tuna fish vs saltwater fish

tuna fish and saltwater fish both are nouns.

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

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

tuna fish: important warm-water fatty fish of the genus Thunnus of the family Scombridae; usually served as steaks
saltwater fish: flesh of fish from the sea used as food

tuna fish (noun)	saltwater fish (noun)
important warm-water fatty fish of the genus Thunnus of the family Scombridae; usually served as steaks	flesh of fish from the sea used as food