Protein and Essential Amino Acids (EAA)
Optimal dietary protein content has been shown to vary with dietary energy levels and the size of fish. Most studies suggest a crude protein (CP) requirement between 45 percent and 55 percent (Glencross, 2006). Requirements for protein and amino acids decreases with increasing fish size according to the change in energy demand of the fish. Glencross (2003) determined a digestible protein requirement of 56, 42 and 31 percent at a DE content of 15kJ/g for 10, 100 and 1000g fish, respectively. Protein utilisation efficiency has been estimated at 46–50 percent (Williams and Barlow, 1999; Lupatsch, 2003). Nankervis & Southgate (2006b) have shown that the optimum diet for L. calcarifer larvae from 14 to 28 days post–hatch (DPH) should contain 50 percent protein and a minimum of 21 kJ/g dietary energy. Some EAA requirements have been determined (Table 2).
Lipids and essential fatty acids
Best performance by 80g fish was obtained with a diet containing 18 percent lipid and 60 percent protein and protein retention may be improved with increased lipid levels (Williams et al., 2003a). Survival at metamorphosis is influenced by ω3 Highly Unsaturated Fatty Acids (HUFA) levels in the live food (20:5 3, 22:6ω3) (Rimmer et al., 1994; Dhert et al., 1990). Dietary n-3 fatty acids levels of 1.0 percent have been recommended for juvenile fish (Wanakowat et al., 1993). Williams and Barlow (1999) suggested an optimal ratio of n-3 to n-6 fatty acids of between 1.5 and 1.8:1.
Carbohydrates
Barramundi have shown limited capacity to utilise dietary carbohydrates for energy. Starch digestibilities are below 30 percent at even low inclusion levels (Anderson, 2003; cited in Glencross, 2006). However, for juvenile barramundi Catacutan and Coloso (1997) showed that carbohydrates have a sparing effect for lipid as an energy source in practical diets. They recommended a dietary carbohydrate level of 20 percent for diets containing lipid levels ranging from 6 to 18 percent with 42.5 percent CP.
Vitamins and minerals
Certain dietary vitamins have been identified as essential for barramundi (Table 2). Those that have not been specifically identified are still thought to be required and this is consistent with findings from other fish (Glencross, 2006). Vitamin C requirement is 500–700 mg/kg of diet (Boonyaratpalin et al., 1989; 1994) or 25–30 mg/kg of diet using crystalline ascorbic acid or ascorbyl-2-monophosphate-magnesium, respectively (Phromkunthong et al., 1997). Wanakowat et al. (1989) have shown that 5 mg/kg diet of pyridoxine was required for normal growth and 10 mg/kg for normal lymphocyte levels. Pantothenic acid requirement is 15 mg/kg diet for normal growth (Boonyaratpalin and Wanokowat, 1993). Dietary phosphorus requirement ranges between 0.55 percent and 0.65 percent (Boonyaratpalin and Phongmaneerat, 1990). T he addition of salt (NaCl) to the diet at a level of up to 4 percent has been reported to generate a better feed utilization (Harpaz et al., 2005b).
Energy
Energy utilisation by barramundi has been estimated as 68 percent (Lupatsch, 2003). Based on the changes in somatic demand for energy by the fish, Glencross (2006) suggested a lower-energy, higher-protein diet (15 kJ DE/g, 50 percent CP, 14 percent lipid) for small fish (<200g), and a lower-protein, medium-energy diet (17 kJ DE/g, 46 percent CP, 20 percent lipid) for large fish (>200g).
At 20–22°C a feed high in DE (19 kJ/g) and a digestible protein to DE ratio of 22.5 g/MJ was recommended by Williams et al. (2006) to improve productivity.
