The US FDA sets the limits of tuna’s mercury content to 1000 parts per billion (ppb). However Defenders of Wildlife have just release a large document showing that tuna’s mercury content from some countries is at 1500 (ppb).
According to the document Is Our Tuna “Family-Safe”? Mercury in America’s Favorite Fish [ PDF ] tins of fish from Ecuador, Mexico and Costa Rica had dangerously high mercury concentrations. It is worrying as people are being encouraged to eat more fish because of the benefits of omega 3. The whole tuna / mercury problem makes having a safe but effective omega 3 diet hard.
This study does not appeared to have peer-review and the document is more like a glossy sales brochure than a scientific study so please read with a suitably sized pinch of salt.
For more information on the tuna / mercury health risks, see The Omega 3 Diet.