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.