Historical age-class diet changes in South American fur seals and sea lions in Uruguay

Pinnipeds are effective bioindicators of the abundance of their prey and changes in marine productivity due to natural oceanographic phenomena or fishery exploitation. In Uruguay, two pinniped species breed sympatrically: Arctocephalus australis, with a growing population, and Otaria flavescens, which has a declining population. Given their contrasting population growth trends, we explored how their trophic patterns varied across age-classes over seven decades using dentin collagen delta C-13 and delta N-15 values in teeth sampled from stranded organisms. Dentin collagen accumulates isotopic information in annual growth layers, providing sequential information on an animal's diet throughout its life. Dentin collagen annual growth layers in 50 A. australis teeth and 37 O. flavescens teeth from individuals stranded over similar to 70 years were processed for isotopic analysis. Importantly, delta C-13 decreased over time in both species (0.024-0.027% year(-1) in A. australis, and 0.028-0.035% in O. flavescens); this pattern reflects the influence of the Suess Effect, a global phenomenon of decreasing delta C-13 values in marine ecosystems. Isotopic values were relatively constant over time, suggesting that these species maintained a stable trophic niche during the seven decades examined, with O. flavescens exhibiting higher delta C-13 and delta N-15 values. Within each species, there was considerable isotopic niche overlap between different age-classes; however, both species occupied different isotopic niches regardless of age-class during all decades. The isotopic niches of the two species converged during the 2000s. Primarily responsible for this convergence were adult A. australis, which overlapped with juvenile and young adult O. flavescens. Moreover, we propose that before the 2000s A. australis fed on prey from different trophic levels while O. flavescens varied its feeding habitat. Our results suggest that these top predators reflect anthropogenic and environmental changes that have occurred over the last seven decades in Uruguayan waters.