young fish vs whitebait

young fish and whitebait both are nouns.

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

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

Other hyponyms of young fish include parr, brit, britt, parr.

young fish (noun)	whitebait (noun)
a fish that is young	the edible young of especially herrings and sprats and smelts
	minnows or other small fresh- or saltwater fish (especially herring); usually cooked whole