When you sit down to a plate of sushi, or more correctly to a plate of sashimi (raw fish), the presentation is usually reliably appetizing – sliced fish with vinegared rice neatly rolled inside leaves of fragrant nori. Of course no serving of sushi would be complete without a side-dish of bright green wasabi, sometimes referred to as ‘Japanese horseradish’.

Oddly enough though, the term sushi doesn’t in fact refer to the fish itself. In Japan the term means ‘snack’ and references the rice. Sushi, taken to mean raw fish, is purely a Western adaptation.

The bad news for sushi lovers is that alarm levels are being raised about the mercury in tuna. This is a concern that should also be shared by people who have a fondness for canned tuna.

Problem is tuna consumption is a bit of a crap shoot. There is no way of being sure if the tuna you’re eating has high or low levels of mercury. Most of the fish we consume arrives at the table untested – both by the Feds and by retailers. The other worrisome concern is that there is virtually no way to ascertain where the fish you are eating came from. When it crossed oceans, it was handled by multiple agents en route to your home or restaurant. No documentation is available detailing the point of origin or the time that elapsed between the date when the fish was netted and when it made it onto your plate.

Some waterways are more contaminated than others. Mercury gets into the water most often via the burning of fossil fuels, and once in the water bacteria convert it to toxic methyl mercury. Tiny fish easily absorb the methyl mercury. The mercury becomes steadily more concentrated as it moves up the food chain. When it is absorbed by large predatory fish such as tuna there is no way to remove it from the flesh of the fish.

Pregnant women should be the most cautious about where to buy tuna and the amount of tuna they consume. The fetus is particularly vulnerable to contamination. Researchers have established a link between mercury exposure in pregnant women and later behavioral disorders in their children. FDA cautions include other types of fish also. Women of childbearing age and small children should say clear of shark, swordfish, king mackerel and tilefish. FDA guidelines recommend no more than 12 ounces of seafood a week for people in this vulnerable group – with no more than 6 ounces of that being tuna.

How will the tuna scare impact the sushi business? Well already European sushi bars and restaurants are giving bluefin tuna the thumbs down and experimenting with alternative species of tuna. Some innovative chefs are even willing to experiment with whale and horse meat.

The greatest concern from the consumers point-of-view is the absence of any index that would reliably inform them of toxic levels in the fish they are consuming. This ‘unknown factor’ makes fish consumption a bit of a gamble. That’s why it is wise to pay close attention to where to buy tuna and type of fish being consumed over a given period.