pollack vs saltwater fish

pollack and saltwater fish both are nouns.

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

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

pollack (noun)	saltwater fish (noun)
important food and game fish of northern seas (especially the northern Atlantic); related to cod	flesh of fish from the sea used as food
lean white flesh of North Atlantic fish; similar to codfish