herring vs saltwater fish

herring and saltwater fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
herring	Yes	No	No	No
saltwater fish	Yes	No	No	No

As nouns, saltwater fish is a hypernym of herring; that is, saltwater fish is a word with a broader meaning than herring:

herring: valuable flesh of fatty fish from shallow waters of northern Atlantic or Pacific; usually salted or pickled
saltwater fish: flesh of fish from the sea used as food

herring (noun)	saltwater fish (noun)
commercially important food fish of northern waters of both Atlantic and Pacific	flesh of fish from the sea used as food
valuable flesh of fatty fish from shallow waters of northern Atlantic or Pacific; usually salted or pickled