Bioavailability and interactions with other micronutrients of three dietary iron sources in Atlantic salmon, Salmo salar, smolts

Atlantic salmon, Salmo salar L., with mean initial weight of 60 g were fed a diet based on cod muscle meal supplemented with elemental iron, iron sulphate or haem-bound iron in concentrations of 0, 25, 50, 100, 500 and 1500 mg iron kg(-1) for 8 weeks. No significant differences in growth or mortality were found, except in fish fed 1500 mg haem iron kg(-1), which showed reduced growth. In fish fed diets supplemented with elemental iron below 1500 mg iron kg(-1), blood haemoglobin and hepatic iron concentration decreased compared with fish fed the unsupplemented diet. Fish fed diets supplemented with iron sulphate showed increased blood haemoglobin and hepatic iron concentrations between 25 and 100 mg iron kg(-1). Fish fed diets supplemented with haem-bound iron showed increased hepatic iron at all dietary iron levels, while blood haemoglobin concentration decreased in the group fed 1500 mg haem iron kg(-1). The bioavailability of haem iron relative to sulphate iron was calculated by the slope ratio method to be 239% and 148% using blood haemoglobin and hepatic iron, respectively. Relative bioavailability of elemental iron was zero when dietary supplementation levels were between 25 and 500 mg iron kg(-1), while a small part was utilized when 1500 mg elemental iron kg(-1) was supplemented. Additions of 500 and 1500 mg haem-bound iron kg(-1) resulted in a complete loss of ascorbic acid in these diets. When these groups were discounted, no significant relationship between hepatic iron and hepatic ascorbic acid was found. There was no significant effect of dietary iron on whole-body manganese concentration and only a weak effect on whole-body zinc concentration. No significant correlations between dietary iron and hepatic copper concentration were found in any of the dietary treatments. This study also showed that the level of inorganic iron supplementation may be reduced by inclusion of 20 g blood meal kg(-1) in the diet.