Tuna could mean anything, but tuna fish always means the stuff in a can. I suppose you could put it on bread without mayo, but to me it would still be tuna fish/salad.
And besides cans, doesn’t it also now come in envelopes?
Oddly enough, when I was a kid my mother used to make tuna casserole, but it was made with tuna fish in a can.
My brain is starting to hurt.
If I were as finicky, I would have asked if it was a tuna steak sandwich, which sounds awesome.
I liked him better when he was Eric Iverson.
Oh yeah? Well, explain grapefruit to me.
Shrimpbug is already taken. At least, that’s what crawfish are called in MY house.
Some Southern Americans call ground beef “hamburger meat”, if that makes you feel any better.
Canadian comedian Tim Steeves had a bit about how Americans always specify “tuna fish”, as opposed to, what, tuna horse? They also say “ice hockey”. If you just say hockey, they’ll be confused and ask you to specify a surface.
Dogfish. 
“Tuna salad” to me would mean a nice seared tuna steak served atop a bed of green salad leaves. Flaked tuna with mayonnaise is “tuna mayo”, as in a tuna mayo sandwich.
I don’t think I’ve ever come across flaked tuna in a can with the mayonnaise already mixed in, though. That’s just weird and lazy.
Max, if I hear “hockey”, I think of field hockey, played by girls in shorts on astroturf. The Canadian thing is ice hockey, because it’s played on ice 
I’ve wondered this many times. Makes no sense whatsoever to me.
Same thing with “Navy SEALs.”
:: Goes into kitchen, checks label on tin. ::
Chicken of the Sea Chunk Light Tuna in Water
Pervert.
How about catfish fish?
By prepared for disappointment if you ever order tuna salad at a deli. Likewise, don’t get your hopes up that the “chicken salad” will consist of an entire roast chicken plopped atop a plate of romaine.
Apparently it grows in bunches, like grapes. That’s according to Trivial Persuit. Web searches on the subject sort of bear this out, but I have yet to find a trustworthy cite.
Shit sandwich?
As others have said in the thread, if someone just says tuna, I’m going to think steak or sushi related. Tuna fish is generally the canned variety. So I’ll tend to say tuna fish sandwich, but a tuna sandwich is the same thing. Tuna Salad is something altogether different, it’s not just mayo, but has a few other things in it, usually some celery and stuff. Thus, there is a tuna fish sandwich, which may or may not have mayo, but there’s also a tuna salad sandwich, or just play tuna salad which can be sort of a side dish. There are, of course, salads with chopped up tuna steak in it, I might also call that a tuna salad, but it’s usually some kind of salad with tuna because you could often just as lightly have had chicken or shrimp or whatever in there as protein as well and, obviously, chicken salad is something very different than a salad with chopped up chicken in it.
Intravenus de Milo.
Did you know the titmouse has nothing to do with either tits or mice?