common mackerel vs mackerel

common mackerel and mackerel both are nouns.

Word	Noun	Adjective	Verb	Adverb
common mackerel	Yes	No	No	No
mackerel	Yes	No	No	No

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

common mackerel: important food fish of the northern Atlantic and Mediterranean; its body is greenish-blue with dark bars and small if any scales
mackerel: any of various fishes of the family Scombridae

common mackerel (noun)	mackerel (noun)
important food fish of the northern Atlantic and Mediterranean; its body is greenish-blue with dark bars and small if any scales	any of various fishes of the family Scombridae
	flesh of very important usually small (to 18 in) fatty Atlantic fish