7. POLYCULTURE OF AFRICAN CATFISH WITH NILE TILAPIA

Nile tilapia (Oreochromis niloticus) is currently the most widely cultivated finfish species in Africa. However, a major disadvantage of this species and Tilapia in general is their excessive reproduction and at harvest up to 23% of the biomass may consist of fingerlings (Figure 20). The main problem with the existence of the fingerlings is that they compete for and consume the feed provided for the adult tilapia and consequently the growth rate of the adults is reduced (de Graaf et al., in press).

Figure 20. The percentage of the total harvest weight consisting of males, females and fingerlings of Oreochromis niloticus obtained in a mixed culture in the Republic of the Congo (stocking density 2.2 fingerlings/m², wheat bran as feed, production 7-8 t/ha/year, de Graaf et al., (1996) in press).

Furthermore the same authors found that the growth of adult fish was improved by reducing fingerling stocking density (Figure 21).

Figure 21. Growth rate of pond reared male and female O. niloticus in relation to the presence of fingerlings in the Republic of the Congo (de Graaf, et al., (1996) in press).

However, the number of tilapia fingerlings can be reduced by rearing tilapia in combination with a predator fish. For example, the following predator fish have been successfully used in polyculture with Nile tilapia in Africa:

· Clarias gariepinus: generally considered to be a “lazy” predator but works well as long as they are stocked at high densities (8 000-9 000 fingerlings/ha).

· Ophiocephalus obscuris: highly effective predator which is completely piscivorous and eliminates all the tilapia fingerlings if stocked at a density of 800-1 000/ha.

· Hemichromis fasciatus: good predator but has a low market value and is rather fragile with high mortalities often occurring at harvest.

· Lates niloticus: good predator, but difficult to obtain fingerlings.

Large catfish (6.8-130 g) were able to control the recruitment of Nile tilapia completely, with less than 0.15 % of fingerlings (as % of total harvested biomass) remaining at a stocking density of 8 300 catfish/ha.

However, small catfish (less than 3.6 g) were not able to completely control the recruitment of Nile tilapia (a fingerling percentage of 3.7 % persisting at the end of the culture period). The major reason for this was that small-sized catfish have a specific food preference for zooplankton and probably shift to a more piscivorous behaviour once they reach a weight of 7-8 g. Consequently all of the Nile tilapia larvae hatched at the beginning of the rearing period usually escape predation. From a practical point of view this has the advantage that at harvest a small number of nicely calibrated tilapia fingerlings (30-35 g) are usually obtained.

Figure 22. The relationship between stocking density of C. gariepinus and the remaining quantity of fingerlings of Nile tilapia in polyculture (expressed as % of total harvested biomass).

Fish may be fed all kinds of available agriculture by-products, including rice bran, wheat bran, cotton seed cake, etc. However, the feeding rate must be adjusted to the presence and appetite of the catfish or else they will be underfed. In practice, for the monthly adjustment of the feeding rate, the weight of catfish can be considered to be that of the tilapia. Some basic parameters for the polyculture of Nile tilapia and African catfish are presented in Table 14.

Table 14. Key parameters for the polyculture of Nile tilapia with African catfish as obtained in large-scale polyculture operations in the Republic of the Congo (de Graaf et al. (1996) in press).

Parameter
Stocking rate: tilapia (20-30 g)	2.2/m2
Stocking rate: catfish (8-10 g)	1/m2
Feed used	wheat bran
Feed Conversion Ratio	6.6
Rearing period	6 months
Weight of male Tilapia at harvest	200-250 g
Weight of female Tilapia at harvest	100-130 g
Weight of catfish at harvest	200-300 g
Net yield	7-8 t/ha/year

An advantage of rearing African catfish in polyculture with Nile tilapia is that the lower-valued tilapia fingerlings are replaced by a more or less equal quantity of the higher-valued catfish¹². Another advantage is that larger adult tilapia are obtained as the growth rate of the stocked adults increases (Figure 23).

¹² Provided the feeling level is adjusted for the stocking rate of C. gariepinus