By: Dana Sackett
Mercury contamination in fish and aquatic systems has received a lot of attention because of the potential health concerns for people and wildlife. In fact, my own research has focused on mercury dynamics in aquatic systems and fishes over the last several years. Here I wanted to share some of my expertise and try to clarify the mixed messages often sent by popular news articles on eating fish in relation to mercury, but first let us begin with some background information.
|Sign warning fish consumers about the risks of mercury from California. Source: http://www.electricbluefishing.com/eb_sub_menu/mercury_in_fish__shellfish.htm|
The primary way people are exposed to mercury is through eating fish, and most of the mercury that finds its way into fish comes from coal-fired power plants. Coal contains mercury naturally and when it is burned for electricity, that mercury is released into the atmosphere. Some other sources of mercury which have been a concern in the past include chloralkali plants, which used mercury to extract chlorine from salt and gold mining operations that used mercury to extract gold from sediment. Though these sources still exist, regulations have made them less of a concern in the United States. Natural sources of mercury, such as volcanoes, contribute about one third of the total mercury released into the atmosphere.
|A simplified schematic of the mercury cycle. Source: http://action.sierraclub.org/site/PageServer?pagename=event_FLD_WV_Mercury_reminder|
After being released to the atmosphere, mercury can fall out of the sky immediately or can travel around the globe before depositing on the surface of the earth (this is largely driven by the form of the mercury). Because mercury can travel so far, even sites with no visible sources of mercury can be contaminated. Once in the water, bacteria can transform mercury into a highly neuro-toxic form of mercury known as methylmercury. Unfortunately, methylmercury makes-up most of the mercury in fish tissue (95-99%) and its characteristics allow it to enter the base of the food web very easily and increase in concentration as it moves from prey to predator up the food web (through bioaccumulation). This means that mercury will be highest in the oldest and largest top predators in a system, such as sharks or swordfish.
|A schematic of mercury bioaccumulation in an aquatic system. The size of the red dot signifies the increase in mercury up the food web. Source: http://www.mercury.utah.gov/health_effects.htm|
Fish advisories for mercury focus on women of child-bearing age and children because developing children, inside and outside the womb, are more vulnerable to the toxic effects of mercury than adults. Even a small amount of mercury can cause neurological damage to a developing brain. More recently, mercury from fish consumption has also been linked to heart disease in adults. These negative consequences are often chronic, meaning it takes time for the symptoms to manifest and are often not seen until later in life.
On the other hand, fish are stock full of important vitamins and minerals and are a large source of omega-3 fatty acids, which are known to improve heart and brain health, the opposite of mercury. These contradictory effects have lead to mixed messages when it comes to fish consumption. For example, one recent popular news article released in February of this year was titled, “Women who eat fish during pregnancy ‘more likely to have brainy and sociable children” while another released at the same time was titled, “Report: Some Minn. babies born with high mercury.” One of the articles encouraged pregnant women to eat fish, while the other discouraged fish consumption.
|Recommendations from Washington Department of Health. Source: http://www.doh.wa.gov/CommunityandEnvironment/Food/Fish/HealthyFishGuide.aspx|
Providing more confusion are advocate groups that either try to protect fishery demand by suggesting mercury is not that bad and you should eat whatever you like as often as you like, or groups that suggest eating any fish will cause horrible things to happen to you. Neither of these messages are true. The goal is to balance the benefits with the risks, not to stop eating fish entirely.
|Recommendations from the Natural Resources Defense Council. Get the complete card from the link below. Source: http://www.nrdc.org/health/effects/mercury/walletcard.PDF|
The US Environmental Protection Agency (EPA) and Federal Drug Administration (FDA) have suggested that it is safe for anyone (even children, pregnant women or women who could become pregnant) to eat two six ounce meals a week of low mercury fish over a lifetime with no adverse affects. These recommendations balance the benefits of eating fish while avoiding exposure to concentrations of mercury high enough to cause damage. But what is a low mercury fish? The EPA states that fish that have concentrations less than 0.3 parts per million (ppm) are considered low for these recommendations.
|Recommendations from Vermont Department of Environmental Conservation, Department of Fish and Wildlife, and Department of Health. Sources: http://www.mercvt.org/fish/index.htm|
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3 Comments Add yours
How much correlation is there between fish diet and mercury contamination? Because it seems that larger, piscivorous fish tend to come with more warnings then smaller fish that rely more on plankton and insects.
Angler you're right. The correlation is high. Because fish mainly accumulate mercury from their diet and because mercury is bioaccumulative (meaning it increases in concentration up the food web) fish that are at the top of the food web in a system will generally have higher mercury than those fish at the bottom of the food web.
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