Different production ponds
Ponds can be separated into those for fry production and those for on-growing production (Fig. 14.1); the difference is normally the size of the ponds. However, full production ponds are also possible. In such ponds, spawning, fry production and on-growing all occur, although harvesting can be quite difficult. Full production ponds may, for instance, be used in crayfish production (Fig. 14.2).
In a pond for fry production, it is especially important to have a well functioning ecosystem, including photosynthesis. Eggs or newly hatched fry are released into the pond where the on-going ecosystem will produce natural prey for the fry. As the fry grow they will gradually feed on other prey that are also available in the pond. Depending on the desired production, additional feeding of the fry may not be necessary.
To stimulate and increase the development of the natural ecosystem, it is possible to fertilize the pond. This increases production of algae and hence production of higher organisms that function as natural prey for the growing fry. It is, however, easy to lose control of the ecosystem, and total breakdown may occur. If fertilizing, it is therefore of major importance regularly to monitor and control changes in the water quality, for instance by monitoring the oxygen content in the pond water.
In on-growing ponds, there is often some type of additional feeding, but this depends on the species. Some species will utilize the plants growing in the pond and the organism created by the ecosystem, but this is normally not enough if high production is wanted; an example here is grass carp. Other species may only use supplied artificial feed, such as catfish and rainbow trout. In on-growing ponds it is easy to overload the system when adding formulated feed and cause problems in the ecosystem which will be put out of balance so that the pond functions in an uneconomic way.
The water flow through a pond having a natural ecosystem must not be too high, otherwise algae and natural prey may flow out with the outlet water, and an imbalance in the ecosystem will occur. Many earth ponds are, however, used in this way. If the fish densities are high the water requirements increase with the fish density and the ponds function as raceways, for which the pond construction is sub-optimal. The results are large variations in pond water quality and accumulation of faeces and feed loss.