Despite the importance of European seabass to Mediterranean marine finfish production, studies on nutrient requirements are rather limited. Very early studies suggested that European seabass might have very high dietary protein requirements, ranging from 52 to 60 percent of the diet. Much fine tuning has occurred since then to show that dietary crude protein levels can be decreased to between 42–48 percent of the diet, given that adequate dietary digestible energy (DE) levels are provided. The digestible protein (DP) to DE ratios decrease with increasing size of fish, going from 23–24 mg DP/kJ DE for fish of 5–20 g to 1–19 mg / kJ in fish of 150 g onwards. Although early data indicated values above 22 mg DP/kJ DE, more recent studies suggest that the optimal protein to energy ratio of the diets for seabass should be 19 mg/kJ, in diets with at least 21 MJ/kg DE, with the major portion of non-protein energy being supplied by dietary fats.
As regards essential amino acids (EAA), European seabass require the same ten amino acids (EAA) as other finfish or crustaceans. Quantitative data on requirements for some of these EAA based on dose-response curves using semi-purified diets have been obtained for arginine, lysine, sulphur amino acids, tryptophan and threonine. For other EAA, indirect estimations have been made using methods such as those based on overall body protein accretion and an ideal protein composition has been provided (Kaushik, 1998b; Tibaldi and Kaushik, 2005) (Table 2).
With regard to the effect of water temperature, available data do not show any effect of water temperature on protein requirements. Although European seabass are very euryhaline, there is, so far, no information on the effect of water salinity on nutrient requirements.
Data on the energy requirements for maintenance and growth of European seabass have been made available (Lupatsch et al., 2001; Lupatsch, 2005). Daily energy and protein requirements of European seabass can be estimated using the following series of equations:
Lipids and fatty acid requirements
An increase in efficiency of protein utilization has been observed with increasing the dietary fat levels up to 18–20 percent, although much higher fat levels (30 percent of dry diet) appear to lead to a growth depression. As regards essential fatty acids (EFA), the long chain polyunsaturated fatty acids (PUFA) of the w3 series (eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)) are essential for European seabass, and current estimates suggest a requirement of about 1 percent of w-3 PUFA (Coutteau, Van Stappen and Sorgeloos, 1996). A number of studies have been undertaken with larval stages of European seabass fed PUFA-enriched live prey. Supplementation with about 1 to 2 percent of phospholipids is also considered to have a beneficial effect on growth and retention of fatty acids in juvenile seabass. Recent studies with the partial substitution of fish oil by single or mixtures of vegetable oils (Mourente and Bell, 2006) have shown that the fatty acid composition of the neutral lipids in seabass, as in many other teleosts, reflects that of the dietary fatty acid profile. Seabass are however susceptible to having a high fat deposition in the liver. Given that marine oils rich in w-3 PUFA are susceptible to oxidation, the consequences in terms of muscle quality and oxidative status are issues of concern.
Carbohydrate utilization
Digestibility of carbohydrates in seabass is affected by the nature and complexity of the carbohydrate supplied; use of pre-cooked or pre-treated plant products then becomes necessary for improving starch digestibility and hence DE supply. Very high levels of starch (> 30 percent) appear however to induce some growth depression. At the post-absorptive level, a prolonged hyperglycemia is also reported in seabass with postprandial patterns comparable to those observed in other species.
Vitamin requirements and deficiency signs
Data on quantitative vitamin requirements of European seabass are extremely scarce. A dietary requirement for vitamin C below 50 mg/kg diet has been reported for seabass. Requirements for growth of European seabass juveniles appear to be low (< 10 mg/kg feed), but a higher dietary supply can ensure maximal ascorbate concentration in the liver (Fournier, Gouillou-Coustans and Kaushik, 2000). There is also indirect but clear evidence that the requirements of European seabass for most water-soluble vitamins would not differ from those established for the salmonids and that the recommendations on requirements for water-soluble vitamins established for salmonids (NRC, 1993) can be applied to European seabass fed practical diets (Kaushik, Gouillou-Coustans and Cho, 1998). More recently, in studies with first-feeding larvae of European seabass fed formulated feeds, maximal growth and overall quality of larvae were achieved with feeds containing several times the NRC recommendations (Mazurais et al., 2008). Although no quantitative data on requirements are available as regards fat-soluble vitamins, a dietary supply of vitamin E at levels of 500 mg/kg diet has been recommended as a measure of defence against peroxidative damage and to improve shelf life of filets. During the early larval development, a supply of optimal levels of vitamin A is considered essential, especially with regard to skeletal development and deformities.
Minerals and trace elements
There is a definite lack of data on the requirements of European seabass for minerals and trace elements. Of the minerals, only that for phosphorus has been estimated to be 0.65 percent of the dry diet (Oliva-Teles and Pimentel-Rodrigues, 2004). The possible improvement of the availability of phosphorus in plant feedstuffs by dietary exogenous phytase has also been demonstrated in European seabass.