Identifying critical habitat of the endangered vaquita (Phocoena sinus) with regional δ13C and δ15N isoscapes of the Upper Gulf of California, Mexico

The vaquita (Phocoena sinus) is the world's most endangered cetacean and has experienced a 60% reduction in the size of its population in the past decade. Knowledge of its basic ecology is essential for developing successful management plans to protect and conserve this species. In this study, we identified vaquita foraging areas by creating an isoscape of the Upper Gulf of California (UGC) based on sediment and zooplankton carbon (delta C-13) and nitrogen (delta N-15) isotope values. Our results confirm that this species is confined to the western region of the UGC, which is characterized by relatively high delta N-15 values (sediments: 10.2 parts per thousand +/- 2.0 parts per thousand, zooplankton: 15.8 parts per thousand +/- 1.3 parts per thousand), higher sea surface temperatures (similar to 16 degrees C-25 degrees C), higher concentrations of silt in sediments, and the highest turbidity. In contrast, the eastern region of the UGC had relatively low sediment (7.7 parts per thousand +/- 2.4 parts per thousand) and zooplankton (14.6 parts per thousand +/- 1.0 parts per thousand) delta N-15 values, and the highest concentrations of sand in sediments. Our approach is an effective use of marine isoscapes over a small spatial scale (<200 km) to identify the environmental characteristics that define the critical habitat for an extremely endangered marine mammal.