rockfish vs saltwater fish

rockfish and saltwater fish both are nouns.

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

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

rockfish: the lean flesh of any of various valuable market fish caught among rocks
saltwater fish: flesh of fish from the sea used as food

rockfish (noun)	saltwater fish (noun)
marine food fish found among rocks along the northern coasts of Europe and America	flesh of fish from the sea used as food
marine food and game fish with dark longitudinal stripes; migrates upriver to spawn; sometimes placed in the genus Morone
the lean flesh of any of various valuable market fish caught among rocks