So is the weird Swedish delicacy ever called "sursill"? Eller säger ni bara strömming?
"Sursild" is a word in norwegian, though - but I don't think it's the same as surströmming.
No, we do not say "sursill" because the delicacy is produced in northern Sweden. In northern Baltic and the Bothnian bay, Clupea harengus is called "strömming", whereas it is called "sill" in the southern Baltic and on the west coast of Sweden.
''Herring is a fish'' is completely unnatural to my ear, even if it's the most grammatically fateful translation. A name of whichever species begs an article before it (commonly ''the herring is a fish'') or else it sounds like a proper noun.