The concentration of mercury in fish becomes higher as you move up the food chain. Even the EPA says "Do not eat Shark, Swordfish, King Mackerel, or Tilefish because they contain high levels of mercury." Albacore consumption should be limited. Info here:
http://www.epa.gov/ost/fishadvice/advice.html

Salon.com did a couple of good articles on mercury in fish as well: