Increased methylmercury concentrations in water and fish have been detected after flooding of soils associated with reservoir creation (e.g. for hydroelectric power generation) and in thermokarst wetlands that form after permafrost thaw.There are various sources of inorganic mercury that may indirectly contribute to the production of methylmercury from microbes in the environment. Natural sources of mercury released to the atmosphere include volcanoes, forest fires, volatilization from the ocean and weathering of mercury-bearing rocks. Anthropogenic sources of mercury include the burning of wastes containing inorganic mercury and from the burning of fossil fuels, particularly coal. Although inorganic mercury is only a trace constituent of such fuels, their large scale combustion in utility and commercial/industrial boilers in the United States alone results in release of some 80.2 tons (73 metric tons) of elemental mercury to the atmosphere each year, out of total anthropogenic mercury emissions in the United States of 158 tons (144 metric tons)/year.In the past, methylmercury was produced directly and indirectly as part of several industrial processes such as the manufacture of acetaldehyde. However, currently there are few direct anthropogenic sources of methylmercury pollution in the United States.Whole-lake ecosystem experiments at IISD-ELA in Ontario, Canada showed that mercury falling directly on a lake had the fastest impacts on aquatic ecosystems as opposed to mercury falling on the surrounding land. This inorganic mercury is converted to methylmercury by bacteria. Different stable isotopes of mercury were added to lakes, wetlands, and uplands, simulating rain, and then mercury concentrations in fish were analyzed to find their source.
According to one study, based on modeled concentrations, pre-Anthropocene tissue-bound levels in fish may not have differed markedly from current levels. However, based on a comprehensive set of global measurements, the ocean contains about 60,000 to 80,000 tons of mercury from pollution, and mercury levels in the upper ocean have tripled since the beginning of the industrial revolution. Higher mercury levels in shallower ocean waters could increase the amount of the toxicant accumulating in food fish, exposing people to a greater risk of mercury poisoning. See also
Canadian Reference Materials include some with methylmercury, e.g. DORM
Dimethylmercury, mercury with a second methyl group
Ethylmercury, a related cation
Mercury poisoning
Mercury regulation in the United States

References
External links
ATSDR - ToxFAQs: Mercury
ATSDR - Public Health Statement: Mercury
ATSDR - ALERT! Patterns of Metallic Mercury Exposure, 6/26/97
ATSDR - MMG: Mercury
ATSDR - Toxicological Profile: Mercury
National Pollutant Inventory - Mercury and compounds Fact Sheet
Methylmercury-in-fish exposure calculator provided by GotMercury.Org, which uses FDA mercury data with the EPAs calculated safe exposure levels. Methylmercury Contamination in Fish and Shellfish Archived 2013-11-02 at the Wayback Machine
nytimes.com, Tuna Fish Stories: The Candidates Spin the Sushi
U.S. Environmental Protection Agencys mercury site
U.S. Geological Surveys mercury site Archived 2013-11-02 at the Wayback Machine
Environment Canadas mercury site
Health Canadas mercury site
International Conference on Mercury as a Global Pollutant 2006-Madison, WI USA  2009-Guizhou, China  2011-Halifax, NS Canada