bluefish vs saltwater fish

bluefish and saltwater fish both are nouns.

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

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

bluefish (noun)	saltwater fish (noun)
bluish warm-water marine food and game fish that follow schools of small fishes into shallow waters	flesh of fish from the sea used as food
fatty bluish flesh of bluefish