cusk vs saltwater fish

cusk and saltwater fish both are nouns.

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

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

cusk (noun)	saltwater fish (noun)
large edible marine fish of northern coastal waters; related to cod	flesh of fish from the sea used as food
elongate freshwater cod of northern Europe and Asia and North America having barbels around its mouth
the lean flesh of a cod-like fish of North Atlantic waters