Lifetime coastal and oceanic foraging patterns of male Guadalupe fur seals and California sea lions

Abstract

Carcasses of adult and subadult male Guadalupe fur seals (GFSs) are beached on Isla Magdalena, representing about 73% of all GFSs beachings since 2003. Analysis of carbon and nitrogen stable isotopes in dentin collagen from male GFSs was used in an attempt to distinguish animals from two possible locations of origin: the oceanic Guadalupe Island (GI) and the coastal San Benito Islands (SBI). Samples of conterminous and contemporary California sea lions (CSLs) were included as representative of coastal predators. Two distinct groups of GFSs were evident based on δ13C values: coastal GFSs with higher δ13C values than CSLs, and oceanic GFSs with lower δ13C values than CSLs. Significant temporal trends were observed in the δ15N values; values for oceanic GFSs and CSLs increased annually (0.082‰ and 0.089‰, respectively), while those for coastal GFS decreased annually by 0.183‰. The isotopic segregation of oceanic and coastal GFS males, presumably from GI and SBI respectively, has been present at least since the mid‐1990s. This segregation may be due to dissimilar ingestion of prey from oceanic vs. coastal origin prior to their seasonal co‐occurrence in the Gulf of Ulloa.

Foraging segregation by sex and age class in the Guadalupe fur seal from Guadalupe Island, Mexico

“…This δ 13 C difference could be related to more coastal foraging habits by males relative to females. This is supported by the established foraging area in the west coast of the Baja California Peninsula for subadult and adult male GFSs in the Gulf of Ulloa (Aurioles‐Gamboa & Szteren, 2020). Additionally, adult and subadult males had the highest δ 13 C standard deviations compared to all other age classes.…”

Population assessment and feeding ecology on a new colony of the Guadalupe fur seal (Arctocephalus philippii townsendi) on Farallón de San Ignacio Island, Gulf of California, Mexico

“…Squid such as Onychoteuthis banksii and D. gigas were the most important prey items at the oceanic Guadalupe Island (Amador‐Capitanachi, 2015; Gallo‐Reynoso, 1994; Gallo‐Reynoso & Esperón‐Rodríguez, 2013; Hernández‐Montoya, 2009; Juárez‐Ruiz et al, 2018), while the market squid, Loligo opalescens , was the predominant prey species in samples from the inshore San Benito Archipelago (Amador‐Capitanachi, 2015; Aurioles‐Gamboa & Camacho‐Ríos, 2007; Esperón‐Rodríguez & Gallo‐Reynoso, 2012; Pablo‐Rodríguez et al, 2016). Variation in the main prey of Guadalupe fur seals in Mexico reflect the ability of this species to adapt to the environmental conditions in sites where they have established foraging colonies (Aurioles‐Gamboa & Szteren, 2020; Gallo‐Reynoso & Esperón‐Rodríguez, 2013).…”