Ah, where I'm from (US Cali), the guideline for tuna (chunk light) is no more than 6 ounces a week. However, due to my location, I've worried about cesium 137 from the Fukushima fall-out (since a lot of our tuna seems to come from the Pacific)... and have been trying to avoid it altogether.
Nevertheless I think if one does some research they can find a source of tuna that isn't measured to have high levels of contaminants. Healthy tuna can offer so much nourishment, it's so sad to avoid.