Distribution and Trophic Transfer of Mercury and Selenium in European Hake (Merluccius merluccius) from the Northeast Atlantic Ocean: A Stable Isotope Approach

European hake (EH - Merluccius merluccius) are a commercially valuable, high trophic position marine fish species inhabiting the Northeast Atlantic Ocean (NEAO). Here, we investigated total mercury (Hg) bioaccumulation and trophic transfer dynamics, and selenium (Se) Hg interactions in EH fillets at a historic waste disposal site with a known Hg point source (n = 25) and at reference sites (n = 763) throughout Norway. Fillet concentrations of Hg (> 91% methylmercury - MeHg) and Se, were elevated in samples from the polluted site (Hg_polluted = 0.271 ± 0.153, Hg_reference = 0.114 ± 0.096, Se_polluted = 0.425 ± 0.033, and Se_reference = 0.321 ± 0.044 mg/kg wet weight). The opposite was found for Se:Hg molar ratios (Se:Hg_reference = 12.1 ± 7.91, Se:Hg_polluted = 6.39 ± 4.96), however when controlling for Hg concentrations, the EH Se:Hg ratio from the polluted site was higher compared to reference conditions. We further investigated trophic niches using stable carbon, nitrogen, and Hg isotopes and found different isotopic niches between reference and polluted sites. EH from both site types had different Hg isotopic signatures and were enriched in heavier isotopes at reference sites (e.g., δ²⁰²Hg_reference = 0.964 ± 0.177, δ²⁰²Hg_polluted = 0.714 ± 0.115‰). Hg enriched in fillet tissue primarily underwent MeHg photodegradation at both sites. However, due to the local Hg point source, lower MDF isotope values and odd MIF isotope values (e.g., ∆¹⁹⁹Hg_reference = 0.981 ± 0.105, and ∆¹⁹⁹Hg_polluted = 0.829 ± 0.110‰) were observed at the polluted site. Overall, our findings indicate good discriminability regarding Hg source apportionment and highlight the role of point source pollution as an important modulator of the EH Hg bioaccumulation regime in the NEAO study system.