Habitat, trophic level, and residence of marine mammals in the Gulf of California assessed by stable isotope analysis

Approximately 32 species of marine mammals of the Gulf of California (GC) share habitat and resources in this ecosystem. Unusually high delta N-15 values at the base of the food web in the GC permeate up all trophic levels, distinguishing the isotopic signature of the GC from other ecosystems in the Eastern North Pacific. Values of delta C-13 and delta N-15 were acquired from particulate organic matter (POM) of surface sediment, zooplankton, mollusks, crustaceans, fish, and marine mammals in the GC to construct a general trophic structure and were complemented with additional isotope data from the literature. Aims were to: estimate marine mammal trophic levels and habitat preferences; distinguish between GC residents and visitors; and assess potential trophic overlap among the most common and abundant cetacean species. Trophic level 1 (TL1), represented by POM, showed average (+/- SD) delta C-13 and delta N-15 values of -21.4 +/- 0.5 parts per thousand and 9.6 +/- 0.7 parts per thousand, bulk zooplankton (TL2) showed -18.8 +/- 0.7 parts per thousand and 11.8 +/- 1.4 parts per thousand, while TL3, represented by baleen whales, some fish, squid, and seabirds, showed delta C-13 values between -13 and -16 parts per thousand and delta N-15 values between 16.5 and 20 parts per thousand. Marine mammals occupied the entire coastal and pelagic isotope gradient (delta C-13 values from -12 to -18.1 parts per thousand), with most of the species at TL4 (delta N-15 values from 17 to 23 parts per thousand), whereas Orcinus orca occupied TL5 (25.8 parts per thousand maximum value). The odontocetes Mesoplodon peruvianus, Globicephala macrorhynchus, Grampus griseus, Kogia sima, Delphinus delphis, D. capensis, and Tursiops truncatus had both GC resident and visitor representatives, and the latter 3 species showed a significant degree of trophic and habitat overlap.