The volume of water supply depends on the amount of fish in the pond and the intensity of farming in relation to evaporation. In addition, the acceptable time for filling the pond with water is important. Normally this is the value specified for the water supply, because the water flow into a pond under normal farming conditions is very low.
The value used to calculate the necessary water supply should be sufficient to fill the total pond volume within 2–3 days. The whole farm, including all the ponds, should be filled during a 20 day period.12The amount of water needed is, of course, affected by evaporation. The total evaporation depends on temperature, cloud conditions, wind conditions and pond construction; Normally it is in the region of 0.25 to 1 cm per day in temperate areas.9
Calculate the necessary water supply to cover the evaporation loss to a 100 m2 pond where the evaporation is 0.5 cm per day. The water supply is equal to the evaporation volume.
Evaporation volume = 100 m2× 0.005 m
= 0.5 m3/day = 0.35 l/min
If the water supply and exchange are too high, the production of algae will not be adequate. The algae may flow out through the outlet and the ecosystem will not flourish. The ponds are now functioning as raceways, for which the construction is sub-optimal. The ecosystem inside the pond becomes unstable when the daily water exchange is too high, i.e. between 25 and 33% of the total volume, which equates to retention times of 3–4 days.
In addition to supplying water, it may be necessary to supply extra air or oxygen (Fig. 14.6). Control of concentrations CO2 and ammonia in critical situations may also be necessary; additional inlet water may be required in such circumstances.