butterfish vs saltwater fish

butterfish and saltwater fish both are nouns.

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

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

butterfish: any of numerous small flat Atlantic food fish having smooth skin
saltwater fish: flesh of fish from the sea used as food

butterfish (noun)	saltwater fish (noun)
small marine fish with a short compressed body and feeble spines	flesh of fish from the sea used as food
slippery scaleless food fish of the northern Atlantic coastal waters
any of numerous small flat Atlantic food fish having smooth skin