Source: Farmed, some wild
Mercury Risk: Low
Oysters (or kaki in Japanese) are becoming more and more common on menus in U.S. sushi restaurants, becoming as popular as some more traditional sushi fish. These bivalves are presented in a variety of forms and styles: roasted in the shell, raw on the half shell, or glistening with ponzu sauce, yuzu-soy reductions, or any of a thousand other delectable touches.
On a global scale, about ninety-five percent of the oysters we eat are farm-raised. The United States boasts a large number of oyster farms along its coasts, as do Canada, Japan, and many other countries. Oysters are comparatively easy to raise and of high value, so many fish farmers have invested in this industry.
Luckily for us, farmed oysters are also one of the more sustainable options at the sushi bar. As filter feeders, not only do they not require feed added to the water, they actually clean their surrounding habitat by converting nutrients and organic matter into consumable biomass.
When farmers import a foreign strain of oysters, there is a potential problem that can occur. Oysters, like many bivalves, are broadcast spawners; sexually mature oysters emit thousands of reproductive cells into the surrounding waters. When an alien species of oyster is introduced to a new habitat, it may disturb the genetic makeup of native species, or even cause a bioinvasion.
Wild oysters are a different matter. Unless they are from an artisanal fishery (that involves skilled operators but is small and not industrialized) and gathered by hand, it is likely that they were obtained by dredging. Some forms of dredging tear up great swathes of sea floor in order to capture the creatures dwelling in or on it. This practice can be incredibly detrimental to the environmental health of any given area. That being said, it is extremely uncommon to encounter wild oysters at a sushi bar.
In general, farmed oysters are an excellent choice. Consider making kaki sushi one of your staple items when you visit your favorite sushi restaurant.