not much

can someone please tell me why we say tuna fish? i do it. i have always done it and i have no idea why. we don't say cod fish or salmon fish or any other kind of fish - save swordfish but i think that is because that's its name. its even weirder than that though - it only applies to sandwiches, right? who started this? starkist? chicken of the sea?

well it ain't bumble bee because i just consulted my can in the cupboard and that my friends is Premium Tuna.