The lipid composition of Patagonian toothfish from the Macquarie Island region : ecological and dietary implications within a regional food web

Recent commercial interest in the Patagonian toothfish (Dissostichus eleginoides) in the Macquarie Island region has heightened the need for a better understanding of the life history of this species. Of particular interest are dietary considerations. However, despite the importance of toothfish in the deep-sea food web of this and other Southern Ocean regions, relatively little is known of their diet. Current knowledge is based upon the examination of stomach contents, a technique that fails to identify with any certainty long-term dietary composition. The determination of lipid composition may provide a simple and effective means of determining the diet of toothfish, one that complements traditional techniques. Specifically, many higher order species are thought to deposit fatty acids derived from prey into storage with little or no modification. Constituent fatty acids of a predator may therefore represent a temporal integration of diet. The lipid composition of toothfish was investigated in relation to both physical (sex, age/size) and fishing parameters (region, season, depth, time of day). Each parameter was compared using a variety of statistical methods including multidimensional scaling and linear discriminant analysis. Significant variations in total lipid content, lipid class composition and especially fatty acid composition between fish of different sizes (ages) points to a potential shift in diet as fish mature. Eicosapentaenoic and docosahexaenoic acids were those fatty acids most responsible for the observed variation with size. The influence of the remaining parameters on lipid composition is less pronounced. To determine whether these variations in lipid composition were due to possible variations in diet, a comparison with potential prey species was undertaken. This involved the examination of 21 species of midwater fish, including 8 species of Myctophidae, and 6 species of squid. Many of these species are know to contribute to the diet of toothfish in the Macquarie Island region. The results of statistical analysis suggest that the diet of Patagonian toothfish at Macquarie Island may vary to a far greater extent, relative to size increases, than is suggested by stomach content analysis. For instance smaller sized toothfish are more closely linked to squid, based on fatty acid composition, than are larger toothfish. Most notable with regards to potential fish prey is that the fatty acid compositions of Myctophidae are more closely related to larger rather than smaller toothfish, possibly indicating a shift in dietary preference as toothfish mature.