A dietary study of Moroteuthis ingens and other Southern Ocean squid species : combined stomach contents and fatty acid analyses

The squid fauna are a key component of the Southern Ocean ecosystem, although unfortunately little is known of their distribution, biology and ecology. While the biomass of squid in the Southern Ocean must impose a large amount of predatory pressure on lower trophic levels, few dietary data exist for any of the Southern Ocean species. Conventional dietary analyses of squid are fraught with many sources of bias, and therefore this study has employed complementary stomach contents and fatty acid analyses to investigate aspects of the diet of Southern Ocean squid. The diet of the onychoteuthid Moroteuthis ingens was largely comprised of myctophid fish, and the digestive gland of M. ingens was rich in lipid and found to be an ideal source of fatty acid dietary tracers that are unmodified from the diet. Fatty acid dietary tracers were then applied in combination with stomach contents analyses to investigate the temporal, spatial and sizerelated dietary patterns of M. ingens. Using these complementary techniques, the diet of M.ingens was found to vary significantly on an interannual and seasonal basis within the vicinity of Macquarie Island, largely due to fluctuations in the proportions of myctophid fish species in the diet. Moroteuthis ingens also exhibited spatial variations in diet among Macquarie Island, the Falkland Islands, the Chatham Rise and Campbell Plateau (the latter two sites located within New Zealand waters). The diet at the New Zealand sites was characterised by a proportion of temperate myctophid species not identified at other sites, whereas squid collected from the Falkland Islands consumed a much greater proportion of cephalopod prey. Lipid class and fatty acid analyses also indicated that the diet varied considerably between the Chatham Rise and Campbell Plateau, most likely due to the influences of different oceanographic regimes. Stomach contents and fatty acid analyses of squid collected from the Falkland Islands indicated that M. ingens switches from a crustacean- and cephalopod-based diet to a fish- and cephalopod-diet at around 200 mm mantle length. Lipid class and fatty acid analyses of the other Southern Ocean squid indicated that oegopsid species accumulate a large amount of lipid in the digestive gland. This lipid has been determined to be of dietary origin, and will thus provide a rich source of fatty acid dietary tracers applicable to future food-web studies of squid. Further consideration of the mode of lipid and fatty acid storage in the digestive gland will be required before squid fatty acid profiles can be reliably applied to dietary studies of higher trophic predators.