Mercury in Seafood: How Bioaccumulation Works, and How We Test For It

Mercury is a real problem in seafood, and it concentrates in the predatory fish at the top of the food chain — along with sharks and marine mammals — through a natural process called bioaccumulation. Understanding how that process works is the most useful thing a seafood buyer can know, because it explains why two fish sold at the same counter can differ by an order of magnitude.

The federal advice on which species to eat and which to avoid is published and current: FDA/EPA Advice About Eating Fish. The Dietary Guidelines for Americans cover the same ground in the context of a whole diet.

How does mercury bioaccumulation work?

Once mercury enters the marine food chain it is converted into methylmercury, which bioaccumulates in larger predators. Mercury biomagnifies up the food chain, producing toxic concentrations in higher-trophic organisms even when concentrations in the surrounding habitat stay below the threshold for direct toxicity. That is why larger, longer-lived predatory fish generally carry more mercury than smaller ones, and it is why trophic position — where an animal sits in the food chain — is the single best predictor of what a lab will find.

Where does the mercury in seafood come from?

Mercury is a naturally occurring element present throughout the environment. But human industrial activity — coal-fired electricity generation, smelting, waste incineration — raises the amount of airborne mercury that eventually reaches lakes, rivers and the ocean. About two-thirds of the elemental mercury in the ocean comes from human activity. The largest single source is the burning of fossil fuels, especially coal, which releases roughly 160 tons of mercury a year into the air in the United States alone. Rainfall washes it into the ocean, where it is absorbed by plankton and enters the food chain from the bottom.

Trophic level is the variable that matters

With each step up the food chain, methylmercury multiplies. A wild kanpachi eats whole baitfish like mackerel or squid, which were eating smaller fish like sardines, which were eating plankton — three or four steps of accumulation. A fish raised from an egg on a formulated diet built from lower-trophic inputs does not travel the same number of steps, and the difference shows up in the tissue.

That is a general fact about aquaculture rather than a claim about any one farm. Every farm is different and every feed is different, which is exactly why we measure rather than infer. The only mercury numbers we will quote are ones a third-party lab produced from a sample we submitted.

How we quantify it

Every supplier is third-party tested for nutrition, mercury and microplastics, then randomly audited to ensure consistent compliance with the Seatopia Standard. Our Certificates of Analysis are third-party, from Edacious and EMSL — Edacious runs a 140+ analyte nutrient and contaminant panel, EMSL covers microplastics down to 1 micron.

Our limit is 0.1 ppm methylmercury — a tenth of the FDA action level of 1.0 ppm. It is also below every mercury screening value FDA and EPA publish: those screening values are 0.46, 0.23 and 0.15 ppm, corresponding to one, two and three servings a week, and our standard sits under all three. FDA’s technical documentation sets out how those values are derived.

One thing worth being precise about, because the numbers invite confusion: none of those figures is a verdict on a fillet. Each is a concentration tied to a consumption rate. FDA’s action level is an enforcement trigger for adulteration, not a health threshold. EPA’s reference dose for methylmercury is 0.1 micrograms per kilogram of body weight per day, which is a daily intake and not a tissue concentration — the matching digits with our 0.1 ppm standard are a coincidence, not a correspondence.

Farms named, panels published. Scan the QR code on the box and the farm page opens with its panels. If you would rather choose by species than by brand, the FDA/EPA advice linked above lists the low-mercury options, most of which we do not sell.