The suitability of land animal by-products (ABPs) in feed for Atlantic salmon postsmolts (initial weight 372 g) in sea water was studied in a feeding experiment, using poultry by-product meal (PBM) and porcine blood meal (BM) as protein sources and poultry oil as fat source. Four extruded feeds were tested in a 2 * 2 factorial model, with or without ABP protein sources and with or without poultry oil. The control feed contained a mix of marine and plant ingredients. Initial feed intake was highest in the ABP proteinbased diets, whereas poultry oil had a weak opposite effect. No differences were seen in growth rate or body weight. Addition of PBM and BM led to increased FCR, and lower retention of crude protein and energy. This could be explained by lower digestibility of amino acids and crude protein, and a slightly lower energy level in these diets. Reduced igf1 mRNA levels in liver and muscle were seen in fish fed dietary ABP protein and oil. Despite lower protein digestibility of ABP protein, this study confirms the suitability of ABP protein and lipid in combination with plant ingredients in feed for Atlantic salmon growers.

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An experiment with 2(7 − 3) reduced factorial design was conducted to study the biological effects of pro‐ and antioxidant micronutrients and lipid in Atlantic salmon. Vitamins C and E, astaxanthin, lipid, iron, copper and manganese were supplemented at high and low levels. For vitamins and minerals, high levels were chosen to be below the anticipated toxic level and the low levels were just above the requirement (vitamin C, 30 and 1000 mg kg−1; vitamin E, 70 and 430 mg kg−1; Fe, 70 and 1200 mg kg−1; Cu, 8 and 110 mg kg−1; Mn, 12 and 200 mg kg−1). For astaxanthin, the dietary levels were 10 and 50 mg kg−1 and for lipid, 150 and 330 g kg−1. The experiment was started with postsmolts (148 ± 17 g) and lasted for 5 months. The variation in micronutrients had only minor effects on growth, feed conversion and fillet quality, measured as lipid and astaxanthin deposition. High dietary lipid had a profound positive effect on growth and feed conversion but gave fillets nearly two times the fat content that was found in fish fed the low lipid diet. Astaxanthin deposition in the fillet was primarily affected by dietary astaxanthin with a positive effect of high dietary lipid in week 14 but not in week 23. Vitamin E protected the fillet against iron ascorbate stimulated oxidation, with no effect of the other nutrient variables.

The aim of the present study was to examine whether a combined plant protein and vegetable oil replacement (P diet) of fish‐based marine feed ingredients (F diet) imposes oxidative stress on Atlantic salmon after short‐ and long‐term feeding. Oxidative stress was assessed by measuring a suit of hepatic biomarkers, including TBARS, enzymatic and transcriptional levels of a set of antioxidant proteins as well as antioxidant vitamins. Antioxidants assessed regularly in the long‐term study were influenced by the dietary compositions, mostly influencing liver astaxanthin and tocopherols. In the short‐term crossover study, fish were sampled initially, at Day 2 and at Day 17 after transfer to new diet. The results showed that fish relocated from the F diet to the P diet had significantly lower transcriptional levels of CuZn SOD (Day 2, P < 0.006) and Mn SOD (Day 17, P < 0.003), in addition to lower levels of catalase protein (Day 17, P < 0.05) and total glutathione (Day 17, P < 0.05). At Day 17, both groups of fish relocated to new diets had significantly increased transcriptional levels of GSH‐Px (F‐P: P < 0.05, P‐F: P < 0.007). This study showed that switching from a marine‐based to a plant‐based diet affected Atlantic salmon gene expression and the antioxidative defence in the fish.

Atlantic salmon were fed one of four diets with increasing levels (11–58%of total fatty acids (FAs)) of n‐3 highly unsaturated FAs (HUFAs) in order to investigate the effect on muscle and mitochondrial membrane lipids and their susceptibility to oxidative stress. The high n‐3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) groups contained lower percentages of sphingomyelin and cardiolipin in total muscle than the intermediate n‐3 HUFA group. Cardiolipin and sphingomyelin are particularly susceptible to peroxidation, and a reduced percentage of these lipids showed that mitochondrial membranes had been damaged by oxidation. The intermediate n‐3 HUFA group had the highest level of mitochondrial integrity and tendencies of lower thiobarbituric acid reactive substances (TBARS) level than the other dietary groups. The high caspase‐3 activity, in addition to casp3a and bax gene expression levels, in the n‐3 DHA group also suggests that some degree of oxidative stress had occurred. Electron microscopy images showed a higher degree of myofibre–myofibre detachment in fish fed the high HUFA diets than in fish fed the intermediate n‐3 HUFA diet. Our findings show that intermediate levels of n‐3 HUFAs in salmon diets gave the best protection against oxidative damage of mitochondrial membranes and muscle structure.