black rudderfish vs butterfish

black rudderfish and butterfish both are nouns.

Word	Noun	Adjective	Verb	Adverb
black rudderfish	Yes	No	No	No
butterfish	Yes	No	No	No

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

black rudderfish: blackish fish of New England waters
butterfish: small marine fish with a short compressed body and feeble spines

Other hypernyms of black rudderfish include stromateid, stromateid fish.

black rudderfish (noun)	butterfish (noun)
blackish fish of New England waters	small marine fish with a short compressed body and feeble spines
	slippery scaleless food fish of the northern Atlantic coastal waters
	any of numerous small flat Atlantic food fish having smooth skin