The majority of methylmercury is created through water pollution. Water-filtering algae and corals absorb the toxic substance and ultimately get eaten by shrimp, squid, and smaller fish, which store the neurotoxin in their tissues. These types of sea creatures, in turn, are the primary food source for sea predators that are not only larger in size, but also live longer, accumulating even more methylmercury in their bodies.
This chain reaction yields an expected result: the sea life that is on the top of the food chain accumulate the most of the toxin. This process is called bioaccumulation, and unfortunately, humans are not immune to it either.
More specifically, the researchers found that the neurotoxin levels fluctuate in the fish depending on their diet. So, the levels of methylmercury rose in spiny dogfish populations by 33-61% when they were forced to feed on squid instead of their primary diet, cod, due to overfishing of the latter. Squid and shrimp, on average, have higher levels of the toxin in their flesh, which is why the dogfish that ate the squid, too, accumulated significantly more of the toxin.
However, this is only half of the puzzle, as the scientists also report that rising water temperatures require some fish, particularly the fast-swimming tuna, to eat more prey, which, in turn, can also increase the levels of methylmercury in its tissues, so much so that the study found a yearly 3.5% increase of the toxin in Atlantic bluefin tuna populations between 2012 and 2017, rendering the popular fish one of the least healthy choices available.
While the rising levels of the neurotoxin are certainly alarming, a lesson all of us can learn from this is that not all kinds of fish are toxic, and even the majority of those that are can be consumed in moderation. Fish is essential for a healthy diet, being an excellent source of protein, iron, vitamins, and healthy fats, and cutting it out of our diets will do more harm than good, so continue eating fish, but avoid some varieties (see chart above).
The only exception to that rule are pregnant women and children under the age of 2, as the methylmercury is particularly dangerous for brain development, so these populations should stick to the safest fish choices available. The chart above has been updated as of 2019 by the FDA and shows how often you can eat some of the most popular kinds of fish and which ones you should avoid. For an updated version of this chart, follow this link.