young fish vs young

young fish and young both are nouns.

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

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

Other hypernyms of young fish include offspring, fish.

young fish (noun)	young (noun)
a fish that is young	any immature animal
	young people collectively

young fish (adjective)	young (adjective)
	(used of living things especially persons) in an early period of life or development or growth
	(of crops) harvested at an early stage of development; before complete maturity
	not tried or tested by experience
	being in its early stage
	suggestive of youth; vigorous and fresh