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The population of Southern elephant seals (Mirounga leonina) at Macquarie Island has declined since the 1960s, and is thought to be due to changing oceanic conditions leading to reductions in the foraging success of Macquarie Island breeding females. To test this hypothesis, we used a 55-year-old data set on weaning size of southern elephant seals to quantify a decrease in weaning size from a period of population stability in 1950s to its present state of on-going decline. Being capital breeders, the size of elephant seal pups at weaning is a direct consequence of maternal foraging success in the preceding year. During the 1940-1950s, the mean of female pups at weaning was similar between the Heard and Macquarie Island populations, while the snout-tail-length length of male weaners from Heard Island were longer than their conspecifics at Macquarie Island. Additionally, the snout-tail-length of pups at weaning decreased by 3cm between the 1950s and 1990s in the Macquarie Island population, concurrent with the observed population decline. Given the importance of weaning size in determining first-year survival and recruitment rates, the decline in the size at weaning suggests that the decline in the Macquarie Island population has, to some extent, been driven by reduced maternal foraging success, consequent declines in the size of pups at weaning, leading to reduced first-year survival rates and recruitment of breeding females into the population 3 to 4 years later.

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Details of the environmental variables used to explain the weaning mass of southern elephant seal pups. The variables are divided into global and regional variables Global Climate Modes: The high latitudes of the Southern Hemisphere experience large-scale, sub-decadal climatic variation as a result of a number of global modes of atmospheric variability present in the region, including: Southern Annular Mode: The Southern Annular Mode (SAM, or Antarctic Oscillation) is the dominant mode of extra-tropical atmospheric variability in the Southern Hemisphere, that acts over intra-seasonal to inter-annual timescales (Thompson & Wallace 2000, Lovenduski & Gruber 2005). The SAM is characterized by synchronous atmospheric pressure anomalies of opposite sign between the mid-and high-latitudes that drives the northsouth movement of a circumpolar band of strong sub-polar westerly winds (Stammerjohn et al. 2008b). During positive SAM (+SAM) phases, positive pressure anomalies over the mid-latitudes and negative pressure anomalies over the high-latitudes drive the northward expansion and weakening of the sub-polar westerlies, while the opposite is true for negative phases of the SAM, with the subpolar westerlies contracting poleward and subsequently strengthening (-SAM; Stammerjohn et al. 2008a).