Effects of the 2014-2016 marine heatwave on Steller sea lions in the Gulf of Alaska and implications for top-down forcing

Abstract

Marine heatwaves can have rapid and wide-reaching biological and economic impacts. We explored the effects of a recent severe marine heatwave on Steller sea lions in the Gulf of Alaska and Southeast Alaska using biennial counts of pups and non-pups from aerial surveys of terrestrial sites between 2013 and 2019. We combined count data with a bioenergetic model to explore the potential effects of Steller sea lion responses to the heatwave on prey populations. Pup production was anomalously low in the eastern and central Gulf in 2017 following the end of the heatwave in 2016, with a 33.5 and 17.4% reduction from 2015 values respectively. Based on the energetic costs of pregnancy and lactation, these reductions in pup production represent an average difference of 6121 metric tons of prey between 2015 and 2017. Non-pup counts were anomalously low in Southeast Alaska in 2017 and in the central and eastern Gulf in 2019. No effects were detected in the western Gulf, although they could have been masked by the variability in count estimates. Such changes in non-pup abundance had the potential to induce changes in predation pressure on the order of thousands to tens of thousands of metric tons of prey, even when responses were assumed to persist for a relatively short time (1 yr or less). In addition to these short-term effects, declining counts of non-pups from the eastern Gulf and Southeast Alaska occurred following the 2014-2016 heatwave, suggesting more lasting effects on Steller sea lion populations and top-down forcing from these regions.