Calonectris shearwaters reveal a gradient of mercury contamination along the Atlantic and Mediterranean waters of the Iberian Peninsula

This study examines blood mercury (Hg) concentrations in Calonectris spp. shearwaters from three colonies along the Atlantic and Mediterranean waters of the Iberian Peninsula (southwestern Europe), investigating their relationship with foraging ecology through GPS tracking and stable isotopes ( δ 15 N and δ 13 C) data during the breeding season. Hg levels exhibited a spatial gradient, increasing from the Atlantic Ocean (1.8 ± 0.4 μg g −1 dw) towards the Mediterranean Sea, with shearwaters from the Columbretes Islands (NW Mediterranean) showing the highest Hg levels (6.5 ± 2.1 μg g −1 dw). Individuals breeding in the Alboran Sea, a transition area between both basins, had intermediate Hg concentrations (3.1 ± 1.5 μg g −1 dw). All individuals were above the Hg toxicity threshold associated with negative reproductive, body condition, and immune system outcomes. However, all shearwaters had a Se:Hg molar ratio above 4, indicating effective protection of Se against Hg toxicity. Positive significant relationships between Hg concentrations, δ 15 N values, and time spent foraging in deep sea waters were observed in Mediterranean colonies, highlighting the ecological context's role in Hg accumulation. Results suggest that feeding on higher trophic level prey, in deep-sea areas, and geographic location contribute to Hg accumulation in these populations. Given the potential health risks associated with elevated Hg levels, further research is warranted to explore the ecological factors driving Hg accumulation and the implications for the health status of these populations. • Hg levels in shearwaters increased from the Atlantic to the Mediterranean colonies. • Columbretes shearwaters showed the highest Hg levels and moderate toxicity risk. • Hg linked to δ 15 N and deep-sea foraging in Mediterranean shearwater colonies. • Se:Hg molar ratio showed an effective protection against Hg toxicity.