Temporal Increases in Mercury Concentrations are Associated with Increased Risk of Death by Infectious Disease in Harbour Porpoises ( <i>Phocoena Phocoena)</i>

Trace elements, particularly heavy metals like mercury, pose significant risks to marine ecosystems due to their toxic and bioaccumulative properties. Concentrations in some marine species show concerning temporal increases, yet spatiotemporal trends in UK marine mammals have not been comprehensively assessed for nearly two decades. Marine mammals serve as sentinel species for ocean health due to their long lifespan and high trophic level, making them vulnerable to bioaccumulative pollutants. Monitoring trace elements, in highly exposed populations, is critical for assessing trends and indices of health, like infectious disease. Using liver tissue samples of 738 UK-stranded harbor porpoises (<i>Phocoena phocoena</i>) collected between 1990 and 2021, we found mercury and selenium concentrations have increased, in contrast to declines in cadmium and lead. Spatial analyses revealed that mercury concentrations are highest at lower latitudes, while cadmium increases northward. Levels of zinc, mercury, selenium, and cadmium are significantly associated with infectious disease mortality risk. Our findings highlight the importance of monitoring trace elements in sentinel species to inform conservation efforts and evaluate the effectiveness of pollution mitigation policies such as the Minamata Convention. Increasing mercury levels, despite emission reductions, underscores the urgent need for improved measures to protect marine biodiversity and ecosystem health.