I didn't even know tuna were big fish until I read we couldn't eat them

It's because of the mercury build-up, I think. Small fish have a harmless amount of mercury (or whatever it is), but when the bigger fish eat them, it builds and builds. Safe for them, not so much for us

And we all know at least one person who does something "wrong" and comes out with a normal baby - you're it for the fish thing