young fish vs offspring

young fish and offspring both are nouns.

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

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

Other hypernyms of young fish include young, fish.

young fish (noun)	offspring (noun)
a fish that is young	any immature animal
	something that comes into existence as a result
	the immediate descendants of a person