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Two experiments were performed to study the effects of imbalanced levels of arginine and lysine in diets of Atlantic salmon (Salmo salar L.) with Zein as the major dietary protein source. In the first experiment, the effects of high levels of lysine on marginal levels of arginine and high levels of arginine on marginal levels of lysine were compared in fish fed these amino acids at either marginal or high levels. Dietary contents of arginine/lysine were 30.0/29.4, 17.0/29.4, 30.0/14.4 and 17.2/14.5 g kg–1, respectively. In the second experiment, fish were fed increasing levels of lysine in a diet containing arginine at the requirement level. Dietary levels of arginine/lysine in the second experiment were 23.0/17.0, 23.0/34.3, and 23.0/46.4 g kg–1. Atlantic salmon fed marginal levels of lysine showed better growth and feed utilization when fed diets supplemented with high levels of arginine. Fish fed diets marginal in arginine exhibited increased growth and feed utilization when fed diets supplemented with high levels of dietary lysine. Increasing dietary lysine did not significantly affect growth in fish fed arginine at the requirement level. Increased growth in fish fed high dietary arginine levels might be explained by increased tissue concentrations of ornithine acting as a precursor for in situ synthesis of polyamines. A slight correlation between specific growth rate (SGR) and concentration of spermidine in muscle and between condition factor and concentration of total polyamines in muscle was seen. Abdominal injection of U‐C14 arginine may indicate lysine inhibition of arginine availability in muscle tissue.

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Dierent chemical forms of essential minerals and trace elements supplemented to feed may show dierent bioavailability in ®sh. The present experiment was performed to compare an organic zinc form (Zn-gluconate) with zinc sulphate as dietary zinc sources for Atlantic salmon. Triplicate groups of ®sh were fed commercial ®sh meal based diets supplemented with either a low zinc level (50 mg Zn kg )1 ) or medium zinc level (180 mg Zn kg )1 ) as zinc gluconate or zinc sulphate for 6 months. Growth, mortality and feed conversion were recorded through the experiment. Fish were sampled initially and every month for zinc status analyses measured as whole-body zinc concentration. After 4 and 6 months, vertebral zinc concentration was measured and serum zinc was analysed at the end of the experiment. There were no dierences in growth or mortality during the experiment. The results showed a signi®cant eect of dietary zinc supplementation level on zinc status in ®sh for both zinc forms. However, there were no dierences in zinc status between groups given dierent zinc forms. In conclusion, the present experiment showed that zinc sulphate and zinc gluconate are equally ecient as zinc supplements in salmon feeds within the concentrations normally found in salmon feed. KEY WORDS

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