Why do large predatory fish tend to contain more mercury than smaller fish?

Large predatory fish contain higher levels of mercury due to the processes of bioaccumulation and biomagnification.12

Mercury enters aquatic environments and is converted by bacteria into methylmercury, a toxic form that is easily absorbed by organisms at the bottom of the food chain, such as phytoplankton and zooplankton.1

As smaller fish consume these organisms, they accumulate the mercury in their tissues. Because fish typically accumulate mercury more rapidly than they can excrete it, the concentration of the toxin increases as it moves up the food web. When larger predatory fish eat many smaller fish, they ingest the accumulated mercury from all of their prey, leading to progressively higher concentrations at each successive trophic level. This ecosystem-level process, known as biomagnification, explains why top predators often carry the highest mercury loads.312