This study assessed ontogenetic dietary changes in male South American fur seals Arctocephalus australis in northern and central Patagonia (Argentina) using stable isotope ratios (δ 15 N and δ 13 C) in vibrissae and bones. Sucking pups were characterised by higher δ 15 N values and lower δ 13 C values than older specimens. Weaning was associated with a marked drop of δ 15 N values, both in bone and vibrissae. Such a drop was inconsistent with the consumption of local prey and may reveal movement to distant foraging grounds or physiological changes associated with either fasting or rapid growth. Stable isotope ratios indicated that juveniles fed more pelagically than subadults and adults, but that there were no major differences between the 2 latter age categories. As subadults and adults are rather similar in body mass and are much larger than juveniles, body mass may play a role in the ontogenetic dietary changes reported. Nevertheless, demersal benthic prey were always scarce in the diet of male fur seals, which relied primarily on Argentine shortfin squid and small pelagic fish throughout life, though adults also consumed large amounts of decapod crustaceans available at shallow depths. Vibrissae did not reveal regular oscillations of δ 15 N or δ 13 C, except in 1 individual. Thus, male fur seals from northern and central Patagonia do not appear to migrate regularly between isotopically distinct areas, although nomadic displacements cannot be ruled out. KEY WORDS: Foraging ecology • Pinniped • Arctocephalus australis • Ontogenetic dietary change • Stable isotope analysis • δ 15 N • δ 13 C • Patagonia Resale or republication not permitted without written consent of the publisher This authors' personal copy may not be publicly or systematically copied or distributed, or posted on the Open Web, except with written permission of the copyright holder(s). It may be distributed to interested individuals on request.

The human exploitation of marine resources is characterised by the preferential removal of the largest species. Although this is expected to modify the structure of food webs, we have a relatively poor understanding of the potential consequences of such alteration. Here, we take advantage of a collection of ancient consumer tissues, using stable isotope analysis and SIBER to assess changes in the structure of coastal marine food webs in the South-western Atlantic through the second half of the Holocene as a result of the sequential exploitation of marine resources by hunter-gatherers, western sealers and modern fishermen. Samples were collected from shell middens and museums. Shells of both modern and archaeological intertidal herbivorous molluscs were used to reconstruct changes in the stable isotopic baseline, while modern and archaeological bones of the South American sea lion Otaria flavescens, South American fur seal Arctocephalus australis and Magellanic penguin Spheniscus magellanicus were used to analyse changes in the structure of the community of top predators. We found that ancient food webs were shorter, more redundant and more overlapping than current ones, both in northern-central Patagonia and southern Patagonia. These surprising results may be best explained by the huge impact of western sealing on pinnipeds during the fur trade period, rather than the impact of fishing on fish populations. As a consequence, the populations of pinnipeds at the end of the sealing period were likely well below the ecosystem's carrying capacity, which resulted in a release of intraspecific competition and a shift towards larger and higher trophic level prey. This in turn led to longer and less overlapping food webs.

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.

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Stable carbon and nitrogen isotopes in skin and bone of South American sea lions from Brazil and Uruguay were analysed to test the hypothesis that trophic overlap between the sexes is lower during the pre-breeding season than throughout the rest of the year. The isotopic values of skin and bone were used to infer the trophic relationships between the sexes during the pre-breeding period and year round, respectively. Prey species were also analysed to establish a baseline necessary for interpreting the stable isotope ratios of skin and bone. Standard ellipse areas, estimated using Bayesian inference in the SIBER routine of the SIAR package in R, suggested that males and females used a wide diversity of foraging strategies throughout the year and that no differences existed between the sexes. However, the diversity of foraging strategies was largely reduced during the pre-breeding period, with all the individuals of each sex adopting similar strategies, but with the two sexes differing considerably in stable isotope values and the ellipse areas of males and females not overlapping at all. Nevertheless, the results revealed a general increase in the consumption of pelagic prey by both sexes during the pre-breeding period. The progressive crowding of individuals in the areas surrounding the breeding rookeries during the pre-breeding period could lead to an increase in the local population density, which could explain the above reported changes.

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.

We analyzed the δ13C and δ15N values in the vibrissae of captive adult breeding South American sea lions (Otaria byronia) fed at a constant diet and then used this information to analyze the change in stable isotope values along the vibrissae from wild individuals. The overall diet‐to‐vibrissa discrimination factor of the captive animals was 3.0‰ ± 0.1‰ for δ13C and 3.6‰ ± 0.1‰ for δ15N, but the stable isotope ratios fluctuated periodically despite constant diet. The δ13C and δ15N values of the captive male declined at the end of the breeding season, whereas the δ13C values of the female increased during the central part of pregnancy and the δ15N values peaked during lactation. The δ13C and δ15N values of adult wild specimens also fluctuated periodically and vibrissae growth rate (0.15 mm/d in both sexes) was slightly lower than in captivity (0.17 mm/d), assuming an annual periodicity for oscillations. Similarities in the amplitude of the cycles of captive and wild males suggested that fasting was probably the main source of periodic variability in the δ15N of wild males, whereas pregnancy and lactation were probably the main source of periodic variability for the δ13C of wild females.

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