The increasing importance of tilapia as an aquaculture candidate makes it necessary to understand their food preferences and feeding regimes in their natural habitats, in order to prepare suitable diets for them and adopt appropriate feeding regimes under culture conditions. Tilapia are generally herbivorous/omnivorous (i.e. they are low on the aquatic food chain). This characteristic is extremely important for the economics of tilapia culture. Although they feed at low trophic levels and feed costs are lower than for carnivorous fishes, tilapia are still a source of high-quality protein suitable for human consumption, at a relatively low cost. The feeding habits and dietary preferences of tilapias depend, among other factors, on tilapia species and size, time of day, photoperiod, water depth and geographical location. The feeding habits of tilapia in relation to these factors are briefly discussed in this section, with emphasis on economically important species.
During larval stages, tilapia feed initially on zooplankton, especially crustaceans (copepods). When Florida red tilapia fry were stocked in fertilized seawater pools containing different food resources, copepods were selectively ingested. Frequent consumption of phytoplankton also takes place during early larval stages of some tilapia species.
The transition period from planktivore stageto a typical, specialized diet(s) is usually short(Bowen, 1976), but in some cases it occurs graduallyover an extended period of a year or more(Whitefield and Blaber, 1978). The food of juvenile and adult tilapias consists of a considerable variety of aquatic vegetation, phytoplankton, zooplankton, periphyton and detritus of plant origin, depending on tilapia species, as mentioned earlier.
Many research studies have been carried out on the feeding habits of different tilapias in their natural habitats, under a variety of environmental conditions.
Generally speaking, tilapia can be classified into one of the following broad categories according to their feeding patterns:
1. Tilapia of the genus Oreochromis are primarily microphagous, feeding mainly on phytoplankton, periphyton and detritus. Oreochromis niloticus, O. aureusand O. mossambicus are examples of this category.
These species can efficiently ingest the food sources mentioned through ‘filter-feeding’. For example, Harbott reported that all size classes of O. niloticus in Lake Turkana are primarily phytophagous, grazing on the dense algal plankton found in sheltered, inshore waters, while little selectivity of food items was recorded. Similarly, Spataru and Zorn (1978) found that O. aureus in Lake Kinneretare mainly zooplankton feeders, while vegetable detritus, mixed with plankton and benthos forms, served as additional and alternative foods.
2. Tilapia of the genus Sarotherodon are also primarily phytoplankton feeders, but they are more selective. For example, Spataru found that the din flagellate Peridinium cinctum was the most abundant food in the stomachs of S. galilaeus in Lake Kinneret, sometimes comprising > 95% of the phytoplankton biomass, especially during the blooming season of these din flagellates (March–April).
3. Tilapia of the genus Tilapia are generally macrophyte feeders. This explains why they are used for biological control of aquatic weeds.
However, they cannot avoid ingesting the algae, phytoplankton, zooplankton, bacteria, benthic invertebrates, insect larvae, fish and vertebrate eggs and detritus that are attached to the macrophytes they feed on. These attached materials are therefore an important food component for Tilapia species (Bowen, 1982). Fagade found that the principal components in the stomachs of Tilapia guineensis and Tilapia melanotheron in Lagos Lagoon (Nigeria) were algal filaments, diatoms, sand grains and unidentified organic material, and these components were very similar in both species. Adult T. rendalli have also been found to consume a wide variety of food items, including macrophytes, phytoplankton, zooplankton, insect larvae, fish eggs and larvae and detritus.
It should be realized that the above-mentioned categories of feeding habits are not rigid, but extensive overlap may also occur in tilapia diets. Intraspecific variations may also occur in these diets between environments over time, as has been reported with O. mossambicus in Sri Lankan reservoirs.
Since light penetration into the water column is affected by water depth and turbidity, natural food productivity in the water is likely to vary between different depths. Accordingly, the feeding patterns of tilapia may vary with varying water masses. Bowen (1982) stated that deposit feeders that feed in the littoral zone frequently ingest algae, detritus and bacteria, which cannot be distinguished from the material attached to macrophytes.
On the other hand, deposit feeders that feed in deeper waters ingest a mixture of precipitated phytoplankton and detritus that are indistinguishable from the diets of suspension feeders. By this mechanism, tilapia can switch from one source of food to another with little change in diet composition.
Tilapia feeding patterns and efficiency may also be subject to diurnal and seasonal changes. Extensive studies have considered the diel feeding patterns of tilapia, both in their natural habitats and under aquaculture conditions. Harbott (1982)found that all size classes of O. niloticus in LakeTurkana followed a daytime feeding cycle, but fish fry infrequently consumed invertebrates during the hours of darkness. The feeding activity of O. niloticus adults reared under conditions of self feeding(using demand feeders) was also observed exclusively during the light phases. Moreover, Nile tilapia in Lake Rudolf exhibited a regular diurnal feeding rhythm, commencing between 05.00 h and 08.00 h and ceasing between 14.00 h and 18.00 h, and the largest fish individuals appeared to feed longer.