Mercury in fish

What is mercury, why is it in my food, and why should I care?

Mercury is a heavy metal that occurs naturally in our environment, generally in a sequestered or inert state. Over the last century, however, industrial pollution from power plants, waste incinerators, and mining operations have contributed to a pronounced increase in our oceans’ mercury levels. This mercury is gobbled up by the animals that call these waters home.

Sea life accumulates mercury mainly by consuming contaminated food. In areas near runoff sites, bottom-dwelling animals often have high mercury levels; these “detritivores” generally eat whatever they find on the seabed around them, much of which may be contaminated. In the open ocean, the highest mercury levels are usually found in the long-lived fish and predators that eat at the top of the food chain.

A debate rages over whether or not the mercury levels in fish have an impact on human health, and if so, whether or not the benefits of eating fish are outweighed by the mercury risks. Researchers have demonstrated links between contaminated fish consumption and numerous health issues, including developmental delays in children, increased cardiovascular risk, and decline in motor and brain function.

In the United States the Environmental Protection Agency (EPA) has defined a “reference dose,” which describes the amounts of mercury that can be safely ingested without appreciable health risks. While some dismiss the reference dose as unnecessarily cautious, many others, among them notable scientists and environmentalists, criticize it for not being stringent enough.

Bluefin tuna, king mackerel, shark, marlin, and swordfish are some of the species that accumulate the highest levels of mercury. They eat voraciously and may live for many years. In fact, these fish often contain many times more mercury than what is considered to be safe by the EPA.

Many experts claim that mercury is more dangerous for young and unborn children than it is for the average adult. This is due to the characteristics of the human blood-brain barrier, a semipermeable membrane that controls the flow of blood and nutrients between the bloodstream and the brain. In a fetus or young child, this barrier is more easily penetrated by mercury, which is then able to affect the entire brain. This process is inhibited in adults by a more developed and restrictive blood-brain barrier. Mercury is also concentrated in umbilical cord blood, further increasing exposure for unborn children.


Sustainable Sushi aims to give the average sushi diner accurate and honest information about the likelihood of high mercury levels in various marine animals. This information is presented for many fish to help sushi patrons make their own informed decisions about the benefits and risks of eating fish.

The mercury notifications in Sustainable Sushi assume a three ounce portion of fish (about four pieces of nigiri or one good serving of sashimi). Rankings are based on scientific data compiled by the Environmental Defense Fund (EDF) and the Environmental Protection Agency’s Guidance for Assessing Chemical Contaminant Data for Use in Fish Advisories.

Mercury recommendations correspond to the number of servings that an average adult (weighing 154 pounds) can safely eat per month and are structured as follows:

Low — More than 10 servings

Moderate — 6–10 servings

High — 3–6 servings

Extreme — Less than 3 servings

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