The reduction and replacement of fishmeal with plant-derived ingredients have affected the availability and supply of dietary zinc to salmonids which also has an important role in osmoregulation mechanisms that affect smoltification.
Norwegian researchers from the Institute of Marine Research, together with Skretting, assessed low fishmeal feeds supplemented with zinc in two trials in Atlantic salmon. In trial I, Atlantic salmon parr were fed six graded zinc levels (40 to 249 mg kg−1 as ZnSO4) for eight weeks in freshwater followed by a four-week seawater phase. In trial II, Atlantic salmon post-smolt were fed for ten weeks in seawater with ten dietary zinc levels (45 to 280 mg kg−1), either as ZnSO4 or Zn-glycinate.
Results showed that growth was unaffected by dietary zinc in both trials. Dietary zinc affected the concentration of Na+ and K+ ions in the plasma, branchial and intestinal expression of sodium potassium ATPase, tissue and body zinc status, and cataracts. Atlantic salmon post-smolt in seawater improved body and tissue zinc status with increasing dietary zinc levels, irrespective of the zinc source.
One of the interesting findings was that dietary zinc need for Atlantic salmon in seawater was 30-40% higher than in freshwater. Seawater transfer significantly reduced the apparent availability of zinc and increased endogenous zinc loss, subsequently reducing body and tissue zinc levels. In accordance, body or tissue saturation of zinc occurred at dietary zinc levels between 137 and 156 mg kg−1 with smolts in freshwater and 181 to 218 mg kg−1 in seawater post-smolts.
Increasing zinc supplementation is limited by the current upper limit for zinc in salmonid feeds (180 mg Zn kg−1 diet) (European Commission, 2016) and further reduction to 150 mg Zn kg−1 diet has been suggested by EFSA. Researchers concluded that dietary zinc levels below 180 mg kg−1 in low fishmeal feeds compromised the zinc status, health and welfare of Atlantic salmon, especially in seawater. It was further suggested that “early nutritional programming of fish towards low zinc diets can provide insights into mechanisms for effective utilization of dietary zinc. Better utilization of dietary zinc would enable a reduction in total zinc levels in the diet, without compromising fish health and subsequently decrease environmental zinc load.”
Check out the study here.