The mercury (Hg) form of most concern is monomethylmercury (MMHg⁺) due to its neurotoxicity and ability to bioaccumulate in seafood. Bioaccumulation in seafood occurs via bioconcentration (direct uptake) and biomagnification (trophic transfer). Our study separates these processes, showing that bioconcentration increases MMHg⁺ in high trophic level fish by 15 % per level, contributing 28–49 % of MMHg⁺ in Atlantic cod. These findings can be used to inform efficient Hg modeling strategies.