Here's sad news for all you sushi lovers out there. Recently the New York Times did a random test of popular sushi restaurants in New York City and found that in some cases only 6 slices (that's SIX!) of sushi tuna will put a person over the government's established level of safe mercury consumption.
Now, I've always known that sushi grade tuna such as Bluefin, which is one of the more common tunas used for sushi, contains more mercury than albacore. A couple of months ago I wrote a post about the Tuna Calculator which helps you know how much canned tuna you can eat every week before you get more Mercury than is safe in your diet. At that time based on my weight I found out I could eat less than 1/2 of a 4 ounce can of Albacore Tuna a week before getting too much mercury.
Now, I've alway been a huge sushi fan and tuna was one of my favorites. A few years ago I generally stopped eating tuna, raw oysters (which I also loved) and clams. Ecodefense lists here the best and worst types of tuna to ingest. Just keep in mind that the larger and usually the more higher grade of tuna, the greater amounts of mercury the fish will likely contain. This has to do with the length of time the fish has been swimming around in the ocean and the greater length of time the fish gets to consume mercury contaminated foods. Similarly this too is the reason that predatory fish such as sharks and swordfish are always unsafe to consume, especially for women of child bearing age and children.
Mercury gets into the water through pollution generated by coal burning power plants, mining, some factories and waste incinerators. Rain run-off leads to movement of the pollution from our streets into rivers and streams thus eventually ending back in the ocean contaminating our fish and ourselves. The organic mercury is converted by bacteria into methylmercury which then lodges in the fish and our muscle tissues if we consume the fish, inhale mercury vapors or drink water that has mercury contamination. Mercury toxicity is difficult to detect and is not commonly found in blood tests that your doctor uses. If you are experiencing memory problems, emotional lability such as anxiety, depression and excessive anger as well as fatigue and repeated colds and flus consider being tested for heavy metals. Just remember that a blood test for mercury or lead is neither accurate nor preferred for overall exposure to mercury.