Mercury Content In Fish: What You Need To Know

By Jenn Maxwell. May 7th 2016

Seafood is considered an important part of a healthy, balanced diet. Low in saturated fat and high in good-quality protein and omega-3 fatty acids, fish is making its way on more and more dinner plates. But, unfortunately many fish contains damaging mercury, which, if consumed in high enough quantities, can cause serious health complications.

What Is Mercury And Why Is It Found In Fish?

While mercury is a naturally-occurring mineral, additional amounts of the element can be released into the environment through industrial pollution. When mercury enters water supplies such as lakes, rivers and streams, it becomes methylmercury (sometimes written as methyl mercury). According to the USGS (U.S. Geological Survey), methylmercury is “the only form of mercury that accumulates appreciably in fish.”

Nearly all fish and shellfish contain trace amounts of mercury. Fish that have a longer lifespan generally have higher levels because they have had more opportunity to accumulate mercury, according to the EPA (Environmental Protection Agency). For most people, eating fish and shellfish occasionally or even one or two times a week does not pose a health concern.

How Dangerous Is It?

The biggest danger associated with the consumption of mercury in fish is due to mercury’s effect on developing nervous systems. Pregnant women and young children should avoid some types of fish altogether and instead choose types of fish and shellfish with lower mercury levels. Not every agency believes the mercury found in fish poses a health risk, however. The four large fish (shark, King mackerel, swordfish and tilefish) most often cited as the types to avoid completely add up to a very small amount combined (about one percent) of the fish Americans eat, according to UC Davis’ Seafood Network Information Center.

Also, the UC-Davis SNIC notes that most commercial fishermen catch large fish at sea, far away from industrial pollution. Furthermore, studies have shown that many fish such as tuna can actually block and reduce mercury’s toxicity on their own. SNIC posits that the FA recommendation of acceptable mercury levels to ingest “has a considerable margin of safety built into it.”

Who Is Most At Risk?

The EPA advises some individuals to limit their seafood intake to reduce the effects of mercury. Those include:

  • Women who might become pregnant
  • Women who are pregnant
  • Nursing mothers
  • Young children

Because you accumulate mercury in your blood stream over time, women of child-bearing age might want to limit their seafood consumption as a precaution, as should nursing mothers. Methylmercury is removed from the body naturally, but it is a gradual process and can take years for the levels to drop significantly.

Should All Fish Be Avoided?

Luckily, you don’t need to eliminate fish from the menu completely. While all fish contains mercury, there are definitely some types that should be avoided and some that are relatively safe for even pregnant women and young children to eat. Mercury accumulates in the muscles so the larger the fish, the more mercury it contains. According to the EPA, the following are large fish types that should be eaten with caution due to their mercury content:

  • Swordfish
  • Shark
  • King mackerel
  • Tilefish

These fish should also be eliminated because they are predatory fish that that consume smaller fish that also contain mercury. Also, while canned tuna is generally low in mercury, albacore, or white tuna, contains higher levels, as do tuna steaks. Keep this in mind when making your meal selections. Packaged foods such as fish sticks and fish sandwiches are generally made with fish low in mercury. The EPA recommends limiting fish and shellfish to two meals (or about 12 ounces total) of seafood low in mercury per week. Don’t stress too much if you go on a week-long tuna binge, however. Just take it easy for the next few weeks so you’re averaging about two meals a week of seafood.

The American Pregnancy Association reports that the FDA (Food and Drug Administration) expands the EPA’s recommendations by stating that fish with the "highest" mercury levels should be avoided completely while seafood with "high" mercury levels should be eaten only three times per month (six-ounce servings) at the most. Fish with “Highest” mercury levels include the EPA’s list of large fish as well as:

  • Marlin
  • Orange roughy
  • Tuna (bigeye and ahi)

“High” mercury fish includes:

  • Chilean sea bass
  • Bluefish
  • Grouper
  • Spanish and Gulf mackerel
  • Canned tuna
  • Yellowfin tuna

Seafood with the lowest levels of mercury include:

  • Catfish
  • Crawfish
  • Flounder
  • North Atlantic mackerel
  • Oysters
  • Salmon (canned and fresh)
  • Scallops
  • Sole
  • Tilapia
  • Trout
  • Whitefish

A full list of types of fish and their mercury levels can be found at Natural Resources Defense Council’s Guide to Mercury Contamination in Fish. You can also check local advisories (especially handy if you travel and want to sample the local seafood) to check the mercury levels before consuming seafood.


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