young fish vs fish

young fish and fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
young fish	Yes	No	No	No
fish	Yes	No	Yes	No

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

young fish: a fish that is young
fish: any of various mostly cold-blooded aquatic vertebrates usually having scales and breathing through gills

Other hypernyms of young fish include offspring, young.

young fish (noun)	fish (noun)
a fish that is young	any of various mostly cold-blooded aquatic vertebrates usually having scales and breathing through gills
	the flesh of fish used as food

young fish (verb)	fish (verb)
	catch or try to catch fish or shellfish
	seek indirectly