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Methylmercury Contamination in Fish and Shellfish
(Released February 2007)

 
  by Laura Griesbauer  

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Mercury in the Environment

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Once in the atmosphere, mercury vapor (Hgo) is slowly converted by oxidative processes to divalent mercury (Hg2+), which is then returned to the earth's surface by rainfall, where it accumulates in soils and in surface waters. Some of the mercury load is then converted back into mercury vapor (Hgo) and returned to the atmosphere. However, another fraction of the mercury load (Hg2+) is washed into rivers, streams, and eventually the ocean where it accumulates in aquatic sediments. It is here that inorganic mercury is converted to methylmercury (MeHg) by microorganisms living in the sediments by a process called methylation (Clarkson & Magos, 2006).

mercury enters and exits the environment
Mercury in the Environment

bioaccumulation of methylmercury
The Bioaccumulation of Methylmercury
This methylmercury then enters the food chain when it is absorbed by phytoplankton species. Phytoplankton are eaten by plankton consumers, which then are eaten by larger and larger fish. Methylmercury accumulates in the tissues of fish and shellfish via a process called biomagnification, through which methylmercury concentration increases as it moves up from one trophic level to the next. Within each organism, methylmercury bioaccumulates as the organism consumes more and more organisms containing methylmercury. Thus, smaller fish that are lower down in the food chain have lower concentrations of mercury in their tissues while larger fish that are higher up in the food chain have higher concentrations. For example, sardines contain about .01 ppm of mercury while sharks contain from 1 ppm to as much as 4 ppm (EPA, 2006). Fish with the highest levels of mercury include sharks, swordfish, and king mackerels. Large marine mammals such as whales have levels similar to these fish.

charts of mercury concentration
The regression between the mercury concentration in pike (Esox lucius) and body weight (left) or the age of the fish (right).

Go To Mercury in the Body and Health Effects

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