rock salmon vs fish

rock salmon and fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
rock salmon	Yes	No	No	No
fish	Yes	No	Yes	No

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

rock salmon: any of several coarse fishes (such as dogfish or wolffish) when used as food
fish: the flesh of fish used as food

rock salmon (noun)	fish (noun)
any of several coarse fishes (such as dogfish or wolffish) when used as food	any of various mostly cold-blooded aquatic vertebrates usually having scales and breathing through gills
	the flesh of fish used as food

rock salmon (verb)	fish (verb)
	catch or try to catch fish or shellfish
	seek indirectly