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¹ Instituto Politécnico Nacional, Centro Interdisciplinario de Ciencias Marinas

Pinnipeds are effective bioindicators of the abundance of their prey and changes in marine productivity due to natural oceanographicphenomena 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 populationgrowth trends, we explored how their trophic patterns varied across age-classes over seven decades using dentin collagen d¹³C and d¹⁵N values in teeth sampled from stranded organisms. Dentin collagen accumulates isotopic information in annualgrowth layers, providing sequential information on an animal’s diet throughout its life. Dentin collagen annual growth layersin 50 A. australis teeth and 37 O. flavescens teeth from individuals stranded over ~ 70 years were processed for isotopicanalysis. Importantly, d¹³C 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 d13C values inmarine ecosystems. Isotopic values were relatively constant over time, suggesting that these species maintained a stabletrophic niche during the seven decades examined, with O. flavescens exhibiting higher d¹³C and d¹⁵N values. Within eachspecies, there was considerable isotopic niche overlap between different age-classes; however, both species occupied differentisotopic niches regardless of age-class during all decades. The isotopic niches of the two species converged during the2000s. Primarily responsible for this convergence were adult A. australis, which overlapped with juvenile and young adultO. 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 environmentalchanges that have occurred over the last seven decades in Uruguayan waters.

Palabras clave: Diet changes; decadal; Arctocephalus australis; Otaria flavescens; Uruguay

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