Whether you’re expecting a baby, caring for young children, or simply love seafood, it's important to understand mercury in fish. It may seem like there are conflicting messages about the health benefits of fish and the potential risks of mercury poisoning.
Taking a closer look at the facts can help to clarify the benefits and risks of eating seafood. The reality is that most fish contain at least some mercury, but species that contain low to moderate concentrations can safely be enjoyed on a regular basis.
Why is mercury bad for you?
Regarding mercury in fish, a highly toxic form called methylmercury is the primary concern because it’s harmful to humans in large quantities. High levels of mercury can damage the nervous system, especially in developing fetuses and young children.
This type of mercury exposure in the womb can lead to cognition, memory, and language issues. In adults, mercury poisoning can impact coordination, muscle strength, speech, hearing, and vision. However, most people have a little bit of mercury in their bodies at low levels that doesn’t cause problems.
According to the Environmental Protection Agency (EPA), in the United States, almost all mercury exposure happens when people eat fish high in mercury.
How Mercury Gets into Fish and Shellfish
Mercury emissions come from various sources like volcanoes, forest fires, and burning fossil fuels. Once in the water, mercury transforms. Tiny organisms, like bacteria, can convert it into methylmercury, which small aquatic plants and animals absorb.
Mercury accumulates in fish by sticking to proteins. Generally, as bigger, longer-lived fish eat smaller ones, mercury builds up in their bodies over time, amplifying the presence of this toxin.
Is there mercury in all fish and shellfish?
According to the FDA, most fish and shellfish have a little mercury in their bodies, though many don’t have substantial amounts. Different fish have different levels of mercury.
Factors that affect the amount of mercury in fish:
Size and species: Larger and older fish high in the food chain tend to have the most mercury.
Habitat contamination: A fish is more likely to have a lot of mercury in its body if it lives near sources of mercury pollution, like coal-fired power plants.
Fish with the Lowest and Highest Amounts of Mercury
Mercury concentration measures how much mercury is in a substance compared to its total mass. It’s usually given in parts per million (ppm), meaning there’s one mercury part for every million parts of the sample. Here’s a look at some fish that are high and low in mercury based on the FDA’s rating. The numbers are in ppm.
Fish High in Mercury:
Tilefish (from the Gulf of Mexico), 1.123
Swordfish, 0.995
Shark (such as Mako), 0.979
Mackerel King, 0.73
Bigeye Tuna, 0.689
Fish Low in Mercury:
Sardines, 0.013
Anchovies, 0.016
Salmon, 0.022
Pollock, 0.031
Atlantic mackerel 0.05
Shellfish like scallops (.003), crab (.065), and shrimp (.009) are all relatively low in mercury. Other fish such as cod (0.111), halibut (0.241), sablefish (.361), and ocean perch or rockfish (.121) fall more towards the middle of the spectrum.
For a more in-depth measure of Alaskan species, check out the downloadable PDF from the Alaska Department of Environmental Conservation that reports the mercury content of Alaskan fish and shellfish samples.
How often should you eat fish?
How often you should eat fish depends on a variety of factors, including age, weight, and medical conditions. Our bodies can naturally eliminate mercury over time. Dangerous mercury levels occur when people eat large quantities of fish with high mercury content over an extended period.
Like all things, moderation is key. Eat more types that are low in mercury (like salmon and wild Alaska pollock) more often, and fewer types of fish that are high in mercury (like swordfish) less often.
Who should be careful about mercury in fish?
Developing fetuses, infants, and young children can be particularly susceptible to mercury poisoning, so health authorities warn pregnant and nursing people against eating fish that have higher mercury levels.
But, overall, fish is part of a healthy, balanced diet. According to this FDA and the EPA-created chart, pregnant people and children should eat 4-ounce servings of a variety of fish, several times a week.
