LAUSR

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 (dC) and nitrogen (dN) isotope values. Our results confirm that this species is confined to the western region of the UGC, which is characterized by relatively high dN values (sediments: 10.2& 6 2.0&, zooplankton: 15.8& 6 1.3&), higher sea surface temperatures ( 168C–258C), higher concentrations of silt in sediments, and the highest turbidity. In contrast, the eastern region of the UGC had relatively low sediment (7.7& 6 2.4&) and zooplankton (14.6& 6 1.0&) dN 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.