Well my kitties at my parents Catuary love tuna, they go crazy just with the smell. They eat cat food normaly, but sometimes they treat them with tuna or sardines and they go nuts all of them.
There is an island in a western state of Mexico populated by a lot of cats and they fish. They're feral cats and they fish from the river close to the sea.
But that is an unusual occurrence. Somehow they adapted to their environment.
Anyways yeah in the wild they don't eat fish, but all the kitties I've known so far (except Scully) love fish as in they go evil about it.
The levels of mercury are worth to give it a thought. Who knows what fish they use for the canned food, you're right about that.
I won't try fish with her anyway