croaker vs saltwater fish

croaker and saltwater fish both are nouns.

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

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

croaker: the lean flesh of a saltwater fish caught along Atlantic coast of southern U.S.
saltwater fish: flesh of fish from the sea used as food

croaker (noun)	saltwater fish (noun)
any of several fishes that make a croaking noise	flesh of fish from the sea used as food
the lean flesh of a saltwater fish caught along Atlantic coast of southern U.S.