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 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.
