Vaquita's habitat suitability in the Upper Gulf of California between two contrasting environmental years: 1997 and 2008

The vaquita (Phocoena sinus) is the most endangered marine mammal in the world, with an estimated population of around ten individuals in 2018 and with the smallest geographic range. We aim to describe the differences in the preferred vaquita habitat in the Upper Gulf of California between 1997, which was classified as a warm phase of the El Niño-Southern Oscillation (ENSO), and the year 2008 classified as a neutral phase of ENSO. We used vaquita's acoustic and visual records in 1997 and 2008; environmental factors were derived from satellite imagery (sea surface temperature and depth) and measured during oceanographic cruises (salinity, sediment) through the Maximum Entropy algorithm (MaxEnt). The MaxEnt model estimated an area of 649 km² in 1997 and 546 km² in 2008, where the probability of vaquita's occurrence was equal to or higher than 50%. Thus, suggesting an essential reduction in the preferred habitat (∼16% approx.) between both periods. In both years, the area with the highest probability of occurrence was defined by silty sediment. The habitat characteristics were coastal and shallow water. The temperature and salinity exhibit significant differences (p < 0.05) by year, with higher temperatures in 1997 (∼24–25 °C) and lower salinities in 1999 (35.5–36.4 psu) compared with 2008 (23–24 °C; 35.6–36.2 psu). We consider that the reduction of the vaquita's habitat demonstrates the urgent interannual monitoring to assess the environmental changes that occur in the region since any change would affect the Vaquita habitat and its distribution.