Africa was originally a land of rivers and streams and in vast areas there were no natural lakes. Many of the rivers dried out during the dry season and the indigenous fish were adapted to live in streams and pools which held water when rivers did not run. In the struggle for survival some fishes even hibernated through dry periods in holes made in mud and became active again when the rains started.
More recently, the creation of water conservation dams became a vital part of the country’s development and too date a vast number of water storage dams have been planned and built. Many species of fishes have established themselves in this changed environment. They often thrive extremely well enabling these impounded waters to be used for food production as well as for the purposes for which they were originally built.
Until recently, in the construction of dams, no provision was made for fish culture, and even now many farm dams are not solely built for fish farming. In fact, they are never as good for fish production as a fish pond, but may, however, provide a wide range of fishing activities and be used for recreation as well as for profit.
Dams vary considerably in the amount and quality of fish that can be produced in them, but there are really very few dams which are completely unsuitable for fish production. From the production point of view there are many factors which affect the suitability of dams for fish farming:
- Altitude and water temperature;
- Shape, size and depth of dam.
- Locality and soil of the bottom;
- Through-flow and fluctuation of water level;
- Fertility of the water catchment and;
- Silting and water weeds.
Altitude and the temperature of water are closely related factors. High altitude areas often have cold periods in winter when water temperature in dams may become lethal for fishes, especially tilapia species, and considerable losses may occur. In the warm low-veld areas, although the water temperatures may drop during winter, the drop generally does not harm fish. Altitude and temperatures are, as a rule, the factors which decide the kind of fish for stocking a dam. For example, higher temperatures in the low lying areas are favourable for tilapia culture, but regions such as mountain highlands are only suitable for trout culture.
The size, depth and shape of a dam have to be taken into account when commencing with fish farming. In very large dams extensive farming is suitable, in medium size dams semi intensive and in smaIl dams intensive farming methods should be used. Large dams are usually not very productive and the farming of fish means the catching of fish which grow there on natural food. Smaller dams are often more productive, and more easily managed.
Locality of the dam site as well as soil on which the dam is built is important as it affects the fertility of the water and quality of the dam. Excessive through-flow of water in the dam has an adverse influence on fish farming, although a certain amount of new water is needed to prevent the dam from becoming stagnant.
Water fluctuation in the dam may harm the breeding of fish, but, on the other hand, the drying out of a part of the basin has a beneficial effect on the exposed bottom and results in increased fish food production when re-flooded. The natural fertility of water depends on the amount of dissolved salts in the water usually brought in from the catchment area or through-inflow. Of these salts, phosphate is by far the most important followed by nitrates, calcium, and trace elements. The quality of water depends also on the amount of dissolved oxygen, on the pH value, and the density of plankton in the dam.
Silting of damsmay also affect the fishing and water-weeds may become a very serious problem.
The above-mentioned factors are the basic ones with which one has to deal when starting fish farming in dams. They should be borne in mind when new dams are constructed and steps should be taken to promote fishing facilities as far as possible.
In small dams all obstacles, such as shrubs, tree stumps, roots, possibly rock sand boulders, should be stumped below the ground level and removed. Deep holes should be filled in, and mounts leveled out in order to provide favourable facilities for seine netting.
In medium and large dams, where clearing of the whole dam area may be difficult and too costly, certain areas should be prepared for fishing. The number of these places may vary depending on the characteristics of the dam. All smooth gently sloping areas where water depth at 60 m from the shore does not exceed 3 – 5 m, seine netting is possible. Large areas of very shallow water should be avoided if possible, because these are likely to become choked with waterweeds. These parts may however, successfully be used for rice culture.
New dams, as a rule, have exceptionally high fish production for 3 – 4 years after completion, due to the high fertility in water caused by nutrients taken up from the newly submerged soil and vegetation. Thereafter the natural fertility declines used up by fishes and plant production falls to a certain level or may continue decreasing if no .effort is made to improve the conditions in the dam.
New dams, built with connections from rivers or streams, usually need no stocking due to the invasion of the dam by the existing fish population. In the new environment they usually multiply adequately to fulfill the stocking requirements of the dam. In these dams practically all kinds of fish found in rivers or streams may adapt to dam conditions, often only migrating up or downstream to suitable breeding grounds.