Including fish in your diet has a lot of benefits including high amounts of protein and omega-3 oils. Like with most good things, however, there is a catch----almost all fish contain mercury, which can be potentially dangerous. Instead of worrying if your fish has mercury, be mindful of the level of mercury the fish has, as it varies from fish to fish.
Though people sometimes associate mercury with pollution, it also exists naturally in the environment. When mercury falls into the water, it becomes methylmercury, which is dangerous to ingest. Fish absorb methylmercury through what they eat, and it binds to the proteins in their tissue. The amount of methylmercury in a fish depends on its diet, but all fish are prone to ingesting mercury in some amount.
Almost all fish sold in grocery stores are regulated by the government, and therefore contain less than 1 ppm (parts per million) of mercury. This is done so that people who eat and buy a lot of fish are not in danger. Grocery stores generally sell fish that were caught in the ocean, and ocean fish contain less mercury than freshwater fish. Types of ocean-caught fish include salmon, tilapia, tuna, cod and lobster. These contain much less than the limit of 1 ppm of mercury; most average less than 0.1 ppm.
Fish that eat other fish--predators such as shark, swordfish and ray--generally have higher levels of mercury. Even these fish contain less than 1 ppm of mercury (averaging around 0.9 ppm), so they are still safe to consume and be sold in grocery stores. As a general rule, however, eat them only once a week at most.
Locally caught freshwater fish generally contain higher levels of mercury, and since the government does not regulate freshwater fish for commercial consumption, you should know which fish are safe to eat. You should only eat bowfin and chain pickerel about once a month. You can eat perch, carp and most species of trout about once a week. Most freshwater fish species contain around 0.2 ppm of mercury, and you can eat them safely about once a week.
For specific information on fish caught and farmed in your area, check the Environmental Protection Agency's website for local advisories and the lists of safe fish for each body of water.
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