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.
Highlights
Two otariid species inhabit Mexico, the Guadalupe fur seal (Arctocephalus philippii townsendi) (GFS) and the California sea lion (Zalophus californianus) (CSL). The former was hunted to near extinction in the 19th century; once the species received protected status in the 1950s, its population began to recover on Guadalupe Island (GI) [1] to a current abundance of 17,000–20,000 individuals [2]
Based on the fact that 2015 was declared as an El Niño-Southern Oscillation (ENSO) year [24], we examined two of the main variables responsible for shaping the marine mammals distribution patterns [42]; sea surface temperature (SST) and chlorophyll-a (Chl-a) concentration, which is a proxy of primary productivity [43,44]
Differences in the interannual abundance of Guadalupe fur seals (GFS) were observed, during May, July, and September, when the 2015 values were nearly half of those recorded during the same months in 2014
Summary
Two otariid species inhabit Mexico, the Guadalupe fur seal (Arctocephalus philippii townsendi) (GFS) and the California sea lion (Zalophus californianus) (CSL). Based on scat and stable isotope analyses of pup fur during 2013, GFSs from GI and those from SBA had different diets. Scat analysis has revealed that CSL from the SBA consume prey like the Pacific silverweed (Argentina sialis), Panama hake (Merluccius angustimanus), and rockfishes (Sebastes spp.) [17] For both otariids, foraging strategies vary by site and sampling period [15,18]. In order to assess pinniped foraging habits, neonate stable isotopes are commonly used, because at this age, pups are completely milk-dependent This makes them effective indicators for their mothers, who catabolize their tissues to produce milk, emulating a predator-prey relationship [16,37]. Our contribution constitutes evidence that supports the role of these marine mammals as indicators of change in the ecosystem
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