Trophic Variation Between the Two Existing Guadalupe Fur Seal Colonies on Guadalupe Island and the San Benito Archipelago, Mexico.

Scat analysis and stable isotope analysis (δ13C Archipelago, trophic variation, stable isotopes, and δ15N) were used to perform a trophic com- scat analysis parison between two existing Guadalupe fur seal (Arctocephalus philippii townsendi) colonies Introduction on Guadalupe Island (GI) and the San Benito Archipelago (SBA) during the 2013 breeding The Guadalupe fur seal (Arctocephalus philipseason. The stable isotope analysis included 67 pii townsendi) (GFS) is recognized as an endansamples from GI and 36 from the SBA, indicat- gered species under Mexican law (NOM-059- ing that this otariid is a teuthophagous specialist. SEMARNAT-2010). The only well-established The two colonies showed different trophic levels: breeding colony of the species is located on (1) 4.5 for GI, for which the most dominant prey Guadalupe Island (GI), where an estimate of was the jumbo squid, and (2) 4.0 for the SBA, for ~17,000 GFSs are found (International Union for which the main item was the opalescent squid. The Conservation of Nature [IUCN], 2015); meanstable isotope analysis included 15 fur samples while, the San Benito Archipelago (SBA), 268 km from each site. Both analyses were complemen- southeast, has a colony of up to 3,700 to 4,500 tary, identifying prey species from an oceanic individuals that is undergoing a process of recolohabitat, with fur seal δ13C values that are typically nization since the late 1990s (Maravilla-Chávez & associated to that environment. Although assigned Lowry, 1999; Sierra, 2015) and is mainly composed a higher trophic level (scat analysis), GI had lower of juveniles and subadults. Additionally, vagrant δ15N and δ13C values, suggesting latitudinal segre- GFSs on the islands off California have become gation. These differences between colonies may be more frequent since the 1980s (IUCN, 2015). related to their distinct age proportions or a strategy Female GFSs make foraging trips that vary to avoid intrapopulation competition. Our contribu- from 222 to 589 km and around 14 d in duration tion regarding trophic variation is based on a snap- (Gallo-Reynoso et al., 2008). Foraging variation shot regarding scat analysis and a time window may exist between individuals within a given pinof ~16 wks prior to collection reflected by stable niped population because of physiological limitaisotope analysis in fur. Additionally, this species is tions associated with age (Kuhn et al., 2006) or sex currently experiencing population growth, which (Weise & Costa, 2007), and may even occur when increases the possibility of overlap for resources phenotypes and dispersion capabilities are similar between both colonies. Therefore, it is important to (Van Valen, 1965; Bolnick et al., 2003; Estes et al., establish baselines and continue these comparative 2003) as evidenced via telemetry in Antarctic fur analyses over time to determine their significance seals (Arctocephalus gazella; Staniland & Boyd, within its recovery. [ABSTRACT FROM AUTHOR]