red rockfish vs saltwater fish

red rockfish and saltwater fish both are nouns.

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

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

red rockfish (noun)	saltwater fish (noun)
a large fish of the Pacific coast of North America	flesh of fish from the sea used as food
red flesh of large food fish of Pacific coast