pilchard vs saltwater fish

pilchard and saltwater fish both are nouns.

Word	Noun	Adjective	Verb	Adverb
pilchard	Yes	No	No	No
saltwater fish	Yes	No	No	No

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

pilchard (noun)	saltwater fish (noun)
small fishes found in great schools along coasts of Europe; smaller and rounder than herring	flesh of fish from the sea used as food
small fatty fish usually canned