Actually it was from this American tuna calculator:
Tuna Calculator | Environmental Working Group
I love my tuna.. especially as a protein source, but it sounds like you can probably only have 2 big 185g tins a week.. maybe 3 safely. The bigger you are, the more you can take, safely.
I'm interested to hear anyone's thoughts on this.. I personally have been eating tuna for a long time, at least the whole way through uni and my first few years of work..