young fish vs brit

young fish and brit both are nouns.

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

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

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

young fish (noun)	brit (noun)
a fish that is young	the young of a herring or sprat or similar fish
	minute crustaceans forming food for right whales