Murcia coastline sustains high marine biodiversity but has suffered severe impacts from toxic heavy-metal discharges. We report mercury (Hg) and selenium (Se) levels, and the Se:Hg molar ratio, in liver, kidney, and muscle samples from three dolphin species stranded along the Murcia Region coastline between 2009 and 2023. Species studied include 148 striped dolphins (Stenella coeruleoalba), 23 bottlenose dolphins (Tursiops truncatus), and 5 common dolphins (Delphinus delphis). Besides, we analyzed oxidative stress and biochemical biomarkers. Glutathione-S-transferase (GST), catalase (CAT), lipid peroxidation (LPO), lactate dehydrogenase (LDH), alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), acetylcholinesterase (AChE), and paraoxonase 1 (PON1) were assessed. The highest mean Hg concentrations were observed in the liver, with individual values reaching up to 498 μg g⁻¹ ww. Although Se: Hg molar ratio exceeded 1 in most individuals, we found significant associations between trace elements and Se:Hg ratios and multiple biomarkers, indicating enzyme alterations compatible with hepatic, renal and muscular responses already evident even below 100 μg Hg g⁻¹ ww (defined as indicative of liver damage in marine mammals) and highlighting that the Hg-Se balance rather than Hg concentrations alone is critical for early biochemical disruption. Age showed a clear influence on Hg and Se accumulation and on AST, ALT, and PON1 activities, whereas ALP and AChE were higher in younger individuals. Sex effects were more limited, with differences in CAT, LPO, AST, and ALT, but not in metal accumulation. Despite a complex and age‑structured temporal pattern, Hg contamination in western Mediterranean striped dolphins remains high, confirming the region as a mercury hotspot and underscoring the need for long‑term biomonitoring and management.
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