Fish of the genus Tilapia are macrophyte feeders, that is, the adults feed mainly on filamentous algae and higher aquatic plants. Figure 13.9 shows aT. zilliigrown in a pond and fed only plant leaves. Tilapias of the genusOreochromisare microphagous; their feeding regime consists notably of phytoplankton, zooplankton, detritus, and benthic organisms. Species
FIG. 13.9
Tilapia zillii,a macrophyte feeder, grown to marketable size without processed feed.
The fish was fed plant leaves, which can be seen in the background in the culture pond.
of the genusOreochromisare primarily omnivores. However, there is a great deal of overlap among the diet compositions of various species of tilapias.
Bacteria, protozoans, and microcrustaceans attached to detrital particles are important sources of protein, lipids, and vitamins. Some algae are relatively high in protein and energy with a good digestibility.
Most tilapias have short and widely spaced gill rakers but are efficient in ingesting phytoplankton, evenNannochloris, a solitary coccoid green alga measuring less than 5 μm in diameter. The collecting processes of the minute food particles involve entrapment of algae in mucus secreted by mucous glands in the mouth and filtration by microbranchiospines present on the outermost gill arches (Fryer and Illes, 1972).
Most tilapia culture in the world is in ponds, where natural food organisms can supply substantial amounts of nutrients required by the fish. Schroeder (1983) used stable carbon isotope analyses of the fish and the food sources and found that natural foods contributed 50 to 70% of the growth of tilapias in ponds receiving supplemental feeds. Stomach analysis showed that up to 50% of the stomach contents of tilapias are natural food in the intensively fed pond cultures, indicating that the natural pond productivity contributed
a substantial amount of nutrients. The size of this contribution to the fish’s nutrient requirements for maximum growth will depend upon the pond productivity and the fish density in the pond.
13.4.3.2. Practical Feeds
Commercial pond feeds for tilapias usually contain 25 to 28% protein.
Natural pond foods contribute a significant amount of protein, so this level is assumed to be high enough. However, few pond studies have been con- ducted to compare various diet formulations for extensive or semiintensive culture of tilapias. A 25% protein pellet composed of 15% fish meal, 20%
soybean meal, 20% ground wheat, and 45% ground sorghum has been used successfully in this type of production in Israel. The importance of micronu- trient supplementation in pond feeds for tilapias is not well known. Due to the extreme variation in the culture practices used, formulation of practical feeds to supplement the nutrient contribution of the natural food efficiently is practically impossible.
In intensive cultures, such as raceways or cages, where tilapias rely solely on the prepared feeds as a source of nutrients, a nutritionally complete feed containing all essential nutrients is fed. The protein content of cage or raceway feeds is usually 32%; however, there have been relatively few experiments on the protein allowance for production feeds for tilapias. In recirculating systems, where the overhead costs of operating the systems are high and unretained organic and inorganic matter is objectionable, highly concentrated feeds may be desirable. Some recirculating culture operators use 36% protein feeds, with energy balanced with protein, and claim that the additional growth rate, compared to that with a 32% protein feed, is economically beneficial. Model formulas for pond and raceway feeds for tilapias are listed in Table 13.7.
Tilapias accept a variety of feeds, in nonpelleted form and in sinking and floating pellets. The crude feedstuffs, such as rice bran and brewery wastes, are offered in nonpelleted form, whereas the compounded diets are most often processed into pellets. Studies have shown that tilapias can utilize meal-type feeds effectively, however, they obviously do not consume the nonpelleted feeds efficiently. High-quality feeds should be processed into pellets to minimize waste.
Feeds in meal or crumble forms are used for fry and fingerlings. These are made by first pelleting or extruding the feed mixture and then reducing the particles to size by crumbling. Usually only one size pellet is used for feeding tilapias to the marketable size of 500 g. The most common pellet size is approximately 3 to 4 mm in diameter and 6 to 10 mm in length. Tilapias seem to prefer smaller pellets than do channel catfish and salmonids of comparable size. They tend to chew the pellets rather than swallow them immediately as do most finfish species.
Table 13.7
Model Tilapia Feeds for Ponds and Racewaysa Percentage
Ponds
Raceways Ingredient (26% protein) 32% protein 36% protein
Soybean meal 38.3 48.5 50.8
Wheat middlings 4.0 20.0 18.0
Fish meal 4.0 6.0 12.0
Corn 50.8 22.6 16.5
Dicalcium phosphate 1.0 1.0 0.8
Plant oil (sprayed) on pellet surface 1.5 1.5 1.5
Vitamin mixb 0.2 0.2 0.2
Trace mineral mixb 0.2 0.2 0.2
aAdapted from Lovell (1989).
bVitamin and mineral allowances for channel catfish given in Chapter 9 will be sufficient. Vitamin and trace mineral supplements are not necessary in pond feeds if the fish are small and the density is low.
Feeding rates for tilapias are affected by the species, size, energy level in the diet, water quality, feeding frequency, and availability of natural foods.
For example,T. randaliconsume more feed thanO. niloticusof comparable age (Balarin and Haller, 1982). As with other fish, feed consumption rate is inversely related to fish size. Tilapias, like many other cultured fish species, benefit from multiple daily feedings. Kubaryk (1980) found thatO. niloticus grew faster when fed four times daily than when fed two times but did not
Table 13.8
Feeding Rates and Frequencies for Various Sizes of Tilapiasa Daily feeding
Size (% of fish weight) Times fed daily
2 days old to 1 g 30–10 8
1–5 g 10–6 6
5–20 g 6–4 4
20–100 g 4–3 3 or 4
>100 g 3 3
aAt 28◦C. Adapted from Jauncey and Ross (1982), Coche (1982), and Kubaryk (1980).
grow faster when fed eight times. Small fish should be fed more often than larger fish. The feeding frequency is decreased as the fish increase in size.
Table 13.8 lists feeding rates and frequencies for various sizes of Nile tilapias for maximum growth.
13.5