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 Bahía 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‰ (range from −16.4 to −16.7, n=7). Prey in Bahía Magdalena had a carbon isotope value of −18.4‰. Dietary enrichment from prey in Bahía Magdalena would correspond to 2 ± 0.1‰, whereas previously published results for prey in Alaska would result in an enrichment of 3‰, which suggests that whales were more likely feeding on prey from Bahía 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 Bahía Magdalena in winter and Alaska in summer.