pollock vs saltwater fish

pollock and saltwater fish both are nouns.

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

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

pollock (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