Is Surströmming the Swedish version of lutefisk? I've had lutefisk before and it was rather bland, jelly like, etc... but not particularly offensive. By the way, I suspect there are superior and inferior versions of suströmming, or ways of preparing it. What kinds of Surströmming do Swedes find good or "truly Swedish?"
It's fish in the mass noun sense here, much like e.g. "goat milk is milk" functions grammatically even though "goat milk is a milk" doesn't. Whether it's the most idiomatic option is another issue, though - but "surströmming is a fish" would be surströmming är en fisk in Swedish, and it would mean a fish species.