Why Mercury Builds Up in Fish

Mercury contamination increases dramatically as it moves up the food chain.

Small aquatic organisms absorb methylmercury directly from water and sediments. Small fish then eat these organisms. Larger fish eat the smaller fish, and the mercury concentration increases at each step.

This process is known as biomagnification.

By the time methylmercury reaches large predatory fish such as tuna, swordfish, king mackerel, and shark, concentrations can be thousands to millions of times higher than in surrounding water.

Because methylmercury binds tightly to muscle tissue, cooking or cleaning fish does not remove the contamination.

For most people, seafood consumption is the primary source of mercury exposure. The U.S. FDA and EPA both issue fish consumption advisories to help consumers limit intake of species known to contain higher mercury levels.