Determining the unknown in Southern Ocean squid : distribution and diet of Histeoteuthis eltaninae and Martialia hyadesi

In the Southern Ocean, cephalopods are infrequently captured by conventional sampling methods, but this is not a reflection of their abundance. This inadequacy has resulted in their being limited knowledge into cephalopods despite their importance as voracious predators and a key prey resource to apex predators. These predators have allowed for a new technique to be developed through using their stomach contents to obtain cephalopods resistant mouth parts (known as beaks). In the Southern Ocean, juvenile elephant seals have the greatest diving and foraging capacity, resulting in the greatest representation of the cephalopod community in their stomach contents. These beaks are morphologically uniqueto a species, with the dimensions related to the size the whole animal, allowing for species specific knowledge that can be related to size. Diet, potential prey and distribution can be determined through stable isotope analysis. Stable isotope analysis was conducted on two predominate species found in the stomach contents of juvenile elephant seals at Macquarie Island. Histioteuthis eltaninae was the most predominate cephalopod species and is believed to have a life span of approximately one year. Over this year period, ˜í¬•\\(^{13}\\)C signatures indicated that there was no active migration occurring, resulting in all life stages inhabiting the same broader region. H. eltaninae distribution is relatively confined to waters around Macquarie Island and appears to be restricted by temperature. The ˜í¬•\\(^{15}\\)N value indicated a positively linear relationship between trophic level and size of the individual. However, as expected in an annual species, this relationship was effected by season. Despite these relationships, the most predominant prey item appears to correspond with myctophid fish. Martialia hyadesi was also investigated and appears to have a minimum life span of 16 months indicated by the two concurrent size classes occurring between September and December. The stable isotope analysis demonstrated a broad distribution that involved extensive migration from southern, almost Antarctic waters to waters north of Macquarie Island. Therefore this species must withstand a great diversity of temperatures, particularly when compared to H. eltaninae. When considering trophic level and potential prey resources, there appears to be a shift from one tropic level, such as crustaceans and myctophid fish when individuals were smaller, to larger fish and other cephalopods, approximately the next trophic level up in larger individuals. M hyadesi appear to utilize lower trophic levels to a greater extent thanH. eltaninae, potentially the result of M. hyadesi opportunistic nature.