Since the industrial revolution, the burning of fossil fuels has tripled the concentration of mercury in the ocean. All marine life absorbs some mercury directly and indirectly, and once it enters the body, it cannot be discharged.
Mercury content varies from species to species. Predators such as tuna and swordfish, in addition to their longevity, also absorb mercury from the hunting objects, so the body has the highest mercury content.
The concentration of mercury in the bottom of the food chain, such as oysters and shrimps, is relatively low.