is tuna safe for dogs?

I was told recently that I shouldn't feed Penny tuna as often as salmon, because tuna has a high level of mercury. I don't know much about that, but I do know salmon is certainly a bit expensive to be a regualr part of her diet.

So I was interested to read this article on PhysOrg.com (a science and technology news site) that some species of tuna have less mercury than others.