwhile and soon covered by the increased revenues from higher growth/better feed conversion rates and an increased vitality and health status of the fish. 

    BioFarm – BioMar’s Expert Advisory Team 

    In line with our commitment to innovation through partnership and collaboration, our partners receive technical support to drive a sustainable and efficient aquaculture industry. 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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