What is the most common contaminant found in fish?

The most common contaminant found in fish is mercury, especially the organic form methylmercury.

Mercury occurs naturally in rocks, soil and water, but industrial activity can also release it into the environment. Microorganisms convert some mercury into methylmercury, which enters aquatic food webs. Small organisms absorb it, fish eat those organisms, and larger predatory fish can accumulate higher concentrations over time through bioaccumulation and biomagnification.

Nearly all fish contain trace amounts, but levels vary greatly by species and size. Shark, swordfish, king mackerel and some tuna generally contain more methylmercury than smaller fish such as sardines or salmon. Mercury is a chemical contaminant, not an infection or parasite; cooking does not remove it. Fish remains nutritious, so health authorities generally recommend choosing lower-mercury species rather than avoiding seafood altogether.