fish vs rock salmon

fish and rock salmon both are nouns.

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

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

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

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

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