Mercury is bad for you, especially if you are pregnant or are a child, and it's also increasingly present in the food chain. That's why the FDA has recently issue new recommendations for pregnant women and parents to help them reduce their intake of mercury by either reducing their fish consumption, or by switching to species that are less likely to be contaminated, and why Consumer Reports - after doing some lab testing of their own - has decided to recommend that pregnant women abstain from eating tuna altogether.
Most people who eat fish and want to be careful about their mercury exposure will usually try to pick fish species, or fish coming from certain parts of the world, that are known to contain fewer toxins (and hopefully also not be endangered species!). The NRDC has a handy 'Mercury in Fish' wallet card that you can print out and carry with you when you are grocery shopping or ordering at a restaurant.
But unfortunately, things aren't quite that simple. Deciding to only eat certain types of fish from certain places works in theory, but in practice you might not be getting what you think you're getting. A recent study used DNA testing to figure out how much fish was mislabelled in the U.S., and the results are pretty scary. This isn't just a marginal issue, and so it's hard to make informed choices when you get bad information...
In fact, a brand new study that analyzed fish bought in 10 different states in the U.S. concludes that "Seafood mislabeling distorts the true abundance of fish in the sea, defrauds consumers, and can also cause unwanted exposure to harmful pollutants." The problem is that there's a big incentive for the people who catch and sell fish to label less desirable - and thus less profitable - species as something else to make more money.
Here's the scary part: The study looked at fish that are labeled as Marine Stewardship Council- (MSC-) certified Chilean sea bass and the regular, uncertified Chilean sea bass. The MSC label is supposed to mean that the fish comes from the less polluted waters near Antarctica.
"In a previous study, the scientists had determined that fully 20 percent of fish purchased as Chilean sea bass were not genetically identifiable as such. Further, of those Chilean sea bass positively identified using DNA techniques, 15 percent had genetic markers that indicated that they were not sourced from the South Georgia fishery."
In this new study, researchers found that the verified MSC-certified fish had similar levels of mercury as the non-certified fish, which isn't what you would expect to see if the fish really come from the cleaner waters around South Georgia (the very south-most part of South-America).
Fish that is certified as being sustainably harvested and coming from cleaner parts of the world should still be better on average, but because there's so much fish substitution and mislabelling, it won't be as healthy as it should be if 100% of that 'certified' fish was truly what it claimed to be.