Agri Kultuur December / Desember 2018 - Page 9

dietary ingredients, fish species and size and feeding frequency. 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. Nile tilapia can utilize 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. It’s been 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. 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, has been reported to spare vitamin E in diets for hybrid tilapia. Choline can be spared to some extent by betaine. β-carotene can be bio-converted to vitamin A with a conversion AgriKultuur |AgriCulture ratio of about 19:1. 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, tilapias can 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. 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. 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 9