Fish contain heart-boosting omega-3 fatty acids and many other nutrients that make them a vital part of a person's diet. All fish, however, contain some level of the element mercury, which is poisonous to the human body when ingested in large amounts. Because some fish contain more mercury than others, it is important to be aware of mercury levels and how much of a fish you can eat.
Significance
Mercury is a naturally occurring element in nature. It does not naturally occur in lakes and streams, however. Rather, it is generated by industrial pollution and emitted into the air. The mercury then falls and begins to build up in streams and oceans. When this occurs, the mercury turns into methylmercury, which is harmful to people--especially pregnant mothers and their unborn children. Because fish and shellfish live in these waters, they naturally absorb the methylmercury. Not all fish absorb methylmercury in the same manner, which is why some fish are more dangerous than others to eat.
Warning
Fish that live the longest and are the largest tend to have the highest amounts of mercury content. Because these fish have had a longer time to accumulate mercury, they contain highly concentrated amounts of mercury expressed in parts per million or PPM. These fish include mackerel king (0.730 PPM); shark (0.988 PPM); swordfish (0.976 PPM); or tilefish (1.450 PPM), according to the U. S. Food and Drug Administration. For this reason, these fish should not be consumed by children or women who are pregnant or may be pregnant in order to avoid harmful effects from mercury, according to the Environmental Protection Agency.
Time Frame
Other forms of fish contain lower amounts of mercury and are considered safe for consumption. However, those in a higher-risk category--such as pregnant women or children--should only eat amounts up to 12 ounces, which is typically the equivalent of two meals per week, according to the American Heart Association. These fish include shrimp (below detection levels), canned light tuna (0.118 PPM), salmon (0.014 PPM), pollock (0.041) and catfish (0.049 PPM), according to the U. S. Food and Drug Administration. Each fish contains low levels of mercury, but still should not be eaten more than twice per week by those in the higher-risk category.
Considerations
Other forms of fish may contain only moderate amounts of mercury and should be consumed at six servings or less per month, according to the National Resources Defense Council. These include Alaskan cod, halibut (Pacific), lobster, mahi mahi, freshwater perch, skate, snapper and tuna. If women are planning on becoming pregnant or are nursing, they should not consume these fish.
Effects
Excessive consumption of mercury has been associated with conditions such as brain damage and seizures, according to the Public Broadcasting System. Mercury also has been shown to cause birth defects, learning problems and lowered intelligence in children and unborn fetuses. A child may experience symptoms such as red cheeks, nose and lips; hair loss; kidney failure; or insomnia. Adults may notice symptoms such as swelling, peeling skin and persistent burning or itching of the skin.
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