Wild Atlantic mackerel, Scomber scombrus, were kept in net cages and fed a high energy salmon diet rich in (n–3) polyunsaturated fatty acids for 8 months. Analyses of whole body and muscle were performed at regular intervals on 1 and 2-year-old fish. The percentage of lipid and dry matter increased in whole body and muscle as the fish weight incrcased from catch to market size over the course of the following year. In all groups of fish examined the sum of fatty acids (n–3) as the percentage of total lipid in muscle decreased from 33 to 26%, while the sum of monoenoic acids increased with 10% from 39 to 49%. Vitamin E concentration of muscle increased substantially during feeding. The oxidation status of the fish muscles was very good at the end of the feeding period, with thiobarbituric acid values of less that 3 µmol kg−1. At the end of the feeding experiment large fish to be sold at market were starved for two weeks. Analyses of the nutritional and oxidative state of muscle showed that transport, slaughter and storage did not affect the lipid content or the fatty acid composition. Low and stable thiobarbituric acid values were observed, while vitamin E content significantly decreased during processing and storage at −30 °C, which indicates that vitamin E was an effective antioxidant in mackerel muscle.