The issue with Mercury is connected to how old fish are when caught.  Older fish have longer to absorb more Mercury.

Whilst I appreciate tinned Tuna is amongst the most convenient ways to add protein to your diet, you'd be amazed how many protein rich alternatives there are, particularly nuts, beans and pulses.

Here are a few worrying links:

Mercury in Fish: your questions answered

Canned Tuna Riskier than Previously Suspected

FDA Urged to Crack Down on Tuna Mercury Levels

As far as I can tell the advice is just don't eat too much.  I reckon two tins per week would be ok