Mackerel is a common name applied to a number of different species of fish, mostly, but not exclusively, from the family Scombridae. They occur in all tropical and temperate seas. Most live offshore in the oceanic environment but a few, like the Spanish mackerel (Scomberomorus maculatus), enter bays and can be caught near bridges and piers. Common features of mackerels are a slim, cylindrical shape (as opposed to the tunas which are deeper bodied) and numerous finlets on the dorsal and ventral sides behind the dorsal and anal fins.
The Century Dictionary and Cyclopedia (New York, NY: The Century Co., 1889)