Although Atlantic salmon is the most successfully farmed salmonid, the nutrient requirements of this species are not well defined, and the available information is based on studies conducted on young fish. Similar to other fish species, salmon require the same nutrients (protein, amino acids, essential fatty acids, vitamins and minerals) for normal growth, reproduction, and immune and metabolic functions (Table 2). Nutrient requirements of rainbow trout (Oncorhynchus mykiss) and chinook salmon (O. tshawytscha) have been used to predict the requirements of certain micronutrients such as amino acids, minerals and vitamins for feed formulation when this information is not available for Atlantic salmon (NRC, 1993; Storebakken, 2001).
Like other fish species, salmon do not have a protein requirement, but they require essential amino acids (arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine) contained in protein for normal growth. Early research based on purified diets showed that juvenile Atlantic salmon reared in seawater required 45 percent protein (Lall and Bishop, 1977). Rapidly growing salmon fry and juvenile fish perform better on high protein diet (~50 percent) and grower diets containing 42–48 percent protein (Table 2). For the most part, the quantitative dietary essential amino acids requirement established for rainbow trout is also used for Atlantic salmon. Lysine, methionine and arginine (or threonine) are the most limiting amino acids in salmon feeds when fishmeal level is reduced and plant protein sources are increased. As is the case with most fish, the optimum protein level in feeds depends upon dietary energy content and the ratio of essential to non-essential (or indispensable to dispensable) amino acids. The disproportionate levels of specific amino acid antagonistics such as leucine and isoleucine and others (arginine/lysine, cystine/methionine) in the diet may result in marginal or severe amino acid deficiency, particularly when fish are under certain environmental and physiological stress. Certain essential amino acids (e.g. leucine) may also be toxic when present in excess in diets. In Atlantic salmon smolts, dietary histidine appears to be one of the important factors in preventing cataracts, and the beneficial effects are related to high levels of histidine and the build up of N-acetyl histidine (NAH) in the lens, which possess buffering and antioxidant properties (Bjerkås, Breck and Waagbø, 2006).
Atlantic salmon have no specific requirements for dietary carbohydrates. The following two types of carbohydrate are derived from the feed ingredients of plant origin used in salmon feeds: starch and non-soluble polysaccharides (NSP). Starches are also added as binders to improve the stability of extruded feed pellets. Raw starch is essentially unavailable to salmonids, but cooking during feed processing improves its digestibility. Atlantic salmon has poor ability to regulate blood glucose when carbohydrate load is excessive (Hemre et al., 1996). NSP are not available to fish (reviewed by Stone, 2003).
The energy requirement for maximum growth is influenced by water temperature, size of fish, and diet composition and nutrient availability. The efficiency of energy utilization is improved by reducing dietary protein content and increasing dietary lipid, thereby reducing the digestible protein (DP) to digestible energy (DE) ratio. The ratios of DP to DE for maximum growth have been measured using practical diets: fingerlings, 23 g/MJ; smolts, 20 g/MJ; grower (0.2 –2.5 kg), 19 g/MJ ; and grower (2.5–4 kg), 16–17 g/MJ (Storebakken, 2001). Recent genetic improvements in growth and feed formulation strategies to provide optimum DP:DE ratio in feeds at different stages of the life cycle of Atlantic salmon have resulted in higher growth and feed utilization.
Dietary lipids supply energy and essential fatty acids (EFA). Increasing levels of dietary fat (up to 24 percent) increases the efficiency of protein utilization. The EFA requirement of Atlantic salmon can only be met by supplying the long-chain highly unsaturated fatty acids, eicosapentaenoic acid (EPA), 20:5n-3, and/or docosahexaenoic acid (DHA), 22:6n-3. Based on total body and tissue fatty acid composition data, the estimated EFA requirement of salmon is 1 percent of the diet for 20:5n-3 and 22:6n-3 fatty acids combined (Ruyter et al., 2000). EFA deficiency causes reduced growth, increased mortality, and reduced concentrations of EPA and DHA in the blood and liver phospholipids and an increase in 20:3n-9 level. Although marine fish oils (MFO) have been traditionally used in salmon diets, the overexploitation of marine resources has resulted in limited supply of this oil supplement. Recent research has shown that it is possible to replace the major proportion of MFO with vegetable oils (VO) and still maintain optimum growth and feed utilization over the major part of the life cycle. The partial substitution of MFO in fish diets with vegetable and animal lipid sources affects tissue and cellular lipid composition. Finishing diets based on MFO can be used to tailor the desired level of EPA and DHA in the final product. To date, any significant effects of either partial or full replacement of MFO with vegetable oils (canola, rapeseed and flaxseed oils) on flesh and sensory quality of fish have not been observed. Diets containing high levels of n-3 and n-6 fatty acids from fish and vegetable oils modify the tissue and cellular phospholipid’s fatty acid composition (Bell, Dick and Sargent, 1993).
Qualitative and quantitative requirement values of most fat-soluble (A, D, E and K) and water-soluble (thiamin, riboflavin, niacin, pyridoxine, pantothenic acid, biotin, folic acid, vitamin B12 and vitamin C) vitamins established for rainbow trout and chinook salmon have been used for the feed formulation of Atlantic salmon with some exceptions (NRC, 1993). For juvenile salmon, the minimum requirement of vitamin E has been estimated as 60 mg/kg dry feed (Hamre and Lie, 1995), a value higher than that for other salmonids. A dietary supplement of 500 mg/kg has been recommended as a measure to prevent oxidative damage of salmon fillet during storage, as well as to maintain optimum flesh pigmentation. It appears that requirements of water-soluble vitamins are lower than values recommended in early studies (NRC, 1993; Woodward et al., 1994). The requirement values determined by maximum liver storage or certain enzyme activity data are often higher than values based on weight gain and the absence of deficiency signs data. There is evidence of improvement on health immune function and disease resistance in salmon with higher supplementation of vitamin C and other vitamins; however, the response under farming conditions is not always consistent with laboratory findings. Ascorbic acid appears to protect phagocytic cells and surrounding tissues from oxidative damage. An increased immune response due to high levels of ascorbic acid supplementation has been demonstrated in several fish species (reviewed by Gatlin, 2002). Dietary and environmental contaminants such as heavy metals increase the ascorbic acid requirements of fish. Reduced reproductive performance has also been reported in rainbow trout fed ascorbic acid-deficient diets (Sandnes et al., 1984). Ascorbic acid reserves are rapidly depleted during the embryonic and larval development of certain fish, suggesting essentiality of this vitamin during early life stages as well as a higher requirement than in juveniles and adult fish. Liver and kidney ascorbic acid concentrations of less than 25 μg/g have been suggested as an indicator of ascorbic acid deficiency in salmonids (Sandnes et al., 1992).
Generally, fat-soluble vitamins function as an integral part of cell membranes; in addition, some of them may have hormone-like functions. Water-soluble vitamins act as coenzymes accelerating enzymatic reactions and often serve as carriers for specific chemical groupings. Diseases due to vitamin deficiencies are a gradual process. When the deficiency persists, the level in cells falls and the metabolic processes involving a particular vitamin are impaired. However, the changes do not occur at a uniform rate throughout all tissues of the body because some retain particular vitamins more strongly, while other tissues, by virtue of their metabolic peculiarities, are sensitive to change in vitamin availability. Therefore, vitamin supplementation of feeds takes into account genetic strains, physiological status, growth and stress.
Most essential elements required by terrestrial animals are also considered essential for Atlantic salmon, and thus requirements have been reported for phosphorus, magnesium, iron, copper, manganese, zinc, selenium and iodine (Lall, 2002, 2008). The exchange of ions from the surrounding water across the gills and skin of fish complicates the measurement of mineral requirements, and uptakes of water-borne minerals were not taken into account in requirement studies. Mineral deficiency signs in salmon and other fish include reduced bone mineralization, anorexia (potassium), lens cataracts (zinc), skeletal deformities (phosphorus, magnesium, zinc), fin erosion (copper, zinc), nephrocalcinosis (magnesium, selenium toxicity), tetany (potassium), thyroid hyperplasia (iodine), muscular dystrophy (selenium) and hypochromic microcytic anemia (iron).
Bioavailability of dietary phosphorus is influenced by several factors, including chemical form, digestibility of diet, particle size and interaction with other nutrients, feed processing and water chemistry. High concentrations of some minerals can create a mineral imbalance in the diet and cause a pollution problem in effluent waters. The digestibility of P in fishmeals ranges between 40 and 60 percent. Plant proteins contain phytates (inositol hexaphosphoric acid), which are unavailable to salmonids. Supplementation of microbial phytase has been effective in improving P bioavailability of plant feed ingredients provided the activity of this enzyme is maintained and water temperature is optimum for feed utilization. Zinc bioavailability is reduced by plant phytates and higher concentrations of calcium phosphate supplied by bones in fishmeal. Atlantic salmon feeds may contain a high proportion of fishmeal and marine by-products supplemented with trace elements at a higher concentration than required due to limited information on their requirements and bioavailability from feed ingredients. Feeds are often supplemented with zinc, iron, copper, manganese, selenium, iodine and phosphorus; however, they may also contain other trace elements supplied from common feed ingredients. Elevated levels of zinc, copper, cadmium and manganese have been found in sediments under sea cages and in solid wastes generated by fish farms that affect the ecology of benthic organisms (reviewed by Lall and Milley, 2008).
