Effects of different dietary fat levels in cage-fed Atlantic mackerel (Scomber scombrus)

In wild Atlantic mackerel, Scomber scombrus, the lipid content of the later al 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 cus tomers. Wild caught mackerel were kept in salmon cages fed a high energy (3 00 g fat kg(-1)) salmon diet rich in n-3 polyunsaturated fatty acids from O ctober 1997 to April 1998. Fish were submerged at 20-40 m at a temperature above 5 degrees C from January until April. Then the fish were brought to t he 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 mor tality was very low and the body weight doubled concomitant with an increas ed muscle lipid content from 19.5% to 30.9% during the first six months. Bo th groups receiving feed further increased the body weight and muscle lipid content at the same magnitude towards the end of the experiment. During th e first six months the cross sectional area (CSA) of red muscle fibres incr eased 3.3fold and this size was maintained throughout the experiment. In co ntrast, 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 fa tty acid oxidation (beta-oxidation) capacity dominated in the red part of t he lateral muscle. During the winter an increased beta-oxidation capacity w as 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 beta-oxidation capaciti es, particularly in high energy-fed and non-fed mackerel. It is concluded t hat it is possible to feed captive mackerel during the winter and produce m ackerel with a high quality for the market in early spring.