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 δ¹⁵N 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 δ¹³C and δ¹⁵N 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) δ¹³C and δ¹⁵N values of -21.4 ± 0.5% and 9.6 ± 0.7%, bulk zooplankton (TL2) showed -18.8 ± 0.7% and 11.8 ± 1.4%, while TL3, represented by baleen whales, some fish, squid, and seabirds, showed δ¹³C values between -13 and -16% and δ¹⁵N values between 16.5 and 20%. Marine mammals occupied the entire coastal and pelagic isotope gradient (δ¹³C values from -12 to -18.1%), with most of the species at TL4 (δ ¹⁵N values from 17 to 23%), whereas Orcinus orca occupied TL5 (25.8% 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.