mackerel vs saltwater fish

mackerel and saltwater fish both are nouns.

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

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

mackerel: flesh of very important usually small (to 18 in) fatty Atlantic fish
saltwater fish: flesh of fish from the sea used as food

mackerel (noun)	saltwater fish (noun)
any of various fishes of the family Scombridae	flesh of fish from the sea used as food
flesh of very important usually small (to 18 in) fatty Atlantic fish