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Methylmercury Contamination in Fish and Shellfish
(Released February 2007)

 
  by Laura Griesbauer  

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Fish Consumption Advisories and Mercury Levels

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While the danger of mercury poisoning may seem like a good reason to refrain from consuming fish, the benefits of eating fish may outweigh many of the risks. Fish is high in protein, low in saturated fats, and contains important nutrients such as heart healthy omega-3 fatty acids. One fatty acid found in fish oils, Docosahexaenoic acid (DHA), is one of the most important fatty acids for normal brain development and function (Sakamoto et al., 2004). It is possible that DHA may even counteract the negative effects of mercury though this relationship has not yet been proven scientifically. Eating fish has also been found to reduce the risk of heart attacks, lower blood pressure, and improve arterial health (Senkowsky, 2004).

M
Mercury Levels in Commonly Consumed Fish and Shellfish
Species - Fish Mercury Concentration (ppm)
Shark 0.988
Swordfish 0.976
Tuna (Fresh/Frozen, Bigeye) 0.639
Chilean Bass 0.386
Tuna (Fresh/Frozen, Albacore) 0.357
Tuna (Canned, Albacore) 0.353
Halibut 0.252
Tuna (Canned, Light) 0.118
Cod 0.095
Trout (Freshwater) 0.072
Anchovies 0.043
Sardine 0.016
Salmon (Fresh/Frozen) 0.014
Tilapia 0.01
Salmon (Canned) ND
   
Species - Shellfish Mercury Concentration (ppm)
Lobster 0.31
Crab 0.06
Squid 0.05
Crawfish 0.033
Oyster 0.013
Clam ND
Shrimp ND

ND=Mercury concentration below detection level

Source: CFSAN, US FDA, Feb. 2006

The greater concern for mercury exposure is not to an adult human, but to a developing fetus. As seen in the extreme Minamata case, some mothers showed no outward signs of mercury poisoning, but gave birth to children with severe brain damage (Clarkson & Magos, 2006). Studies have shown a correlation between prenatal exposure to mercury and decreased ability of infants and children on neurobehavioral tests including tests of attention, fine motor function, language skills, visual-spatial abilities and memory (Shea et al., 2004). This is because methylmercury readily crosses the placenta through blood circulation, and fetal blood concentration of mercury is slightly higher than maternal levels. Methylmercury can also be passed through breast milk to infants and consumed by young children in their diets. This is of concern since young children are potentially more susceptible to toxicity from mercury and the brain may be more affected as it continues to develop during the first years of life (Shea et al., 2004).

Based on the growing body of evidence concerning the health issues of methylmercury accumulation in the body, the U.S. Environmental Protection Agency (EPA) and the U.S. Food and Drug Administration (FDA) have issued advisories targeting consumption of fish for specific groups. Their advice to women who may become pregnant, pregnant women, nursing mothers, and young children up to age six is to avoid certain types of fish high in methylmercury and limit the amount of fish consumed each week. Specifically, the EPA and FDA advise these groups not to eat shark, swordfish, king mackerel, or tilefish at all because they contain very high levels of mercury (>1 ppm). They also advise these groups to eat up to 12 ounces (or two average meals) a week of fish and shellfish that are low in mercury. Children should only eat six ounces of fish. Low mercury fish and shellfish include shrimp, canned light tuna, pollock, salmon and tilapia. Albacore tuna is a commonly eaten fish but contains moderate amount of mercury. The EPA and FDA advise eating only 6 ounces of albacore tuna a week. Also, if you exceed the suggested amount of fish or shellfish in a week, simply cut back the amount consumed the next week or two. Lastly, the EPA and FDA advise the public to check for local advisories on fish caught from local lakes, rivers, and streams. These fish may be more greatly affected by anthropogenic pollution sources (CFSAN, 2006). The above guidelines are not aimed at adult men, or woman past child bearing age, but individuals concerned with possible exposure to mercury should follow them as well.

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