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

Abstract: Scat analysis and stable isotope analysis (δ13C and δ15N) were used to perform a trophic comparison between two existing Guadalupe fur seal (Arctocephalus philippii townsendi) colonies on Guadalupe Island (GI) and the San Benito Archipelago (SBA) during the 2013 breeding season. The stable isotope analysis included 67 samples from GI and 36 from the SBA, indicating that this otariid is a teuthophagous specialist. The two colonies showed different trophic levels: (1) 4.5 for GI, for which the most dominant prey was the jumbo squid, and (2) 4.0 for the SBA, for which the main item was the opalescent squid. The stable isotope analysis included 15 fur samples from each site. Both analyses were complementary, identifying prey species from an oceanic habitat, with fur seal δ13C values that are typically associated to that environment. Although assigned a higher trophic level (scat analysis), GI had lower δ15N and δ13C values, suggesting latitudinal segregation. These differences between colonies may be related to their distinct age proportions or a strategy to avoid intrapopulation competition. Our contribution regarding trophic variation is based on a snapshot regarding scat analysis and a time window of ~16 wks prior to collection reflected by stable isotope analysis in fur. Additionally, this species is currently experiencing population growth, which increases the possibility of overlap for resources between both colonies. Therefore, it is important to establish baselines and continue these comparative analyses over time to determine their significance within its recovery.
Key Words: Guadalupe fur seal, Arctocephalus philippii townsendi, Guadalupe Island, San Benito Archipelago, trophic variation, stable isotopes, scat analysis
Document Type: Research Article
Page Numbers: 14-25

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