The species is euryphagous and generally regarded as an opportunistic, omnivorous predator. It has the ability to efficiently utilize and/or switch between alternative food sources such as plants and detritus when prey animals become scarce (Groenewald, 1964; Thomas, 1966; Munro, 1967; Willoughby and Tweddle, 1978; Bruton, 1979b; Clay, 1979; Spataru, Viveen and Gophen, 1987; Winemiller and Kelso-Winemiller, 1996; Dadebo, 2000; Potts, Hecht and Andrew, 2008).
Normally catfish are bottom feeders, but their feeding habits are adaptable and they occasionally filter feed in groups at the water surface. There are four recognized feeding modes, viz. individual foraging, individual shovelling, surface feeding and formation feeding. Adoption of any one of the feeding modes depends on food availability (Bruton, 1979b). Catfish in ponds have been observed to snatch sinking pellets before they reach the substratum, then feed off the substratum and finally surface to feed on the floating fines using the gillrakers as a mechanism to filter out small particles (Hecht, Uys and Britz, 1988).
The species is equipped to feed on a variety of food organisms ranging from phytoplankton to fish. The mouth is wide, subterminal and transverse. The buccal cavity is capable of considerable vertical displacement that enables suction feeding. The teeth are numerous, small, cardiform and backwardly directed (Teugels, 1986). The premaxillary, mandibular and pharyngeal teeth are conical and sharp, whereas the vomerine band has mainly granular molar-like teeth with variable numbers of conical teeth, usually on the distal margin (Figure 3). The vomerine teeth band has no ventral partner, so that crushing and gripping of prey take place against the hyoid apparatus, which bulges upwards to form a tongue. Clarias gariepinus has long gillrakers on the anterior borders of the five branchial arches, and additional gillrakers on the posterior margins of the third and fourth arches that interdigitate with those from the anterior row of the next arch. The number of gillrakers increases with length (Bruton, 1979b). The mean width between gillrakers varies between <0.1 and 0.6 mm, but this increases with length (Murray, 1975). Despite this, larger fish are known to filter feed on phytoplankton, zooplankton and surface scum (Bruton, 1979b). The stomach is muscular, and the intestine is thin walled and relatively short, implying a dependence on high-protein foods. The stomach in North African catfish becomes functional 5–6 d (11 mm TL) after the start of exogenous feeding at 27.5 °C (Stroband and Kroon 1981; Verreth et al., 1992).
Predation is most efficient on relatively slow-moving benthic organisms, but fast prey such as fishes can also be caught individually (Bruton, 1979b) or by using pack-hunting tactics (Merron, 1993). The percent composition of natural food is dependent on the availability and abundance of various food items within systems.
Based on the proximal composition of diets of wild populations, Uys (1989) predicted that the species would have a relatively high protein demand (>45 percent), a lipid demand of around 18.5 percent and a dietary energy requirement of around 18 kJ/g. The predictions were very close to the empirically determined requirements (see later). This serves to illustrate the value of biological and ecological studies.
Early juvenile North African catfish have high levels of amylase, protease and gastric lysozymes, which facilitate its opportunistic feeding habits and its ability to utilize a wide range of nutrients efficiently. The enzyme secretory response is rapid, and there is no rhythmic cycle of digestive activity. This allows the species to efficiently utilize infrequent and irregular meals (Uys and Hecht 1987; Uys, Hecht and Walters, 1987).
A summary of the natural diet of North African catfish is shown in Table 1, while Table 2 shows the detailed diet of the species in Lake Sibaya (South Africa). The species feeds mainly on insects, phyto- and zooplankton, invertebrates and fish, but also takes young birds, rotting flesh and plants (Groenewald, 1964; de Kimpe and Micha, 1974; Bruton, 1979b; Spataru, Viveen and Gophen, 1987; de Moor and Bruton, 1988). Frogs, snakes, fledgling birds, small mammals, seeds and fruits have also been recorded in the stomachs (Bruton, 1979b).
The natural diet is determined largely by prey abundance in any particular habitat. During the larval and early juvenile phase, the natural diet is restricted mainly to zooplankton and chironomids. During this stage, the taste buds (Figure 4) on the circumoral barbels play an important role in prey detection. The species is also able to detect electrical pulses (Lissman and Machin, 1963) and uses this ability to detect prey (Hanika and Kramer, 2000). With increasing size and development of the feeding apparatus, the diet becomes more diverse. Both Bruton (1979b) and Spataru, Viveen and Gophen (1987) recorded over 40 prey species in the stomachs of C. gariepinus in lakes Sibaya (South Africa) and Kinneret (Israel), with fish contributing 75 percent and 81 percent of the dry weight of the diet, respectively, followed by crustaceans. On the other hand, Munro (1967) found that zooplankton becomes more important in the diet with increasing size. This simply illustrates the extraordinary ability of the species to switch diet. Bruton (1979b) and Clay (1979) provide summaries of the very wide natural diet of the species throughout its distributional range.
