Early juveniles and young fish are omnivorous, feeding mainly on zooplankton and zoobenthos but also ingest detritus and feed on aufwuchs and phytoplankton. At around 6 cm TL the species becomes almost entirely herbivorous feeding mainly on phytoplankton, using the mucus trap mechanism and its pharyngeal teeth (Moriarty and Moriarty, 1973; Moriarty et al., 1973). The pH of the stomach varies with the degree of fullness and when full can be as low as 1.4, such that lysis of blue-green and green algae and diatoms is facilitated (Moriarty, 1973). Enzymatic digestion occurs in the intestine where pH increases progressively from 5.5 at the exit of the stomach to 8 near the anus. Nile tilapia exhibit a diel feeding pattern. Ingestion occurs during the day and digestion occurs mainly at night (Trewavas, 1983). The digestive tract of Nile tilapia is at least six times the total length of the fish, providing abundant surface area for digestion and absorption of nutrients from its mainly plant-based food sources (Figure 4) (Opuszynski and Shireman, 1995). Ontogenetic dietary shifts of different size classes of Nile tilapia are presented in Table 1.
