Stable carbon isotope ratios for the gray whale (Eschrichtius robustus) in the breeding grounds of Baja California Sur, Mexico

This work investigated the feeding ecology and behaviour of gray whales in Bahia Magdalena. Underwater observations of bottom feeding were made (n = 4). Skin biopsies of the gray whale had a carbon isotope value of -16.5 +/- 0.1 parts per thousand (range from -16.4 to -16.7, n = 7). Prey in Bahia Magdalena had a carbon isotope value of -18.4 parts per thousand. Dietary enrichment from prey in Bahia Magdalena would correspond to 2 +/- 0.1 parts per thousand, whereas previously published results for prey in Alaska would result in an enrichment of 3 parts per thousand, which suggests that whales were more likely feeding on prey from Bahia Magdalena. Carbon isotopic oscillation along the baleen plate of a stranded 1-year-old whale showed a variation in diet during the year, which suggests continual feeding during this time and corresponding to dietary sample measurements from Bahia Magdalena in winter and Alaska in summer.