δ13C and δ15N values in pup whiskers as a proxy for the trophic behavior of Galapagos sea lion females

The Galapagos sea lion (Zalophus wollebaeki) is one of the few pinnipeds adapted to tropical ecosystems. This adaptation exposes this species to productivity fluctuations associated with oceanographic events, such as ENSO, which has generated a significant population decline during recent decades. This decline represents a considerable challenge for the species, especially for females who invest a large part of their energy in the maintenance of their pups. A comparative analysis of the isotopic signals of whisker samples taken from Z. wollebaeki pups was performed to understand the foraging ecology of their mothers during the pre- and post-partum stages. A significant relationship was observed between the whisker size and the total length of the pups (rs = 0.96, P < 0.05), which enabled a date to be provided for each whisker fragment and for it to be assigned to a specific month. Model-based clustering of isotope ratios identified two distinct clusters in both stages, where the level of uncertainty of the grouping was lower than 40%. The δ13C and δ15N values among clusters within each stage defined differences in the feeding zones and in the trophic levels in both groups (P< 0.01 ). No significant difference was found in the δ13C values between the pre- and post-partum stages, but a significant decrease was observed in the δ15N values in the postpartum period (P= 0.01). There was no correlation of the monthly isotopic values with the superficial sea temperature (SST) and chlorophyll-a (Chl-a); however, the low δ13C values observed during October 2015 matched with an increase in SST and a decrease of Chl-a in the region. Thus, through the analysis of pup samples, we successfully obtained results that give us an insight into the trophic ecology and metabolic processes behind the isotopic signatures of Z. wollebaeki females.