"I know people who had. A while ago we where having a discussion on this at the HST forum and when I and others talked about the concerns one poster was very quick to state that he was on a tuna and rice only diet for one year and nothing wrong happened to him. Curiously, after we started talking about possible symptoms of chronic mercury poisoning the guy actually came out and said that during that year he developed a constant shaking in his hands that he couldn't find what the case was. Guess what's one of the symptoms we where talking about? Mercury functions as a very potent neurotoxin, it's no joke. The study I referenced in cats showed that they became letargic after a few months on a tuna only diet, lost energy, and lost motor skills, including coordination. Other more serious symptoms at a further stage will be muscle wastying, loss of strenght, memory, etc....
The mother of a friend of mine also developed a very weird nervous disease, kind of like Parkinsons, after years of only eating fish and not meat. Two serving a day she had. A friend of this same friend who is a fisherman was ordered by his doctor to completely avoid fish after he developed symptoms of poisoning. Again, two or three servings per day.
Obviously they weren't eating only tuna.
So if your only concern is dying a fulminant death, then fear not and eat at will. If you're somewhat scared of developing a neural disease latter and life that will make you lose your hability to function as a normal human been by your own, then maybe there's reason for concern. Like I said before, there's no research proving that a steady long term low level intake of mercury is safe so in this particular case I think the benefit of doubt is in the side of the cautious faction."