Investigating the use of blubber fatty acids to detect Steller sea lion (Eumetopias jubatus) foraging on ephemeral high-quality prey

One hypothesis for the decline of the western stock of Steller sea lions (Eumetopias jubatus) is dietary related nutritional stress, which may have adversely affected juvenile survival and female fecundity. Fatty acids in the blubber of sea lions are an index of diet. Comparisons of blubber fatty acids between different groups (age, sex, location) of Steller sea lions may indicate if dietary differences exist. Diet estimates can be produced by matching blubber fatty acids to those of potential prey. Two reference databases of Steller sea lion prey fatty acids were compiled for use with captive and free-ranging sea lion samples. The captive and free-ranging databases consisted of 141 and 650 prey items representing 7 and 25 species respectively. Analyses of database fatty acids indicated differences in prey by species and family. Captive Steller sea lions were used to simulate ephemeral feeding events and investigate the use of the Quantitative Fatty Acid Signature Analysis (QFASA) to model diet. The captive study showed that ephemeral events were inconsistently detected unless small schooling fish were grouped together. The captive study also indicated that blubber fatty acids most likely reflect diet from the previous 3-4 months. Blubber samples were collected from adult male (n = 118), adult female (n = 110), and juvenile (n = 35) Steller sea lions in three regions of varying population growth (Western Gulf of Alaska, Central Gulf of Alaska, and Southeast Alaska) in 2002--2004 using a novel remote biopsy system. Analyses of fatty acid data indicated dietary differences among the three regions, and between adult males and females, particularly in areas of stable or increasing population growth. Differences before and after access to ephemeral feeding events were not clearly detected. Differences before and after breeding were evident, with early samples being more similar. Using QFASA, important prey types were determined for free-ranging animals. Pollock, salmon, small schooling fish, and flatfish appear important in all 3 regions. Ephemeral feeding event utilization was evident for some groups, but not significant due to variability between individual animals. In areas of population decline, sea lions fed on a larger range of prey.