Effects of different dietary fat levels in cage-fed Atlantic mackerel (Scomber scombrus)*
In wild Atlantic mackerel, Scomber scombrus, the lipid content of the lateral muscle varies according to the season from around 30% in autumn to less than 5% in spring. To be able to offer mackerel with optimal quality on the market during spring or early summer after overwintering, it is necessary that the muscle lipid content is close to 30%, which is favoured by the customers. Wild caught mackerel were kept in salmon cages fed a high energy (300 g fat kg -1 ) salmon diet rich in n-3 polyunsaturated fatty acids from October 1997 to April 1998. Fish were submerged at 20-40 m at a temperature above 5 °C from January until April. Then the fish were brought to the surface and randomly divided into three duplicate groups, i.e., non-fed, 150 g fat kg -1 , and 300 g fat kg -1 and kept until August 1998. The mortality was very low and the body weight doubled concomitant with an increased muscle lipid content from 19.5% to 30.9% during the first six months. Both groups receiving feed further increased the body weight and muscle lipid content at the same magnitude towards the end of the experiment. During the first six months the cross sectional area (CSA) of red muscle fibres increased 3.3fold and this size was maintained throughout the experiment. In contrast, in non-fed mackerel the increase in CSA of red muscle fibres was 1.5fold. White muscle fibres revealed a much weaker response in fed mackerel and were not affected in mackerel deprived of feed. In wild mackerel the fatty acid oxidation (β-oxidation) capacity dominated in the red part of the lateral muscle. During the winter an increased β-oxidation capacity was found in heart and liver, whereas both red and white part of the lateral muscle showed a low fatty acid catabolism. In contrast, during summer red and white part of the lateral muscle possessed high β-oxidation capacities, particularly in high energy-fed and non-fed mackerel. It is concluded that it is possible to feed captive mackerel during the winter and produce mackerel with a high quality for the market in early spring.