Impact of the 2015 El Niño-Southern Oscillation on the Abundance and Foraging Habits of Guadalupe Fur Seals and California Sea Lions from the San Benito Archipelago, Mexico

Abstract

The abundance of California sea lions (Zalophus californianus) (CSLs) and Guadalupe fur seals (Arctocephalus philippii townsendi) (GFSs) from the San Benito Archipelago (SBA) was determined through nine monthly surveys in 2014–2015. Assessment of their foraging habits was examined based on the isotopic analysis of pups (maternal indicators) (SIAR/SIBER-R). Environmental variability between 2014 and 2015 was also analyzed, in terms of sea surface temperature (SST) and chlorophyll (Chl-a) concentration. Both otariids reached their highest abundance in July of both years; however, relative to 2014, the 2015 survey showed a 59.7% decline in the total GFS abundance and a 42.9% decrease of GFS pups, while total CSL abundance decreased 52.0% and CSL pup presence decreased in 61.7%. All monthly surveys for both otariids showed a similar trend (>50% decrease in 2015). Compared to 2014, the 2015 GFSs isotopic niche was three times larger (2.0 in 2015, 0.6 in 2014) and the δ13C was significantly lower. CSLs also showed significantly lower δ13C and higher δ15N in 2015. Interannual segregation was greater for CSLs, and their pup body mass was also significantly lower during the 2015 breeding season (mean = 8.7 kg) than in the same season of 2014 (mean = 9.9 kg). The decrease in δ13C for both otariids reflected a more oceanic foraging; most likely associated with the decline in primary productivity in surrounding areas to the SBA, related to a higher SST caused by the 2015 ENSO, with a subsequent increase in foraging effort. These would explain the fewer observed individuals on land, especially pups, which showed diminished body condition (CSLs). This study highlights the importance of marine mammals as sentinel species that respond dynamically to changes in environment, providing valuable information on the effect of ENSO on pinnipeds in Mexican waters.

Specialized foraging habits of adult female California sea lions Zalophus californianus

“…This change in foraging behavior may be a step to counter the negative effects of environment variability on prey availability and abundance (Kuhn and Costa , Elorriaga‐Verplancken et al . ).…”

Response to capture stress involves multiple corticosteroids and is associated with serum thyroid hormone concentrations in Guadalupe fur seals (Arctocephalus philippii townsendi)

“…Recent studies of GFS at San Benito Archipelago in Mexico during the 2015–2016 ENSO event showed declines in abundance and an increase of isotopic niche area, as a probable result of increase in foraging effort (Elorriaga‐Verplancken et al . ). Additionally, during this period, unprecedented records of emaciated GFS from the southern Gulf of California were recorded (Elorriaga‐Verplancken et al .…”

Detectability and categorization of California sea lions using an unmanned aerial vehicle

“…, Elorriaga‐Verplancken et al . ). Traditionally, in the Gulf of California, researchers have counted and categorized California sea lions ( Zalophus californianus ) in age and sex classes from boats, while circumnavigating rookeries and haul‐out sites (Le Boeuf et al .…”