tuna vs saltwater fish

tuna and saltwater fish both are nouns.

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

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

tuna: 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 (noun)	saltwater fish (noun)
any very large marine food and game fish of the genus Thunnus; related to mackerel; chiefly of warm waters	flesh of fish from the sea used as food
New Zealand eel
important warm-water fatty fish of the genus Thunnus of the family Scombridae; usually served as steaks
tropical American prickly pear of Jamaica