Why are fish high in mercury?

Fish can have high levels of mercury in their tissues owing to bioaccumulation, in which mercury that is released into water is absorbed by smaller organisms that ultimately are consumed by fish. Mercury makes its way into the environment both naturally—through events such as volcanic eruptions or rock breakdown—and through human actions, including mining and some kinds of manufacturing.

Mercury that enters the water is converted by bacteria into methylmercury, which is more easily absorbed by aquatic organisms but is also highly toxic. Methylmercury is consumed by phytoplankton and zooplankton, which are at the bottom of the ocean food chain. Predators that eat these organisms then take up the methylmercury, which are then eaten by larger fish. This process, known as bioaccumulation, results in higher mercury levels in larger, predatory fish compared to smaller fish.

Industrial pollution has been a significant contributor to mercury levels in aquatic environments. A notable example is the Minamata disease outbreak in the 1950s in Japan, where methylmercury discharged from a factory contaminated local fish and shellfish. The incident led to severe health issues for the local population who consumed contaminated seafood.