Canned tuna does contain mercury, as does fresh tuna. However, it is not enough to put you off this nutritious fish. There have been cases in which people have got mercury poisoning from their tuna, but don’t let this put you off.
To get mercury poisoning from tuna, you’d have to consume a lot of tuna for many days in a row. Eating canned tuna several times a week will not fill your body with enough mercury to get poisoning from it.