Urgent aeration requires the monitoring of dissolved oxygen levels during periods of risk, mainly during the night, so that if necessary the aerators can be switched on.
In an attempt to avoid time-consuming night measurements, a prediction model of the oxygen content at dawn can be used.
The simplest model uses two measurements carried out the previous evening.
The drop in oxygen at night being practically linear, these two values, one taken at dusk, the other 2 or 3 hours later, allow the content at dawn te be estimated.
As a result of the general principle of this method, the precision of the prediction is weak but the fish farmer can improve
this by recording the true values that are observed at dawn.
He can therefore learn to assess the true functioning of his pond and therefore refine his/her predictions with the passing season. This method does not however allow a prediction, for example in the case of severe deoxygenation due to mass mortality of algae.
On the other hand, the elements which increase the risk of deoxygenation must also be taken into account and periods of risk can be specified beforehand.
It is not necessary to carry out oxygen measurements throughout the whole of the year, only in certain critical periods:
- in semi-intensive aquaculture, in our latitudes, it is between 15 July and the 15 September that risks of deoxygenation are the greatest, and the period of May to June can also cause certain risks when the temperature is above average.
- a water temperature greater than 25°C must be considered critical and should prompt an increase in surveillance.
- an increased oxygen content in the evening (greater than 18 mg/l) is the result of intense algal activity which increases the risk of deoxygenation at night.
- stormy periods in summer can be the origin of serious deoxygenation and it pays to take weather conditions into account.
These prediction methods therefore allow time consuming checks of the oxygen levels at night or the unnecessary use of aerators to be avoided.
Their use however carnes the inconvenience of a certain risk of error, which, is all too important in intensive conditions where the flnancial stakes are high, but remains genenally acceptable in semi-intensive or extensive conditions.
In the case of noticed or predicted deoxygenation, fish farmers need to resort to urgent aeration.
The aerators used are either electric aerators already in place
in the tank (ventilators with blades are the most common) or aerators operated by tractor power which are placed only when necessary.