Where to get your Recommended Daily Allowance of mercury—and then some

Tuna from the sea and bass from California lakes have accumulated high levels of mercury.

On Friday we discovered that levels of mercury in the ocean are 30 percent higher than they were a decade ago, and scientists predict they’ll rise 50 percent more by 2050 unless we do something to keep mercury out of the sea. That won’t be easy because much of the mercury that Americans encounter comes from the burning of fossil fuels and waste in Asia. It falls out of the sky off the coast of Asia, drifts eastward in major ocean currents, rising up the food chain as it goes, and ends up lurking within that succulent slice of maguro resting on a bed of sushi rice in a U.S. restaurant, or inside the can of Chicken of the Sea that we mix with a little mayo and celery and layer between two slices of whole-grain bread.

Once it’s in the body, mercury tends to stay there. It contributes to developmental disabilities in children and heart attacks in adults. It’s enough to make you eat lake fish.

But then today comes the news that only 15 percent of California lake fish live in waters free of toxins. The primary culprit? Mercury.

California’s Water Resources Control Board found mercury levels high enough to warrant public health warnings about consuming fish from many of the 152 lakes they sampled. They expect the same is true across the state’s 9,000 lakes.

Only 15 percent of the lakes sampled in 2007 were in the “clean” category. Furthermore, whether these lakes are entirely clean depends upon whether high-methylmercury species such as largemouth bass or self-sustaining trout populations are really absent from these lakes…. Methylmercury was the pollutant primarily responsible for the remaining 85 percent of lakes having at least one species with an average concentration above thresholds.

The highest mercury levels were found in Northern California largemouth bass. The lowest, in mountain trout.

The federal study on the path of ocean mercury is available from the U.S. Geological Survey.

The California study, with its lists of clean and not-so-clean lakes, can be downloaded here as a pdf file.

Chicago’s Shedd Aquarium offers a downloadable, wallet-sized “Right Bite” card that lists top-of-the-food-chain fish to avoid because of mercury bioaccumulation (in addition to tuna, beware of shark and swordfish), as well as fish to avoid because of overfishing or other harm (including cod and halibut). Happily it also lists fish that are “abundant, well managed, and caught or farmed in environmentally friendly ways.”