Changes in the overwintering diet of Steller sea lions (Eumetopias jubatus) in relation to the 2014 – 2016 northeast Pacific marine heatwave

Abstract

Understanding variation in the diets of endangered species during critical time periods in their annual cycle is of utmost importance because of the effects that diet can have on vital rates and population trends. This may be especially true for Endangered Steller sea lions during the winter months when prey can be difficult to find, and many adult females are both lactating and pregnant. This study aimed to examine changes in the overwintering diet of Steller sea lions across an extreme climatic event, the northeast Pacific marine heatwave (PMH) that adversely affected this species and broadly impacted many others throughout the trophic spectrum. Diet analysis was conducted from scat samples using hard part remains and qPCR methods. Prior to the peak impact of the PMH, Steller sea lions fed predominantly on epipelagic and mesopelagic fishes such as capelin, walleye pollock and Pacific cod. Following the warm-water peak, capelin was strongly reduced in their diet, and diet diversity increased with greater presence of demersal and benthic prey such as skates, lumpsuckers, snailfish, and polychaetes. These results suggest that sea lions were having a difficult time finding their preferred prey, spending time searching deeper to find alternative prey with poorer energy content following the peak impact of the PMH. These changes in diet corresponded with fewer pups being born and a reduction in overall sea lion numbers in the study area, providing evidence that overwintering prey fields may have a substantial impact on population health.