Definitely have to consider "the source" on this one (PETA)...Be that as it may, it is no secret that tuna is a major source of heavy metal, specifically Mercury. I have a few friends that are heavy consumers of tuna, and have their levels checked, and they have been VERY high. They stopped eating it, and their levels dropped...definitely a little worrisome. The real question from a medical standpoint, is whether or not the levels were clinically relevant, in that they had no symptoms of "mercury poisoning"...having said that, it couldn't be good...having said THAT, is it any worse than the rest of the CRAP that is in our environment, that ultimately ends up in our bodies: crap in the air, crap in the water, crap in our food (hormones, antibiotics, heavy metals, PCBS, so on and so forth)...one of the most potent carcinogens out there (aflatoxins), is found on the mold in peanuts...
So, you really can't get away from it...I think the key is to avoid overdoing anything, stick with organic foods, and trust your body's ability to cope with toxins in our environment...
Parting shot: how bad for you is the stress from worrying about all of the crap...that couldn't be good for you either...
Andale