Impact of dietary zinc and seawater transfer on zinc status, availability, endogenous loss and osmoregulatory responses in Atlantic salmon smolt fed low fish meal feeds

Atlantic salmon fed low fish meal feeds supplemented with zinc (Zn) were studied in two feeding trials. In trial I, Atlantic salmon parr were fed six graded Zn levels (40 to 249 mg kg(-1) as ZnSO4) for 8 weeks in freshwater followed by a 4-week seawater phase. In trial II, Atlantic salmon post-smolt were fed for 10 weeks in SW with 10 dietary Zn levels (45 to 280 mg kg(-1)), either as ZnSO4 or Zn-glycinate. Growth was unaffected by dietary Zn in both trials. Dietary Zn affected concentration of Na+ and K+ ions in plasma, branchial and intestinal expression of sodium potassium ATPase, tissue and body Zn status, and cataracts. Seawater transfer significantly reduced apparent availability, body and tissue levels of Zn due to increased endogenous Zn loss. Atlantic salmon postsmolt in seawater improved body and tissue Zn status with increasing dietary Zn levels, irrespective of the Zn source. Body or tissue saturation of Zn occurred at dietary Zn levels between 137 and 156 mg kg(-1) with smolts in freshwater and 181 to 218 mg kg(-1) in SW post-smolts. Dietary Zn levels below 180 mg kg(-1) in low fish meal feeds compromised the Zn status and welfare of Atlantic salmon in seawater.