Rapid temporal decline of mercury in Greenland halibut (Reinhardtius hippoglossoides)

Highlights: • Temporal trend of total mercury (Hg) in Greenland halibut (GH) (Reinhardtius hippoglossoides) fillets was assessed from 2006 to 2015. • Hg in GH fillets was primarily present as methylmercury (>77 %) and declined by 35–50 % during 2006–2015. • GH trophic shifts accounted for approximately one third of the observed temporal reduction in Hg. • GH Hg was associated with declines in Hg air emissions, decreasing trophic position, and lower demersal prey use. • Our investigation has important implications for evaluating the impacts of declining Hg emissions on seafood Hg concentrations. Mercury (Hg) pollution in the ocean is an issue of global concern, however bioaccumulation regimes of this ubiquitous pollutant in marine apex predators have important knowledge gaps. Our fish length and stable isotope (δ¹⁵N and δ¹³C) normalized data of Greenland halibut (GH) (Reinhardtius hippoglossoides) showed that Hg bioaccumulation in fillet tissue decreased by ~35–50 %, over a ten-year period from 2006 to 2015 (n = 7 individual sampling years). Hg was predominantly in the methylmercury form (>77 %). Results from a Bayesian information theoretic model showed that GH Hg concentrations decreased with time and its associated declines in Hg air emissions, estimated trophic position, and a potentially lower degree of demersal prey use as indicated by temporal trend shifts in nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotope values. GH trophic shifts accounted for about one third of the observed temporal reduction in Hg. Our study demonstrates the importance of simultaneously considering Hg emissions, food web dynamics and trophic shifts as important drivers of Hg bioaccumulation in a marine, deep water fish species and highlights the effectiveness of Hg regulations on ocean apex predator Hg concentrations and overall seafood safety.