Effect of low dietary magnesium on immune response and osmoregulation of Atlantic salmon

The influences of dietary magnesium on immune response and on osmoregulation in parr of Atlantic salmon Salmo salar were determined. Groups of fish were fed a casein-gelatin diet unsupplemented (containing about 200 mg Mg/kg) or supplemented with either 300 or 500 mg Mg/kg dry diet (as MgSO4) for 12 weeks before vaccination to produce fish with different Mg levels, and the feeding regime was continued throughout the study. No differences were observed between the treatment groups in serum-specific antibody levels every second week for 8 weeks after vaccination against Vibrio anguillarum serotypes O1 and O2. Both lysozyme levels and spontaneous hemolytic activities in serum were elevated in vaccinated Ash compared with unvaccinated fish. Neither lysozyme activity, complement hemolytic activity, total protein in serum nor blood hemoglobin were affected by dietary Mg. The spontaneous hemolytic activity in serum was lower in fish fed the unsupplemented diet and highest in fish fed the diet supplemented with 500 mg Mg/kg. After 28 weeks on the diets supplemented with graded levels of Mg, a salinity tolerance test (32.5 g/L, 24 h) was performed. High mortality and elevated serum chloride concentrations in all groups after 24 h reflected a general salinity intolerance, but the highest serum chloride level was observed in fish fed the unsupplemented dirt. This indicates that low dietary Mg affects the osmoregulation of Atlantic salmon.