Although many types of fish provide good nutrition, consumers are frequently warned to limit the amount of fish they eat or to avoid eating fish altogether. Usually, the warning mentions that fish may be contaminated with mercury, a heavy metal that is toxic to humans. Fish can pick up mercury if it is present in the water they live in or if it appears in their food.
Sources
Mercury is found naturally in some environments, but the toxic levels that appear in fish habitats are usually the result of industrial waste or pollution. According to the Illinois Department of Public Health, mercury can be the result of industrial waste or appear in runoff from landfills. Mercury can also rise into the air as part of the process of burning coal for energy, and then fall along with rain into lakes, rivers and streams. Certain bacteria that live in water can change regular inorganic mercury to methylmercury, which is easily absorbed by living things, including fish.
Ingestion
One way fish can become contaminated with mercury is by eating plants and other fish that contain mercury. For example, methylmercury may be absorbed by algae or other small plants in the water. A small plant-eating fish will absorb the mercury in the algae or plants it eats. The small fish may be eaten by a larger fish, which will absorb all the mercury in all the small fish it eats. The process of absorbing all the mercury in every item eaten along the food chain is called biomagnification. According to the U.S. Environmental Protection Agency, biomagnification can result in fish at the top of the food chain containing hundreds of times more mercury than appears in their water or in any single fish they eat. The EPA advises that people avoid eating shark, swordfish, and tuna. These fish contain the most mercury because they eat contaminated fish.
Absorption
In addition to absorbing mercury from the foods they eat, fish can also pick up mercury present in their environment. Mercury in runoff or rain can become suspended in the water. It enters fish through their gills, which extract oxygen from water and enable the fish to breathe. Once inside the fish, the mercury accumulates just as it does when the fish eats. Protecting fish habitats from pollution is one way to prevent fish from absorbing mercury.
Significance
When human beings eat fish containing mercury, their bodies also absorb the mercury in the fish and store it in the body's fat cells. Too much mercury from fish can result in serious bone and nerve damage, including seizures, paralysis and even death. In the 1950s, residents of the fishing village of Minimata in Japan suffered these symptoms on a wide scale when the fish they lived on became heavily contaminated with mercury. Fifty residents died and over a hundred more were permanently injured by the mercury. Because the body has difficulty eliminating mercury, the descendants of the Minimata villagers often have cognitive and developmental problems related to mercury poisoning, even though they never ate the poisoned fish. The EPA recommends that pregnant women avoid eating fish, as mercury can damage the developing fetus's nervous system and brain.
Considerations
Although the U.S. EPA recommends limiting the amount of fish one eats, most fish can be eaten safely, especially by following a few simple rules. First, the EPA recommends avoiding shark, swordfish, king mackerel and tilefish, which are highest in mercury. Instead, the EPA recommends eating up to twelve ounces per week of fish or shellfish that are low in mercury, such as shrimp, salmon, pollock and catfish. Since mercury is stored in the fish's fat, removing the fat before cooking and choosing younger fish with less fat can also reduce the amount of mercury present. For fish caught in your area, check local advisories about mercury and other contamination to determine if the fish is safe to eat.
Member Comments