Cleaning and re-use of water
In some situations it can be necessary to transport live fish over large distances and for longer periods of time (12–16 h). In these circumstances there must be either a water exchange or continuous flow-through of water as in well boats, depending of the species being transported. It is, however, also possible to design a transport tank system that includes a circuit for cleaning and re-using the water (Fig. 18.7) so that high concentrations of carbon dioxide and ammonia in the water can be avoided. Such transport tanks really function as a recycling plant with 100% re-use of water. Complete removal of metabolic waste products is impossible from an economic view, so even here the water quality will gradually decrease, but over a much longer time period. Compared to a re-use plant, the degeneration of the water quality will be slower because fish respiration is reduced due to prior starvation.
Airlift pumps may be used for removal of carbon dioxide in addition to adding oxygen and creating water flow. Biofilters are not efficient systems for removal of TAN during transport because these are biodynamic systems that require a start-up time to establish a culture of bacteria before use. However, an ion exchanger can be used to remove ammonia. When the water passes through the ion exchanger the ammonia is removed (NH4+ exchanged with Na+); after use the ion exchanger must be regenerated, and this can be done between journeys.
Long journeys and high degrees of water circulation may also cause the water temperature to increase as a result of the pumping required for water re-use and fish metabolism. Installation of a cooling system on the transport tank can therefore be advantageous. The oxygen supplied to the water may be either produced by generators or by bottles of oxygen gas attached to the truck.
In Norway salmon smolt and juvenile turbot have been transported for up to 5 days without water exchange using a specially designed tank with a water re-use system.