The major nutrient requirements of cultured tilapia are reasonably well established and are summarized in Table 2 and 3. Most of the values were determined under controlled laboratory conditions and may not be directly applicable in a commercial set-up. Even though information on the exact quantitative nutrient requirements for other life stages of tilapia is lacking, it can be expected that early juvenile fish (0.02-10.0 g) would require a diet higher in protein, lipids, vitamins and minerals and lower in carbohydrates. Sub-adult fish (10-25 g) require more energy from lipids and carbohydrates for metabolism and a lower proportion of protein for growth. Adult fish (>25.0 g) would require even less dietary protein for growth and can utilize even higher levels of carbohydrates as a source of energy. Comprehensive reviews of tilapia nutrition are available in various publications including that by Jauncey (2000), Shiau (2002), El-Sayed (2006) and Lim and Webster (2006).
Nile tilapia requires the same ten essential amino acids as other finfishes. Protein requirements for optimum growth are dependent on dietary protein quality/source, fish size or age and the energy contents of the diets and have been reported to vary from as high as 45-50 percent for first feeding larvae, 35-40 percent for fry and fingerlings (0.02-10 g), 30-35 percent for juveniles (10.0-25.0 g) to 28-30 percent for on-growing (>25.0 g) (Table 2). The best protein digestibility occurs at 25 °C (Stickney, 1997) and the optimum dietary protein to energy ratio was estimated in the region of 110 to 120 mg per kcal digestible energy respectively for fry and fingerling. Tilapia broodfish require about 40-45 percent protein for optimum reproduction, spawning efficiency and for larval growth and survival.
The lipid nutrition of farmed tilapia has been reviewed by Ng and Chong (2004). The minimum requirement of dietary lipids in tilapia diets is 5 percent but improved growth and protein utilization efficiency has been reported for diets with 10-15 percent lipids (Table 2). Both n-3 and n-6 polyunsaturated fatty acids (PUFA) have been shown to be essential for maximal growth of hybrid tilapia (O. niloticus x O. aureus). For Nile tilapia the quantitative requirement for n-6 PUFA is around 0.5-1.0 percent (Table 2). Unlike marine fish species, tilapia appear not to have a requirement for n-3 highly unsaturated fatty acids (HUFAs) such as EPA (20:5n-3) and DHA (22:6n-3) and its n-3 fatty acid requirement can be met with linolenic acid (18:3n-3).
The exact carbohydrate requirements of tilapia species are not known. Carbohydrates are included in tilapia feeds to provide a cheap source of energy and for improving pellet binding properties. Tilapia can efficiently utilize as much as 35-40 percent digestible carbohydrate. Carbohydrate utilization by tilapia is affected by a number of factors, including carbohydrate source, other dietary ingredients, fish species and size and feding frequency (El-Sayed, 2006). Complex carbohydrates such as starches are better utilized than disaccharides and monosaccharides by tilapias. Hybrid tilapia (O. niloticus x O. aureus) showed the carbohydrate (44 percent) digestibility in the following progression: starch>maltose>sucrose>lactose>glucose (Stickney, 1997). Carbohydrate utilization by tilapia species have been reviewed by Shiau (1997). Nile tilapia are capable of utilizing high levels of various carbohydrates of between 30 to 70 percent of the diet. It has also been demonstrated that larger hybrid tilapia (O. niloticus x O. aureus) utilized carbohydrates better than smaller sized fish. Stickney (2006) reported that the inclusion of soluble non-starch polysaccharides (NSP) in the form of cellulose in the diet of Nile tilapia increased the organic loading of the culture system, while insoluble NSP (guar gum) placed less organic load on the system by increasing nutrient digestibility and improving faeces recovery.
Vitamin supplementation is not necessary for tilapia in semi-intensive farming systems, while vitamins are generally necessary for optimum growth and health of tilapia in intensive culture systems where limited natural foods are available. Several vitamin requirements of tilapia are known to be affected by other dietary factors and these must be taken into consideration in diet formulations. For example, the vitamin E requirement is influenced by dietary lipid level with Nile tilapia requiring 50-100 mg/kg when fed diets with 5 percent lipid and increased to 500 mg/kg diet for diets with 10-15 percent lipid (Table 3). Apart from dietary lipid level, the unsaturation index of the dietary oil will also affect the amount of vitamin E required. The presence of other antioxidants in the diet, such as vitamin C, have been reported to spare vitamin E in diets for hybrid tilapia. Choline can be spared to some extent by betaine. b-carotene can be bio-converted to vitamin A with a conversion ratio of about 19:1 (Hu et al., 2006). Pyridoxine requirement level has been shown to vary with the level of protein in the diet: 1.7-9.5 and 15-16.5 mg/kg diet for fish fed 28 and 36 percent protein diets, respectively for hybrid tilapia. The source of dietary carbohydrates influences niacin requirement for hybrid tilapia which was reported to be 121 mg/kg for dextrin-based diets and 26 mg/kg for fish fed glucose-based diets. Vitamin requirement values are also dependent on the stability and bioavailability of the vitamin compound that was used. For example, the phosphate forms of ascorbic acid are more available than the sulphate forms.
There is little information on the mineral requirements of tilapia. Like other aquatic animals, tilapia are able to absorb minerals from the culture water which makes the quantitative determination of these elements difficult to carry out. For example, when Nile tilapia reared in fertilized ponds were fed with diets either containing complete mineral mixes or one deficient in Ca, P, Mg, Na, K, Fe, Zn, Mn or I and it was found that only the addition of phosphorous significantly affected weight gain, food conversion ratio and protein efficiency ratio (Stickney, 1997). Despite its ability to absorb minerals from the culture water and the presence of minerals in feed ingredients, tilapia feeds should contain supplemental mineral premixes. This is to ensure that sufficient levels are available to protect against mineral deficiencies caused by reduced bioavailability such as when plant phosphorus sources are used in tilapia feeds. Like vitamins, the amount of minerals to be added in the diet will also depend on the source of the element. For example, Shiau and Su (2003) reported that ferric citrate is only half as effective compared to ferrous sulphate in meeting the iron requirement of tilapia.
Many of the plant-based feed ingredients have high phytic acid content which appears to bind metal ions such as calcium, phosphorus, magnesium, manganese, zinc and iron rendering them unavailable. The ability of phytic acid to bind metal ions is lost when the phosphate groups are hydrolyzed through the action of enzyme phytase. Although phytase activity has been shown to be present in ruminants, animals with a simple stomach such as fish lack this enzyme in their gastrointestinal tracts and hence cannot utilize the phytate bound phosphorus or other metal ions. Therefore, feeds are often supplemented with phosphorus in the form of mono or di-calcium phosphate. Phosphorus and calcium requirements are interdependent. Addition of microbial phytase in the diet of Nile tilapia significantly improved the growth of fish (Portz et al., 2003; Furuya et al., 2003). Variations in the quantitative values reported in literature can also be expected due to differences in dietary ingredients used.