I was never told not to eat tuna until my 3rd pregnancy and I ate it everyday with my daughter its all i craved and she is super healthy so I don't honestly think the doctors know what they are talking about when they say not to eat it its not bad it may have traces of mercury in but not enough to hurt or baby if I can eat it literally everyday for 8 months I'm sure its fine :)
Not very often, no more than once a week (I went to once a month). That is because of the heavy-metal levels in the tuna, not because it is canned or anything like that. I also stopped eating tuna in sushi for that reason.