Holocene changes in the trophic ecology of an apex marine predator in the South Atlantic Ocean

Predators may modify their diets as a result of both anthropogenic and natural environmental changes. Stable isotope ratios of nitrogen and carbon in bone collagen have been used to reconstruct the foraging ecology of South American fur seals (Arctocephalus australis) in the southwestern South Atlantic Ocean since the Middle Holocene, a region inhabited by hunter-gatherers by millennia and modified by two centuries of whaling, sealing and fishing. Results suggest that the isotopic niche of fur seals from Patagonia has not changed over the last two millennia (average for the period: δ¹³C2200-0BP = –13.4 ± 0.5‰, δ¹⁵N2200-0BP = 20.6 ± 1.1‰). Conversely, Middle Holocene fur seals fed more pelagically than their modern conspecifics in the Río de la Plata region (δ¹³C7000BP = –15.9 ± 0.6‰ vs. δ¹³CPRESENT = –13.5 ± 0.8‰) and Tierra del Fuego (δ¹³C6400-4300BP = –15.4 ± 0.5‰ vs. δ¹³CPRESENT = –13.2 ± 0.7‰). In the latter region, Middle Holocene fur seals also fed at a higher trophic level than their modern counterparts (δ¹⁵N6400-4300BP = 20.5 ± 0.5‰ vs. δ¹⁵NPRESENT = 19.0 ± 1.6‰). Nevertheless, a major dietary shift was observed in fur seals from Tierra del Fuego during the nineteenth century (δ¹³C100BP = –17.2 ± 0.3‰, δ¹⁵N100BP = 18.6 ± 0.7‰), when marine primary productivity plummeted and the fur seal population was decimated by sealing. Disentangling the relative roles of natural and anthropogenic factors in explaining this dietary shift is difficult, but certainly the trophic position of fur seals has changed through the Holocene in some South Atlantic regions.