Magnesium requirement of Atlantic salmon (Salmo salar L.) parr in seawater-treated fresh water

This experiment was undertaken to establish the magnesium (Mg) requirement in young Atlantic salmon, Salmo salar L., in seawater-treated fresh water. In Norwegian hatcheries it is a common practice to add sodium hydroxide and/or sea water (1-2\%) to improve pH and conductivity of the natural fresh water. Parr with initial weight of 8 g, were divided in six triplicate groups in brackish water containing 54 mg Mg L-1 and fed a basal casein-gelatine diet supplemented with minor amounts of krill and fish meal (containing 200 mg Mg kg(-1)) for an initial period of 3 weeks. Thereafter the fish were fed this diet supplemented with either 0, 100, 200, 300, 400 or 500 mg Mg kg(-1) (as MgSO4) for 12 weeks. Growth and feed efficiency were recorded. Concentrations of Mg and other divalent cations (Ca and Zn) were measured in whole fish, serum and vertebrae. Sodium concentration in vertebrae was also measured. Growth and feed efficiency were unaffected by the levels of dietary magnesium used in the experiment. Magnesium concentrations in the whole body, serum and vertebrae Mg appeared to be more sensitive than growth and feed efficiency to differences in dietary Mg intake. The group fed the unsupplemented diet showed significantly lower Mg concentration in these tissues than the other groups. Whole-body calcium concentration was negatively correlated with dietary Mg and Ca: Mg ratios in the vertebrae were significantly affected by the dietary Mg levels. Zinc concentration in whole body, serum and vertebrae was not altered by the dietary Mg levels. Further, vertebral Na concentration did not vary between the dietary treatments. In conclusion, a minimum Mg supplementation level of 100 mg kg(-1) dry diet (in total, 326 mg kg(-1)) was needed to maintain Mg concentration in the whole body and serum and for proper bone mineralization.