Certain species of fish contain mercury, and mercury is a strong neurotoxin. If you eat enough fish of certain types or caught certain waters, you take in enough mercury for it to start being dangerous.

The obvious first question is, “why do fish contain mercury?” This is actually a case where it is mostly a natural process. The state of California’s Office of Environmental Health Assessment offers this perspective:

Mercury, a metal, is widely found in nature in rock and soil, and is washed into surface waters during storms. Mercury evaporates from rock, soil, and water into the air, and then falls back to the earth in rain, often far from where it started. Human activities redistribute mercury and can increase its concentration in the aquatic environment. The coastal mountains in northern California are naturally rich in mercury in the form of cinnabar ore, which was processed to produce quicksilver, a liquid form of inorganic mercury. This mercury was taken to the Sierra Nevada, Klamath mountains, and other regions, where it was used in gold mining. Historic mining operations and the remaining tailings from abandoned mercury and gold mines have contributed to the release of large amounts of mercury into California’s surface waters. Mercury can also be released into the environment from industrial sources, including the burning of fossil fuels and solid wastes, and disposal of mercury-containing products.

Once mercury gets into water, much of it settles to the bottom where bacteria in the mud or sand convert it to the organic form of methylmercury. Fish absorb methylmercury when they eat smaller aquatic organisms. Larger and older fish absorb more methylmercury as they eat other fish. In this way, the amount of methylmercury builds up as it passes through the food chain. Fish eliminate methylmercury slowly, and so it builds up in fish in much greater concentrations than in the surrounding water. Methylmercury generally reaches the highest levels in predatory fish at the top of the aquatic food chain.

Because mercury accumulates in fish, fish can be poisonous. If you eat too much, it becomes a problem. But how much is too much? Here is the answer:

How to Tell If You’re Poisoning Yourself With Fish

It’s not a simple question. From the article:

Scientists have assured me that one serving of halibut contains nowhere near a dosage that might cause harm. These are the same scientists, though, who admit that no one knows for sure what the threshold dose is that causes mercury to subtly poison cells in the brain and the liver, two organs where it tends to accumulate.

See the article for details on how you resolve this conundrum.

Why is mercury toxic to the brain? Here is the answer to that question:

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