Organic and inorganic fertilizers are commonly used to fertilize ponds for the rearing of fry and fingerlings under supplementary feeding conditions. Bok and Jongbloed (1984) showed the benefits of intensive pond fertilization using chicken manure under supplementary feeding, semi-intensive pond-culture conditions. It is however pointless to fertilize the ponds when the fish are fed a nutritionally complete diet. It should be noted that pond liming and fertilization rates vary widely and are determined by water quality and pH, as well as soil type and pH. There are no specific pond fertilization schedules for North African catfish, and the recommendations made here are based on the general literature.
De Graaf and Janssen (1996) emphasize the importance of high zooplankton abundance in nursery ponds, and this requires good fertilization. Tried and tested techniques should be followed. Prior to stocking a new batch of fry, the pond bottom should be allowed to dry (to enhance mineralization (oxidation) of nutrients) for a few days and then limed. Liming has several beneficial effects, such as disinfecting (quicklime) the substratum by raising the pH of the water. The pH should be allowed to stabilize between 7–9 before fingerlings are stocked into the pond. Liming also assists as a pH buffer and increases the availability of carbon for photosynthesis. The most commonly used liming compounds are quicklime (CaO), caustic or slaked lime (Ca(OH)2) and agricultural lime (CaCO3) The quantity of lime to be used depends on soil pH and hence varies between regions. Heavy loam or clay soils require more lime than sandy soils, and newly excavated ponds require more lime than older ponds. Suggested liming rates are shown in Table 10.
For newly constructed North African catfish ponds, Viveen et al. (1985) suggest agricultural lime at a rate of 200–1 500 kg/ha and mixing this with the upper layer (5 cm) of the dried pond bottom. The pond is then filled with water (till 30 cm) and left for one week prior to fertilization. For older ponds, they recommend the use of 100–150 kg/ha quicklime added to the damp pond bottom to eliminate pathogens, parasites and invertebrate predators. The pond is then left for 7–14 d, where after it is filled with water to a depth of 30 cm and the pH of the water adjusted by adding agricultural lime.
An abundant supply of zooplankton is necessary to successfully rear catfish fry to fingerlings in ponds. Zooplankton abundance is dependent on phytoplankton blooms. Primary production in freshwater is limited by phosphate, in particular, and nitrogen, while potassium (K) also has a favourable action (de Graaf and Janssen, 1996). To ensure a good zooplankton bloom requires adequate fertilization with chemical or organic fertilizers. The choice of either is largely determined by availability and the economic circumstances of the farmer. The main chemical fertilizers used are super-phosphate (approx 20 percent P2O5), triple super-phosphate (45 percent P2O5), urea (about 45 percent N) and NPK 15:15:15 (15 percent N, 15 percent P2O5 and 15 percent K2O). It is strongly recommended to dissolve the fertilizers and to then spread the solution evenly over the water surface. If not, then the phosphorous pellets may easily be absorbed by the mud and hence lost for primary production. De Graaf and Janssen (1996) also suggest that phosphorous fertilizers should be placed on submerged platforms or in hanging bags to promote the slow release of the minerals into the water column. In the absence of any specific work on North African catfish fry or fingerling rearing ponds, it is advised to follow the findings and recommendation made by Hepher (1963, 1967) and Boyd (1982). The work by Hepher (1962, 1967) has shown that applying fertilizer doses greater than 0.5 mg P/litre or 1.4 mg N/litre has no biological or economic justification. This is equivalent to a fertilizer application rate of 60 kg/ha single super-phosphate (11 kg P2O5/ha) and 60 kg/ha ammonium sulphate (13 kg N/ha) applied at 2-weekly intervals (0.8–1.0 m water depth, 8 000–10 000 m3 water/ha) (Tacon, 1987, 1988). Similarly Boyd (1982) and ASEAN (1978) recommend chemical fertilization strategies to maintain soluble nitrogen and orthophosphate at 0.1–0.5 mg P/litre and 0.9 5 mg N/litre and 0.11 mg P/litre (ASEAN, 1978).
The most commonly used organic fertilizers are chicken, duck, pig and cow manure. The N:P ratio and NPK content of these manures are listed in Table 10, while Table 11 shows several application schedules. It is important to note that the application rates are not absolute and only serve as a guideline. If an adequate phytoplankton bloom is not achieved within 6–8 sunny days, then more should be added. Plankton levels are adequate when the water has a green colour and a Secchi disk reading of around 20–25 cm. The biological effect of organic fertilization is improved if the manure is spread evenly over the water surface in a liquid or semi-liquid form (Tacon, 1988). It should be noted that fish ponds can only assimilate a certain amount of manure per day. Hence to prevent de-oxygenation in manure-loaded ponds (due to unchecked peaks in bacterial growth and phytoplankton blooms), manure should be added frequently (daily, if possible) and in small amounts and during mid-morning when oxygen levels are rising (Hepher and Pruginin, 1981).
Amisah, Adjei-Boateng and Afianu (2008) demonstrated the beneficial combined effect of pond fertilization and the use of bamboo poles for the development of periphyton. In their study, bamboo pole density was 4 per m2 and ponds were fertilized with pig manure at an initial rate of 20 kg/100 m2 and then every second week at 10 kg/100 m2.
