The use of fatty acid profiles in determining the diet of steller sea lions (Eumetopias jubatus)

Food preferences in carnivores can be examined using the fatty acid (FA) profiles of their blubber or milk. This technique is particularly useful for examining food preferences of animals for which stomach contents or observations of feeding are difficult to obtain. In monogastric animals, dietary FA remain intact during digestion and are deposited into blubber stores or mobilized for milk production in a relatively unchanged form. Milk samples were collected from 35 adult female Steller sea lions in the late spring of 1993-1996 from three locations in the Gulf of Alaska. All females sampled were believed to be within the first 40 days postpartum. Milk lipids were extracted in chloroform using a modified Folch procedure. Fatty acid methyl esters (FAME) were prepared from the purified lipid extract and analyzed using temperature programmed capillary gas-liquid chromatography. The fatty acid data was analyzed using classification and regression trees in S-plus (Stat Sci, Seattle), a non-parametric multivariate technique for classifying data. The best fitted tree had 5 terminal nodes in which only 2 of the 35 observations were incorrectly classified by location of the animal. In addition, the fatty acid profiles of these animals appeared to resemble the fatty acid profiles of prey from their respective regions. Thus milk fatty acids indicate that Steiler sea. lions are feeding differently in different regions of their home range. The next step in these investigations is to further classify prey items by region, species and size class.