Here is a simple rule for life: the food you eat is only as safe as the environment it comes from. This is narrowly true, in that food from a dirty kitchen is likely to be unsafe. But it’s also true in the broadest sense. A good example is the tuna in sushi. Many New Yorkers have come to love the convenience, taste and aesthetic appeal of sushi. But as The Times reported Wednesday after testing tuna from 20 Manhattan stores and restaurants, sushi made from bluefin tuna may contain unacceptable levels of mercury, which acts as a neurotoxin. Every piece of that tuna, glistening on its bed of rice, is a report on the worrisome state of the oceans.

Bluefin tuna contain higher levels of mercury than other species of tuna because they live longer and, like humans, accumulate more mercury in their body tissues. The trouble for sushi-lovers is that it is very hard to tell what kind of tuna you’re getting, whether you’re dining at an exclusive restaurant or picking up some sushi at the market on the way home from work.

The owners of the establishments whose tuna was tested, including some very familiar names, said they would talk to their suppliers, and the suppliers will no doubt talk to their fishermen. For all this talk, no one is going to be able to find a mature bluefin tuna that is mercury-free, at least not until the oceans are mercury-free.

It’s easy enough to understand the transfer of mercury from fish to diner. If you regularly eat as few as six pieces of tuna sushi a week, you may be consuming more mercury than the levels considered acceptable by the Environmental Protection Agency. What is harder to keep in mind is how mercury is transferred from the environment to the fish themselves.