Role of iron in Atlantic salmon (Salmo salar) nutrition: Requirement, bioavailability, disease resistance and immune response

The role of iron in the nutrition of Atlantic salmon (Salmo salar ) was investigated in this study because of recent information on its clinical role in aquatic and terrestrial animals, detrimental effects of iron on nutrient stability in feeds, discharge of nutrients in aquaculture effluents and the increase in the prevalence of infectious diseases. Six experiments were designed to investigate the quantitative iron requirement of Atlantic salmon, its biological availability from potential inorganic and organic feed supplements and to examine the role of this element in immune function and disease resistance. The estimated quantitative dietary iron requirement of salmon based on broken line analysis of tissue iron concentration data was 60 mg iron/kg of diet. Atlantic salmon fed a low iron semi-purified test diet developed hypochromic microcytic anemia. Fish growth and survival during the feeding trial were similar among six dietary treatments (0,15, 30, 60 and 120 mg iron/kg) and no other overt gross deficiency signs were observed. Results show that serum ferritin levels could be used as an indicator of the physiological iron status of Atlantic salmon. The biological availability of iron from various inorganic feed supplements (ferric chloride, ferric oxide) and animal protein sources (fish meal, blood meal) was measured by the hemoglobin regeneration assay. The relative bioavailability of iron from ferric chloride, ferric oxide, blood meal and herring meal was 98.9, 17.8, 52.3 and 47.1%, respectively. Vitamin C (L-ascorbyl-2-polyphosphate) supplementation (300 mg/kg) of a fish meal based diet had no significant effect on iron absorption and deposition in groups receiving either no inorganic iron supplement or diet containing iron from an organic source (blood meal).; Furunculosis and vibriosis diseases experimentally induced in Atlantic salmon by Aeromonas salmonida and Vibrio anguillarum infections caused mobilization of iron from liver and spleen into plasma. Plasma iron concentrations were significantly higher in fish infected with V. anguillarum than A. salmonicida. Feeding fish during the V. anguillarum infection caused higher mortality than those fish receiving no food. An extensive experiment was conducted to determine the effects of heme (blood meal) and non heme (FeSO₄.7H ₂O) iron on immune response, resistance to bacterial diseases ( Aeromonas salmonicida and Vibrio anguillarum) and tissue iron accumulation. The following immune responses were investigated: bactericidal activity of serum, differential blood cell counts, serum lysozyme activity, phagocytosis by macrophages, myloperoxidase assays and respiratory burst activity of phagocytes and lymphocyte proliferation. No significant disease resistance against virulent V. anguillarum and A. salmonicida was observed among Atlantic salmon fed diets containing different levels of non-heme (0,100, 300, 600 or 1200 mg/kg) and heme (0,100 or 300 mg/kg) iron. Although, variability in several immune responses was observed in fish fed diet with varying concentrations and forms of dietary iron, the responses were not consistent. Excessive amounts of iron in salmonid feeds should be avoided because of its potential detrimental effects on nutrient stability and its influence on plasma iron concentration which may promote growth of bacterial pathogens.