Maternal–Pup Mercury Relationship in South American Sea Lions and Fur Seals

Mercury (Hg) is a non-essential, globally distributed toxic metal that bioaccumulates in aquatic organisms and biomagnifies across trophic levels, posing a threat to top predators such as marine mammals. Our aim was to evaluate total mercury (THg) relationships in mother–pup pairs of two pinniped species, South American sea lions (Otaria flavescens) and South American fur seals (Arctocephalus australis), from Isla de Lobos, Uruguay. We quantified THg in maternal hair and pup lanugo from 11 O. flavescens and 46 A. australis mother–pup pairs. THg concentrations were significantly higher in O. flavescens than in A. australis for both adult females (30.5 ± 9.3 μg/g vs. 5.7 ± 3.4 μg/g, dry weight) and pups (25.3 ± 13.2 μg/g vs. 4.4 ± 2.5 μg/g, dry weight). Hg levels in O. flavescens pups are among the highest recorded for pinnipeds. Pup THg concentrations were positively associated with maternal THg concentrations in both species, although the explanatory power of the final models differed markedly between species (R² = 0.75 in O. flavescens and R² = 0.20 in A. australis). In O. flavescens, male pups had lower THg concentrations than females for a given maternal THg concentration, whereas no sex effect was detected in A. australis. The presence of THg in lanugo formed in utero provides evidence of prenatal Hg exposure, while the variability in mother–pup relationships limits the use of pup lanugo as a quantitative proxy for THg concentrations in individual mothers. Instead, pup lanugo may provide a minimally invasive matrix for characterizing prenatal Hg exposure and broader species- or population-level patterns.