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The results of Micha (1976) clearly indicated that the growth of C. gariepinus decreases with increasing stocking density while fish standing crop remains more or less the same (Figure 19). Stocking rate therefore depends upon the desired market size and varies from 2 to 10 fingerlings/m², which in turn corresponds to a market size of approximately 500 and 200 g respectively after a six month rearing period.

In the Central African Republic static ponds were stocked at a density of 10 fingerlings/m², and were harvested after 6 months when the standing biomass reached 10 000 kg/ha and the catfish attained an average weight of 200 g (Janssen, unpublished data. See also Table 13). Higher stocking densities were not used in the Central African Republic because the adverse water conditions at the end of the production cycle were difficult to manage.

In South Africa and Zambia standing crops of 40 000-100 000 kg/ha were attained in ponds with a water exchange of 25%/day (Hecht et al., 1988). However, this system is difficult to manage, and it is recommended by these authors to stock the ponds at a maximum density of 10 fingerlings/m² and to thin out the population at regular intervals in order to maintain a maximum standing crop of 40 000 kg/ha with a constant daily water exchange rate of 25 %. The latter is essential at these high standing crops because the accumulation of waste (uneaten feed, excreta, etc.) will stress the fish (due to deteriorating water quality) and may provoke the outbreak of diseases.

Figure 19. The relationship between stocking density and growth of C. gariepinus in earthen ponds in the Central African Republic (After Micha, 1976).

6.2. Feeding

C. gariepinus has a relatively high dietary protein requirement. Feeding with a formulated feed is a prerequisite for intensive monoculture of the African catfish. The best growth rates and food conversions are achieved with diets containing 35-42% crude protein and a calculated digestible energy level of 12 kJ g^-1 (ADCP, 1983). Recommended dietary nutrient levels for C. gariepinus are presented in Table 8.

Table 8. Recommended dietary nutrient levels for C. gariepinus (ADCP, 1983).

Nutrients (% of dry matter)	Fry and Fingerlings	Growers	Broodstock
Digestible protein	35-40	30-35	35-40
Digestible energy (kcal/g)	3.0-4.0	2.5-3.5	3.0-4.0
Ca (min-max)	0.8-1.5	0.5-1.8	0.8-1.5
P (min-max)	0.6-1.0	0.5-1.0	0.6-1.0
Methionine + Cystine (min)	1.2	0.9	1.0
Lysine (min)	2.0	1.6	1.8

Artificially formulated diets are generally composed of a mixture of vegetable and animal feedstuffs (usually agricultural and mill by-products) supplemented with vitamins and minerals. An example of a vitamin premix used in diets for C. gariepinus, based on levels used in commercial diets for channel catfish (Robinson, 1984) is given in Table 9.

Table 9. Composition of a dietary vitamin premix for C. gariepinus diets (Hecht et al., 1988).

Thiamin	11 g
Riboflavin	13 g
Pyridoxine	11 g
Pantothenic acid	35 g
Nicotinic acid	88 g
Folic acid	2.2 g
Vitamin B12	0.09 g
Choline	550 g
Ascorbic acid	350 g
Vitamin A (I.U.)	4 400 (I.U.) × 1000
Vitamin D (I.U.)	2 200 (I.U.) × 1000
Vitamin E (I.U.)	55 (I.U.) × 1000
Vitamin K (I.U)	11 (I.U.) × 1000
Filler, maize meal	2 kg

One kilogram of this mixture is sufficient for one metric tonne of pelletized feed (0.1%) or 500 kg of extrusion processed feed (0.2%)

It is not possible to give a standard formulation for a balanced diet for catfish since the composition of artificial diets will depend upon the availability and prices of locally available feedstuffs, which in turn vary considerably between countries. Least-cost formulation methods are commonly used for commercial feed manufacture and two examples of least-cost formulated diets for C. gariepinus are given in Tables 10 and 11.

Table 10: Least cost formulation for on-growing African catfish in the Central African Republic (Janssen, 1985)

Ingredients (kg)	Dry or moist pellets		Moist pellets
	1	2	3	4	5	6
Wet brewers waste (25% dry matter)	-	-	78	60	61	44
Dried brewers waste	15	10	-	-	-	-
Wet brewers yeast (15% dry matter)	-	-	-	30	-	30
Rice bran/polishing	15	15	15	15	15	15
Maize	5.55	6.05	-	-	-	-
Cotton seed cake	25	25	25	25	25	25
Groundnut cake	25	25	25	25	25	25
Sesame cake	10	10	10	10	10	10
Blood meal	-	5	-	-	5	5
Vitamin/mineral premix9	0.25	0.25	0.25	0.25	0.25	0.25
Bone meal	2	2	2	2	2	2
Salt	0.5	0.5	0.5	0.5	0.5	0.5
Palm oil	1	1	-	-	-	-
L-Lysine	0.5	0.2	0.5	0.3	0.2	-
D-Methionine	0.2	-	0.2	0.2	-	-
Gentian violet (g)	-	-	5	5	5	5
TOTAL	100	100	100	100	100	100
Calculated chemical composition
Protein (%)	35.3	38.2	35.9	37.7	38.9	39.2
Digestible energy (kcal/g)	2.70	2.76	2.62	2.67	2.70	2.71
Methionine+Cystine (%)	1.0	1.1	1.0	1.0	1.1	1.1
Lysine (%)	1.6	1.6	1.6	1.6	1.6	1.6
Costs FCFA per kg of feed	78	88	71	67	81	77

mmercial preparation used for poultry (layer) diets

Table 11: Least cost diets for the on-growing (in ponds) of C. gariepinus used in South Africa, reported feed conversion ratios (FCR) and cost efficiency (Hecht et al. 1988)

Diet No.	1	2	3	4	5	6	7	8	9	Control
Maize (%)	-	4.6	15.0	30.0	-	10.5	-	-	-	18.0
Wheat (%)	-	31.8	15.0	-	30.0	-	14.0	-	-	18.0
Cotton seed cake (%)	-	-	-	-	-	25.0	-	-	-	-
Soy bean cake (%)	10	-	-	-	-	10.0	10.0	-	-	-
Fishmeal (%)	24.7	16.0	10.0	10.0	10.0	20.0	10.0	10.0	-	43.5
Poultry by product meal (%)	10	10	-	10	10	9.0	10.0	26.0	35.2	-
Carcass meal (%)	10.5	27.6	50.2	39.5	31.4	-	22.7	-	-	10.0
Lucerne meal (%)	30.0	-	-	-	-	-	-	-	-	-
Tomato waste (%)	-	-	-	-	9.7	8.0	20.0	50.1	50.0	-
Fish acid oil (%)	6,8	-	-	2.5	0.9	7.5	3.3	5.9	6.8	2.5
Molasses powder (%)	8.0	10.0	9.8	8.0	8.0	10.0	10.0	8.0	8.0	8.0
TOTAL	100	100	100	100	100	100	100	100	100	100
Calculated chemical composition
Crude protein (%)	38.0	38.0	38.0	38.0	38.0	38.0	38.0	38.0	38.0	38.0
Total lipid (%)	13.5	8.0	8.1	11.7	9.8	14.2	12.5	19.8	21.6	9.0
10Digestible energy (kJ/g)	12.0	12.0	12.0	12.0	12.0	12.0	12.0	12.0	12.0	12,0
Price/ton (R)	655	586	580	569	543	603	531	415	376	722
FCR	1.05	1.19	1.16	1.25	1.19	1.13	1.12	1.46	1.54	0.98
Feed cost to rear 1 kg offish	0.69	0.70	0.67	0.71	0.65	0.68	0.59	0.61	0.58	0.71

¹⁰ Calculated on the basis of DE values for channel catfish Lovell, 1984)

In most African countries, raw materials containing high amounts of animal protein such as fish meal and blood meal are scarce and expensive. Hence, it is easier to meet the relatively high protein requirements for African catfish by using feedstuffs containing higher quantities of vegetable protein such as plant oilseed cakes and meals. These agricultural by-products are more common, cheaper and generally available in large quantities in the region. The use of unconventional ingredients such as tomato waste in feeds used in South African diets is a good example of this.

Since there is almost no large-scale intensive aquaculture in most African countries, the present demand for raw materials comes mainly from domestic poultry and livestock industries. Consequently, there are generally no specific vitamin and mineral supplements available for aquaculture species. At present these requirements can only be met using the premixes from the poultry industry.

The recommended feeding levels between 21 and 33°C, corresponding with maximum growth rates and optimum food conversion, have been determined by Hogendoorn et al. (1983). These feeding levels have been calculated based on the results of laboratory experiments in flow-through systems, in which fish were fed with a commercial trout diet (crude protein 50%, gross energy 5,200 kcal/kg food) and are shown in Table 12.

Table 12: Recommended feeding levels (% of body weight/day) for C. gariepinus at different temperatures (Hogendoorn et al., 1983).

Temperature (°C)	Body weight (g)
	1	5	25	50	100	200
21	3.6	2.5	1.7	1.4	1.2	1
23	5.1	3.7	2.6	2.3	2.0	1.7
25	6.5	4.7	3.4	3.0	2.6	2.3
27	7.4	5.4	3.9	3.4	3.0	2.6
29	7.9	5.6	4.0	3.5	3.0	2.6
31	8.0	5.5	3.8	3.2	2.7	2.3
33	7.8	5.1	3.4	2.8

In practice it has been found that slightly higher feeding levels may be applied during the first months of culture in order to acclimatize the fish to the feed and feeding place, while lower feeding levels should be applied during the last three months of culture due to the deteriorating water quality conditions in static ponds (Janssen, unpublished results). Moreover, the quantity of feed distributed is generally calculated for a two week period and adjusted every four to six weeks after recording the average body weight of the fish. The latter is usually carried out by taking a sample with a cast net. The biomass of the catfish and the daily quantity of feed are then calculated according to the recorded average body weight and estimated survival rate. It is still difficult to predict growth rates and survival as they depend on many factors such as density, feed quality, temperature, predation by birds, etc. Despite this, some data on monoculture of African catfish is presented in Table 13.

After about six months the pond can be harvested with a net production of 9-16 t/ha/year. The main problems encountered with intensive monoculture of the African catfish have been related to water quality and predation

For example, overfeeding leads to adverse environmental conditions, including low oxygen, high ammonia, high suspended solids, etc. Adverse water conditions also often coincide with dense phytoplankton concentrations followed by the occurrence of a scum of phytoplankton on the water surface. This in turn will cause low oxygen levels at night and pre-dawn. The only remedy for this is to reduce the dietary feeding level and start flushing the pond with fresh water. Moreover, predation by birds and other animals is also another major problem often faced during the intensive farming of African catfish.

Table 13: Biological data on monoculture of African catfish in the Central African Republic, (density 10/m², mean temperature 25-27 °C, Janssen, unpublished data)

Week	Mean body weight (g)	Survival (%)	Biomass (kg/100m2)	Feeding rate11 (%/biomass/day)	Feed (g/100 m2/day)
0	1	100	1	10	100
2	5	70	3.5	7.5	250
4	10	65	6.5	4.5	300
6	18	60	10.8	4.0	400
8	27	60	10.2	3,3	525
10	36	60	21.6	3.0	650
12	52	55	28.6	2.7	775
14	65	55	35.7	2,6	900
16	79	55	43.4	2.4	1025
18	102	50	51.0	2.3	1150
20	130	50	65.0	2.1	1350
22	160	50	80.0	1.9	4500
24	200	50	100.0	1.8	Harvest

¹¹ Diet no 2, 30% digestible protein, 3000 KCal digestible energy/kg feed, (Table 10) was used

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