Do all fish and shellfish contain mercury?

Mercury levels in the oceans have tripled since the Industrial Revolution, thanks to mining and the burning of fossil fuels. All sea creatures absorb some of this heavy metal directly, and once it’s in the body there’s no way of getting rid of it. The amount of mercury in fish varies between species. Longlived predators like tuna and swordfish tend to contain the most, because they also absorb mercury from their prey and they’ve had a long time to accumulate it. The lowest levels are found in shortlived species lower down the food chain, such as oysters and shrimp.