Not sure if its just me, but when I was consuming tuna daily as my main protein source, my mind was really foggy and I had frequent headaches. To the point where I wanted to take a semester off Uni, it was that bad. Had it checked by a doctor who told me to stop consuming tuna for a few weeks.
3 months on and I feel fantastic, no more headaches (ever), head doesn't fog up either. I now eat chicken breasts. Not trying to scare anyone off tuna, but just my personal experience.
good article:
Mercury Rising - Are You Eating Too Much Tuna? - Know It All Health