Declines in marine predator populations have been attributed to anthropogenic activity and environmental change. Southern elephant seals Mirounga leonina are major consumers of biomass in the eastern region of the Southern Ocean and have been declining in numbers since the 1960s. Previous studies have identified evidence for habitat and diet partitioning over a range of spatial and temporal scales between juveniles and adults in the Macquarie Island population. We first analysed the stable isotopes (SI) of 6 entire vibrissae from a dead adult female southern elephant seal from Kerguelen Islands to determine moult and growth patterns. Secondly we analysed the SI from the vibrissae of 102 juvenile southern elephant seals to investigate diet. The results from the growth pattern analysis indicated that vibrissae do not grow or moult simultaneously. However, it is likely that at least part of the vibrissae will have been produced sometime during the most recent trip to sea and will give a broad indication of diet. The subsequent SI analysis confirmed that juveniles are consuming greater proportions of fish species, and identified myctophids as the primary component of juvenile diet. Myctophids are also consumed by king penguins Aptenodytes patagonicus which have greatly increased in numbers recently in the Macquarie Island area. This may have presented the juvenile southern elephant seals with increased competition and may influence survival.

When resources are limited or patchy, a species may develop some degree of resource partitioning to reduce intra-specific competition. Development of intra-specific resource partitioning is more pronounced in species with clear phenotypic variation among individuals (e.g. age or sex). Southern elephant seals Mirounga leonina have pronounced sexual dimorphism and range widely in size and foraging range between juvenile and adult stages. However, hypothesized diet-based resource partitioning has been less clear due to difficulties in sampling diet while seals are away from breeding islands. We analysed fatty acids (FAs) from blubber of 122 juvenile seals and compared them to FA profiles from blubber of 52 adult females, and to FA profiles from 51 prey species (grouped as fish and squid) to examine evidence for diet-based resource partitioning in the seals. FA signature analysis revealed physiological and dietary differences between ages. Principle components of the 21 FAs from seal blubber and prey parts distinguished prey from seals, and clearly separated prey species into fish and squid classes. FA profiles from adult females differed to those from juveniles, with the former more 'squid-like' and the latter more 'fish-like'. Variation in FA profiles of seals was also apparent between sexes and during different seasons. Differences in diet between juveniles and adult females suggest resource partitioning occurs in response to large metabolic and physiological differences with age that limit juvenile dispersal and diving abilities. By consuming a different suite of prey species relative to adult females, juvenile southern elephant seals may reduce intra-specific competition.KEY WORDS: Fatty acid signature analysis · Southern elephant seal · Diet · Resource partitioning Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 384: [303][304][305][306][307][308][309][310][311][312] 2009 distinctive in individuals of different age and sex or life-history stage with obvious morphological differences (Polis 1984, Bolnick et al. 2003. Thus, intraspecific resource partitioning should be more pronounced in species with large phenotypic variation between age and sex classes.The sexually size dimorphic southern elephant seal Mirounga leonina is the largest of all the pinnipeds and exhibits pronounced morphological and physiological differences with age (Field et al. 2005b). Adult body size of males (up to 4 t) is 5 to 10 times that of females (0.5 t). This large body mass and an estimated population size of 757 000 ) make this species a major consumer of marine resources in the Southern Ocean, of which squid, and perhaps to a lesser extent fish, make up the bulk of the diet (Green & Burton 1993, Slip 1995. These factors, combined with the unpredictable and patchy environment of the Southern Ocean in which they spend most of their lives, make southern elephant seals prime candidates for the evolution of mechanisms that reduce competition through intra-specific resource part...