STOCKING A DAM

Dams built in vlei and those with no connection to rivers have the advantage that these may be stocked with desirable species of fish only and therefore usually provide much higher yields. They are also more suitable for proper management because one has control over the fish population in the dam.

All dams should be given anapplication of agricultural lime before filling with water, but it may be difficult in very large dams, although it is a must in dams of a manageable size. In those dams built on soilsofpH5 – pH6, an average of 250 – 450kg lime per acre is needed, and soils with pH6 -pH6.5, approximately 45 – 100kg agricultural lime per acre should be broad cast over the bottom of the dam, and worked lightly into the soil.

Dams, which are not affected by the invasion of the wild fish from natural waters, should be stocked soon after liming and when there is sufficient water to ensure the well-being of the fish. In this case the increase of food from soil being gradually flooded by rising water will be used by the fish most efficiently and the population will be well established when the dam is full. In cold areas, however, the stocking cannot be done during Winter season because of the high mortality which occurs during transport.

The best kinds of fish to stock farm dams with are tilapia mossambica, commonly known as bream and tilapia melanopleura the pink breasted bream. Both of these belong to the Cichlids family.

Fry of tilapia mossambica feed on microscopic crustacea until they reach the size of about 50 mm when they gradually change to a diet of diatomus, algae, plants and insect larvae. Tilapia mossambica start breeding when about 100 mm in length and the number of eggs produced may vary from a few hundred to over 3000 per spawning.

Tilapia mossambica are so-called Mouth breeders, which means that the eggs which are laid, are picked up by the female fish in her mouth and hatch there after a few days. The fry are kept in the mouth for a few dayslonger, and once they are sufficiently advanced, start looking after themselves. Spawning occurs many times during summer and growth can be up to 0.5kg per year.

The diet of tilapia melanopleura fry in the young stage is the same as that of tilapia mossambica but from the size of 50mm they gradually start eating more plant food as they grow larger. When there is no vegetation in the dam, the tilapia melanopleura will eat bottom silt, detritus and insect larvae. Tilapia melanopleura are not mouth breeders and the number of eggs laid may be more than 12000 per spawning.

Both tilapia mossambica and tilapia melanobleura take easily to artificial feeding and the food conversion is very satisfactory compared with farm animals. Therefore these fish are very useful for turning substandard feeds intohigh, quality fish flesh, and are the kinds which should be in every dam.

Experience has shown that suitable stocking rates to establish the necessary fish population in a dam are as follows:-

•   Dams up to 4ha should be stocked with two kinds of tilapia at the rate of 4.5kg fish of mixed sizes – from 20g – 170 g.
•   Dams 4 – 8ha – 9 to 25kg per dam.
•   Over 8 ha – Over 30kg per dam.

This stocking rate allows fishing, or more strictly speaking cropping, after one season.

To increase the natural food in the dam, phosphate fertilizer should be added to the water about 10 – 14 days after liming and .water filling. 100 – 200Kg single superphosphate per ha is a good average dressing although requirements may vary considerably from dam to dam. In smaller dams, the use of animal manures, poultry manure or keeping ducks on the dam will increase the fish production considerably. Waste maize, mill sweepings, and other cheap feeds maybe used for feeding of fish. All fertilizing, manuring and feeding should be done often and in small amounts because it may be harmful to dump large quantities in the dam at a time. In dams with excessive through-flow, or when the spillway is running during rains, fertilization is not recommended because it is wasteful.