EFFECT OF LOW DIETARY MAGNESIUM ON IMMUNE-RESPONSE AND OSMOREGULATIONOF ATLANTIC SALMON

The influences of dietary magnesium on immune response and on osmoregu lation in parr of Atlantic salmon Salmo salar were determined. Groups of fish were fed a casein-gelatin diet unsupplemented (containing abou t 200 mg Mg/kg) or supplemented with either 300 or 500 mg Mg/kg dry di et (as MgSO4) for 12 weeks before vaccination to produce fish with dif ferent Mg levels, and the feeding regime was continued throughout the study. No differences were observed between the treatment groups in se rum-specific antibody levels every second week for 8 weeks after vacci nation against Vibrio anguillarum serotypes O1 and O2. Both lysozyme l evels and spontaneous hemolytic activities in serum were elevated in v accinated Ash compared with unvaccinated fish. Neither lysozyme activi ty, complement hemolytic activity, total protein in serum nor blood he moglobin were affected by dietary Mg. The spontaneous hemolytic activi ty 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 seru m chloride concentrations in all groups after 24 h reflected a general salinity intolerance, but the highest serum chloride level was observ ed in fish fed the unsupplemented dirt. This indicates that low dietar y Mg affects the osmoregulation of Atlantic salmon.