The Collaborative International Dictionary of English v.0.48:

Mackerel \Mack`er*el\, n. [OF. maquerel, F. maquereau (LL.
   macarellus), prob. for maclereau, fr. L. macula a spot, in
   allusion to the markings on the fish. See Mail armor.]
   (Zool.)
   Any species of the genus Scomber of the family
   Scombridae, and of several related genera. They are finely
   formed and very active oceanic fishes. Most of them are
   highly prized for food.
   [1913 Webster]

   Note: The common mackerel (Scomber scombrus), which
         inhabits both sides of the North Atlantic, is one of
         the most important food fishes. It is mottled with
         green and blue. The Spanish mackerel (Scomberomorus
         maculatus), of the American coast, is covered with
         bright yellow circular spots.
         [1913 Webster]

   Bull mackerel, Chub mackerel. (Zool.) See under Chub.


   Frigate mackerel. See under Frigate.

   Horse mackerel . See under Horse.

   Mackerel bird (Zool.), the wryneck; -- so called because it
      arrives in England at the time when mackerel are in
      season.

   Mackerel cock (Zool.), the Manx shearwater; -- so called
      because it precedes the appearance of the mackerel on the
      east coast of Ireland.

   Mackerel guide. (Zool.) See Garfish
   (a) .

   Mackerel gull (Zool.) any one of several species of gull
      which feed upon or follow mackerel, as the kittiwake.

   Mackerel midge (Zool.), a very small oceanic gadoid fish of
      the North Atlantic. It is about an inch and a half long
      and has four barbels on the upper jaw. It is now
      considered the young of the genus Onos, or Motella.

   Mackerel plow, an instrument for creasing the sides of lean
      mackerel to improve their appearance. --Knight.

   Mackerel shark (Zool.), the porbeagle.

   Mackerel sky, or Mackerel-back sky, a sky flecked with
      small white clouds; a cirro-cumulus. See Cloud.
      [1913 Webster]

            Mackerel sky and mare's-tails
            Make tall ships carry low sails.      --Old Rhyme.
      [1913 Webster] mackerel scad