Quantitative data on all the nutrients required by gilthead seabream are rather limited (Oliva-Teles, 2000; Koven, 2002). Very early studies suggested that protein requirements of juvenile (2.5 g) gilthead seabream were about 40 percent for maximum protein and energy utilization (Sabaut and Luquet, 1973), although the best growth was observed in fish fed a diet with 60 percent crude protein. Current data suggest that the crude protein levels can be reduced to about 45–50 percent for juveniles and to 40–42 percent for grow-out. In practice, the digestible protein (DP) to digestible energy (DE) ratios are decreased with increasing size of fish, going from 28–30 mg DP/kJ DE for fish below 3 g to 24 mg/kJ in fish of 200 g and above (Table 2.1).

Energy requirements

Data on the energy requirements for maintenance and growth of gilthead seabream are also available (Lupatsch, 2005). Daily energy and protein requirements can be estimated using the following series of equations:

Essential amino acids
As regards essential amino acids (EAA), initial studies undertaken in France estimated the quantitative requirements based on dose-response curves using semi-purified diets for four amino acids: lysine, methionine, tryptophan and arginine. More recent work (Marcouli et al., 2005) has confirmed that gilthead seabream juveniles would require about 4.9 g/16g N of lysine and 2.8 g/16 g N for methionine, more or less confirming earlier data. For other EAA, indirect estimations have been made using methods such as those based on overall body protein accretion, and an ideal protein composition was proposed (Kaushik, 1998b; Tibaldi and Kaushik, 2005). More recently, Peres and Oliva-Teles (2009) have worked out the optimum dietary essential amino acid profile for gilthead seabream juveniles (Tables 2).

Lipids and fatty acids
As in many other teleosts, an increase in efficiency of protein utilization has been observed with increasing dietary fat levels, with high fat levels leading, however, to increased fat deposition. As regards essential fatty acids (EFA), a dietary requirement for the n-3 long-chain polyunsaturated fatty acids (LC-n-3 PUFA), primarily eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) has been established: current estimates suggest a requirement of about 1 percent of w-3 PUFA (Table 2.2)

 A number of studies have been undertaken with larval stages of gilthead seabream fed PUFA-enriched live prey. During the first weeks of feeding with live prey, LC-n-3 PUFA concentrations of 15 to 20 mg/g dry weight of rotifers or about 30 mg/g dry weight of Artemia have been found to be the most suitable. The nutritional value of DHA is higher than that of EPA for larval stages of fish.

Given the limitations in global availability of fish oil, a number of studies have been undertaken on partial substitution of fish oil by single or mixtures of vegetable oils. Replacement of 66 percent of fish oil by a mixture of plant oils (rapeseed, linseed and palm) leads to similar growth, health or physiological performance as with feeds containing only fish oil (Figure 8). From this study, it is inferred that a dietary EPA + DHA level of 1 percent is sufficient to meet all the vital needs of gilthead seabream. Dietary fatty acid profiles do, however, affect flesh fatty acid composition, which can be tailored by a finishing feeding with fish oil-based feeds.

Carbohydrate utilization
Current feeds contain about 15 percent starch. At the post-absorptive level, a prolonged hyperglycemia is also reported in seabream, with postprandial patterns comparable to those observed in European seabass (Peres, Goncalves and Oliva-Teles, 1999).

Vitamin requirements and deficiency signs
A clear dietary requirement for gilthead seabream for thiamine, riboflavin, pyridoxine, niacin and pantothenic acid has been demonstrated (Morris, Davies and Lowe, 1995). Some rough estimations of quantitative requirements for these vitamins have been made: requirement for niacin is between 63 and 83 mg/kg (Morris and Davies, 1995a), that for thiamine (vitamin B1) in juveniles (>60–200 g BW) is between 0.5 and 5 mg/kg feed (Morris and Davies, 1995c) and that for pyridoxine (vitamin B6) is about 2 mg/kg of dry diet (Kissil et al., 1981). Addition of pyridoxine has also been shown to increase the efficiency of protein utilization in gilthead seabream (Baker and Davies, 1995; Morris and Davies, 1995b). As regards vitamin C, no quantitative data on requirements are available. In seabream fed practical fishmeal-based diets, an absence of any added vitamin C did not affect growth performance (Henrique et al., 1998), but supplementary levels are considered beneficial with regard to resistance to stress, renal function and wound-healing (Alexis, Karanikolas and Richards, 1997; Henrique et al., 1998). In the absence of precise quantitative information, for all practical purposes, the recommendations for salmonids (NRC, 1993) are applied also to gilthead seabream. Indeed, as suggested by Woodward (1994), given that phylogenetically far different groups such as chickens, pigs and rainbow trout have very similar quantitative requirements for almost all water-soluble vitamins, it is considered today that, in the absence of precise species-specific data, the vitamin requirements established for salmonids can be applied to other teleosts.

There are also data available on the quantitative requirements for fat-soluble vitamins; the implications of vitamin E on resistance to stress, immune response and flesh quality are well recognized (Montero et al., 1998, 1999, 2001; Ortuno, Esteban and Meseguer, 2000). Similarly, vitamin A plays a major role in the nonspecific cellular immune system due to its antioxidant properties (Cuesta et al., 2002). During the early larval development, supply of optimal levels of vitamin A is considered essential, especially with regard to skeletal development.

Minerals and trace elements
Data on requirements of gilthead seabream for minerals and trace elements are limited to that for phosphorus. Phosphorus requirements of gilthead seabream juveniles were estimated to be 0.75 percent of the diet by Pimentel-Rodrigues and Oliva-Teles (2001). In another study undertaken by Güthler (2005), the phosphorus requirement of seabream was determined to be about 6.5 g available phosphorus per kg (0.65 percent) diet having a DE level of 18 MJ/kg. It is common practice to include a mineral and trace element premix in commercial feeds.