There’s no doubt at all that fish is one of the healthiest foods you can eat. It’s full of protein, micronutrients and healthy fats, as well as being quick and easy to prepare.

However, there are some fish types that can contain high levels of toxic mercury, which is linked to serious health problems.

So, should you avoid fish altogether, or just avoid certain types of fish? This article aims to answer that question.

What is the Problem with Mercury?

Mercury is a naturally-occurring  heavy metal found in air, water and soil. It’s released into the environment in several ways – often through industrial processes or natural eruptions.

There are three main forms of mercury: elemental (metallic), inorganic, and organic.

People can become exposed to mercury in several ways, such as breathing in mercury vapors while doing industrial work or mining.

There is a risk of exposure from eating fish and shellfish, because these creatures absorb low concentrations of mercury due to the pollution of the water they live in. Over time, methylmercury (the organic form of mercury), can concentrate in their bodies.

Methylmercury is very toxic, and can cause serious health issues if it reaches certain levels in your body.

Which Fish Are Extremely High in Mercury?

The amount of mercury in fish and seafood is dependent on the species and how much pollution is in their environment.

One study, carried out from 1998 to 2005, showed that 27% of fish from 291 streams around the U.S. contained more mercury than the recommended limit.

Another study found that one-third of fish caught on the New Jersey shore had mercury levels higher than 0.5 parts per million (ppm). This level could cause health problems in those who regularly eat these fish.

Generally speaking, the larger and more long-lived fish tend to contain most mercury.

Mercury is measured in parts per million (ppm), and here are the average levels of mercury in different types of fish and seafood, starting with the highest.

Swordfish 0.995 ppm

Shark 0.979 ppm

King Mackerel 0.730 ppm

Bigeye tuna 0.689 ppm

Marlin 0.485 ppm

Canned tuna 0.128 ppm Cod 0.111 ppm

American lobster 0.107 ppm

Whitefish 0.089 ppm

Herring 0.084 ppm

Hake 0.079 ppm

Trout 0.071 ppm

Crab 0.065 ppm

Haddock 0.055 ppm

Whiting 0.051

Atlantic Mackerel 0.050 ppm

Crayfish 0.035 ppm

Pollock 0.031 ppm

Catfish 0.025 ppm

Squid 0.023 ppm

Salmon 0.022 ppm

Anchovies 0.017 ppm

Sardines 0.013 ppm

Oysters 0.012 ppm

Scallops 0.003 ppm

Shrimp 0.001 ppm

Impact on Health

Exposure to mercury can cause health problems, especially brain issues.

A study of 129 Brazilian adults found that higher levels of mercury in hair were associated with a loss of fine motor skills, dexterity, memory and attention.

Other studies also link exposure to heavy metals like mercury with conditions such as Alzheimer’s, Parkinson’s autism, depression and anxiety, but more studies need to be done on this.

Mercury exposure is also linked to high blood pressure, increased risk of heart attacks and higher levels of harmful LDL cholesterol.

The health benefits of eating fish outweigh the risks of mercury contamination, but avoid or limit your consumption of fish that are high in mercury.

Certain Groups of People Are More Vulnerable

Mercury doesn’t affect everyone in the same way, so certain groups of people need to take extra care.

Pregnant women, breastfeeding mothers, and young children need to take special care, as fetuses and children are more vulnerable to mercury poisoning.

An animal study showed that exposure to low doses of methylmercury during the first 10 days of conception impaired brain function in adult mice.

One human study showed a link between exposure to mercury in the womb, and reduced attention, memory, language, and motor function.

Don’t Avoid Eating Fish

Fish, especially oily fish like salmon, are an important source of omega-3 fatty acids, as well as other nutrients.

It’s recommended that most people eat at least two servings of fish per week, but the FDA recommends that pregnant or breastfeeding women should follow these guidelines:

Eat 2-3 servings (227-340 grams) of varied fish every week. Choose lower-mercury fish and seafood, like salmon, shrimp, cod, and sardines. Avoid fish higher in mercury, such as swordfish, king mackerel and tilefish from the Gulf of Mexico. When buying fresh fish, look out for fish advisories on particular streams or lakes.