Stable isotope analyses have become an important tool in reconstructing diets, analysing resource use patterns, elucidating trophic relations among predators and understanding the structure of food webs. Here, we use stable carbon and nitrogen isotope ratios in bone collagen to reconstruct and compare the isotopic niches of adult South American fur seals (Arctocephalus australis; n = 86) and sea lions (Otaria flavescens; n = 49) - two otariid species with marked morphological differences - in the Río de la Plata estuary (Argentina - Uruguay) and the adjacent Atlantic Ocean during the second half of the 20th century and the beginning of the 21st century. Samples from the middle Holocene (n = 7 fur seals and n = 5 sea lions) are also included in order to provide a reference point for characterizing resource partitioning before major anthropogenic modifications of the environment. We found that the South American fur seals and South American sea lions had distinct isotopic niches during the middle Holocene. Isotopic niche segregation was similar at the beginning of the second half of the 20th century, but has diminished over time. The progressive convergence of the isotopic niches of these two otariids during the second half of the 20th century and the beginning of the 21st century is most likely due to the increased reliance of South American fur seals on demersal prey. This recent dietary change in South American fur seals can be explained by at least two non-mutually exclusive mechanisms: (i) the decrease in the abundance of sympatric South American sea lions as a consequence of small colony size and high pup mortality resulting from commercial sealing; and (ii) the decrease in the average size of demersal fishes due to intense fishing of the larger class sizes, which may have increased their accessibility to those eared seals with a smaller mouth gape, that is, South American fur seals of both sexes and female South American sea lions.

En este trabajo se presentan los primeros resultados de las investigaciones realizadas en el Área Natural Protegida Islote Lobos, sector central de la costa oeste del golfo San Matías (provincia de Río Negro, Argentina). Teniendo en cuenta la cronología obtenida y la información generada, se señalan las primeras tendencias observadas a partir del estudio del registro arqueológico recuperado tanto en superficie como en excavación. El análisis de los artefactos líticos, los restos humanos, las evidencias zooarqueológicas y la organización del espacio en general en esta área, aporta información novedosa que permite comenzar a delinear un panorama acerca de las actividades humanas que se pudieron haber desarrollado en la región durante el Holoceno tardío. Así, se propone que en el Área Complejo Islote Lobos se habrían llevado a cabo principalmente actividades tendientes a la explotación de recursos marinos y de ciertos recursos terrestres disponibles en la zona, con una planificación en el uso del espacio. Esto permite, entonces, comenzar a discutir en forma preliminar el uso de los recursos costeros y la circulación humana en la costa oeste del golfo San Matías durante los últimos 3000 años en relación con las tendencias regionales propuestas en trabajos previos.

Abstract—Stable isotope ratios of carbon and nitrogen in archaeological and modern bone samples have been used to reconstruct the dietary changes of the South American sea lion Otaria flavescens from the late Holocene to the present in the southwestern Atlantic. We sampled bones from archaeological sites in northern-central and southern Patagonia, Argentina, and bones housed in modern scientific collections. Additionally, we analyzed the stable isotope ratios in ancient and modern shells of intertidal molluscs to explore changes in the isotope baseline and allow comparison between bone samples from different periods after correction for baseline shifts. Results confirmed the trophic plasticity of the South American sea lion, demonstrated the much larger impact of modern exploitation of marine resources as compared with that of hunter-gatherers, and underscored the dissimilarity between the past and modern niches of exploited species. These conclusions are supported by the rather stable diet of South American sea lions during several millennia of aboriginal exploitation, in both northern-central and southern Patagonia, and the dramatic increase in trophic level observed during the twentieth century. The recent increase in trophic level might be related to the smaller population size resulting from modern sealing and the resulting reduced intraspecific competition. These results demonstrate how much can be learned about the ecology of modern species thanks to retrospective studies beyond the current, anthropogenically modified setting where ecosystem structure is totally different from that in the pristine environments where current species evolved.

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: δC = -13.4 ± 0.5‰, δN = 20.6 ± 1.1‰). Conversely, Middle Holocene fur seals fed more pelagically than their modern conspecifics in the Río de la Plata region (δC = -15.9 ± 0.6‰ vs. δC = -13.5 ± 0.8‰) and Tierra del Fuego (δC = -15.4 ± 0.5‰ vs. δC = -13.2 ± 0.7‰). In the latter region, Middle Holocene fur seals also fed at a higher trophic level than their modern counterparts (δN = 20.5 ± 0.5‰ vs. δN = 19.0 ± 1.6‰). Nevertheless, a major dietary shift was observed in fur seals from Tierra del Fuego during the nineteenth century (δC = -17.2 ± 0.3‰, δN = 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.

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