sardine vs saltwater fish

sardine and saltwater fish both are nouns.

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

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

sardine (noun)	saltwater fish (noun)
any of various small edible herring or related food fishes frequently canned	flesh of fish from the sea used as food
small fishes found in great schools along coasts of Europe; smaller and rounder than herring
small fatty fish usually canned
a deep orange-red variety of chalcedony