“Not frozen fish sticks again, dad!” Nicole knows she is whining but she is tired of the
bland meat, and fish sticks have way too much crunch. “Besides, can’t fish make you sick?” she
asks. “Don’t they have mercury in them?”
Nicole’s dad closes the oven door on the
baking sheet of fish sticks, sighing as he looks
at her. “Mercury in fish can make you ill, but
not these fish sticks,” he assures his daughter.
“How do you know which fish are safe to
eat? Are there guidelines out there for what
“Let’s look it up online.”
Nicole starts searching for “mercury in
fish.” Her search brings up plenty of Web
sites, but two Web pages catch her attention,
one from the U.S. Environmental Protection
Agency (EPA), and the other from the U.S.
Food and Drug Administration (FDA). She is
confused because the two agencies give different numbers for safe levels of mercury.
By Mary Alexandra Agner
“What did you find?” her father asks.
“Well, it turns out all fish have some mer-
cury in them. But the EPA and the FDA give
different kinds of numbers. The FDA says that
grocery stores should not sell fish that has
more than 1 part per million of mercury. The
EPA says not to eat more than 0.1 microgram
of mercury per day per kilogram of your body
mass. Microgram! That’s one millionth of a
gram, so 0.1 microgram is really small!”
“Is there anything else?”
“Hmm, I found a color-coded table that lists
fish by their mercury content—high, average,
low. It’s safer to eat shrimp, sardines, tilapia,
and a dozen other fish. We should avoid tile-
fish, shark, and tuna. Tuna? I eat tuna at least
once a week.”
Her dad clears his throat, eyeing the cans of
albacore in the food pantry. “Anything more
“They just say tuna has high levels of mer-
cury,” she says. “But isn’t tuna safe? And how
do fish get mercury inside them in the first
place?” She resumes her search on the tablet.
It’s easy to find out how mercury gets into
fish, Nicole learns. Tiny organisms, called
plankton, ingest mercury, and they are eaten
by small fish, which are eaten by other fish,
passing the mercury on to each other up the
food chain (Fig. 1).
“It turns out tuna will eat anything. And I
guess that’s why they have so much mercury.
They eat fish that probably already ate some
plankton or other fish with mercury in it. And
the International Union for Conservation of
Nature says the tuna population is decreasing
because they are overfished.”
“So those are two reasons not to eat tuna,”
her dad says.
“But the real culprit is not just plain mercury,” Nicole says. “It’s a form of mercury
called methylmercury (CH3Hg+).”