tuna fish vs bluefin

tuna fish and bluefin both are nouns.

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

As nouns, bluefin is a hyponym of tuna fish; that is, bluefin is a word with a more specific, narrower meaning than tuna fish:

tuna fish: important warm-water fatty fish of the genus Thunnus of the family Scombridae; usually served as steaks
bluefin: flesh of very large tuna

Other hyponyms of tuna fish include albacore, bonito, bluefin tuna.

tuna fish (noun)	bluefin (noun)
important warm-water fatty fish of the genus Thunnus of the family Scombridae; usually served as steaks	largest tuna; to 1500 pounds; of mostly temperate seas: feed in polar regions but breed in tropics
	flesh of very large tuna