The form of mercury that contaminates fish is methylmercury. It is a pervasive neurotoxin formed by aquatic microorganisms acting on inorganic mercury.
Among the most common sources of inorganic mercury: the burning of coal and other fossil fuels, municipal and medical waste incineration, industrial boilers, certain mining practices, volcanic activity, the erosion of mercury-bearing rocks and the evaporation and global cycling of surface mercury vapors.
Inorganic mercury finds its way into lakes, rivers, wetlands and oceans through airborne depositions, in rainwater and as runoff of surface mercury deposition in soils. Once methylated, it works its way up the food chain ' through aquatic flora to herbivorous fish to predatory fish to fish-eating birds and mammals. As it ascends each rung in this ladder, methylmercury accumulates in greater and greater concentrations in muscle tissues, becoming more toxic at each level.
There can be significant variance in methylmercury concentrations from one species of fish to another ' and even within species. In general, the older and larger the fish, the greater the concentration of methylmercury in its tissues. Species such as swordfish, tuna, pike, walleye and bass tend to have higher methylmercury levels than smaller panfish and herbivorous species. But the type, quality, chemistry and mechanics of a habitat can also influence methylmercury levels in individual fish.
Slicing off the skin and fat won't reduce your risk of exposure, because methylmercury concentrates in muscle tissues ' the very part of the fish we eat.
Once ingested as part of a fish meal, methylmercury broadcasts throughout the human body, traveling to the brain and central nervous system, through the bloodstream, residing for extended periods in the hair on one's head, appearing in bile and the liver. Because methylmercury binds to proteins, it is slow to leave the body.
Over a period of months, it is demethylated, excreted, secreted and eliminated. But chronic consumption of the neurotoxin can lead to symptoms that range from alarming to fatal.
The early symptoms are often too subtle to attribute to mercury contamination without specific blood, urine or hair tests. In mature adults, these symptoms may include a tingling sensation or numbness, problems with motor skills, vision and hearing changes, confusion, irritability, balance problems and muscle tremors.
Childhood exposure can result in developmental delays. Prenatal exposure can result in irreversible neurological damage.
Because so many of these symptoms can be caused by other ailments, diagnoses of chronic methylmercury exposure can be difficult to arrive at. And because there are other possible sources of mercury exposure ' such as store-bought fish ' assigning the cause to subsistence fishing requires a thorough inventory of a patient's diet and fishing habits.