The Collaborative International Dictionary of English v.0.48:

mackerel scad \mackerel scad\, mackerel shad \mackerel shad\n.
   (Zool.)
   A small silvery fish (Decapterus macarellus) found from
   Nova Scotia to Brazil.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

Decapterus macarellus
    n 1: small silvery fish; Nova Scotia to Brazil [syn: mackerel
         scad, mackerel shad, Decapterus macarellus]