Mercury is a non-essential element that enters the environment from a variety of sources including coal combustion and artisanal gold mining. In aquatic systems, mercury is transformed by bacteria into toxic methylmercury which biomagnifies in food webs and may reach concentrations that are toxic to both fish and humans. Dusky grouper (Epinephelus marginatus) is a culturally and commercially important fish species in Brazil and elsewhere. Due primarily to overharvest, the species is currently listed as vulnerable by the IUCN. Dusky grouper are large-bodied and long-lived predators – therefore, biomagnification and bioaccumulation may result in high mercury concentrations in dusky grouper, with implications for fish population dynamics and human health through consumption of grouper.