Effect of climate variability on weaning mass in a declining population of southern elephant seals Mirounga leonina

With the global climate predicted to continue to change over the coming century, quantifying how animal populations respond to environmental variation is central for the prediction of future responses and consequences. To quantify how environmental change affects the foraging success of Macquarie Island female elephant seals Mirounga leonina, we related weaning mass—a function of maternal foraging success—to a series of intrinsic and extrinsic (environmental) covariates. We found that the weaning mass of elephant seals was positively related to the number of females ashore during the breeding season and negatively related to maximum sea ice extent in the Ross Sea region. Weaners weighed on average 3 kg more in years with more females ashore and 17 kg less in years of high ice extent. These relationships suggest that in years of poor female foraging conditions (i.e. high ice extent), Macquarie Island population growth is affected not only by a reduction in the number of females ashore for reproduction but also, given that weaning mass dictates first-year survival, by reduced pup survival rates. The decline in the Macquarie Island population over the past 60 yr has occurred concurrently with a positive trend in sea ice extent in the Ross Sea region. The negative relationship between weaning mass and sea ice extent suggests that the decline in the Macquarie Island population trajectory may, in part, be caused by increasing sea ice in the Ross Sea and mediated by a reduction in weaning mass, first-year survival and recruitment rates.