Why did mercury contamination become a concern in tuna, swordfish, and salmon?

Mercury contamination in fish like tuna and swordfish is a significant concern due to the processes of bioaccumulation and biomagnification.

Mercury enters aquatic environments through natural events, such as volcanic eruptions, and human activities, including mining and manufacturing. Once in the water, bacteria convert this mercury into methylmercury, a highly toxic organic compound.12

This methylmercury is absorbed by small organisms like phytoplankton and zooplankton at the base of the food chain. As smaller fish consume these organisms and are subsequently eaten by larger predatory fish, the concentration of methylmercury increases at each successive trophic level—a process known as biomagnification. Consequently, large, long-lived predatory fish such as tuna and swordfish accumulate significantly higher levels of mercury in their tissues than smaller species. Consuming these contaminated fish poses serious health risks, including mercury poisoning, which can cause severe neurological damage.132