The Spanish mackerel has the outline of the slender mackerel rather than of the stout bonito, its body being nearly 4½ to 5 times as long as it is deep. But there is no danger of confusing it with either of the true mackerels, first, because its two dorsal fins (like those of the bonitos) are hardly separated, and second, because of its color pattern, Its high second dorsal, slender form, and spotted sides mark it off at first glance from our bonitos, while its color, slender form, long first-dorsal fin, and the outline of its second dorsal distinguish it from a small tuna. The most clear-cut distinction between the Spanish and its close relatives the king mackerel and the cavalla, is that the pectoral fins of the Spanish are naked but those of the last two are mostly covered with scales. The ventral fins, also, originate definitely behind the origin of the first dorsal in the Spanish, under it or only a very little rearward in the king; and the color differs.
The most distinctive anatomic character of the Spanish, king, and cavalla among our local scombroids is the large conical jaw teeth. The caudal peduncle of the spanish mackerel is keeled; its lateral line wavy; its first dorsal fin (17 to 18 [page 348] spined) is triangular; its second dorsal (14 to 18 rays) is concave and originates a short distance in front of the anal, which is similar to it in form and size. It has 8 or 9 dorsal and as many anal finlets. Its pectorals are naked; its caudal is deeply lunate, with the outer rays decidedly longer than those of the common mackerel. It has 32 teeth, or fewer, in each jaw.
The Spanish mackerel is dark bluish or blue green above, pale below, like all scombroids, and silvery, its sides marked with many small, oblong-oval, dull orange or yellowish, spots, both above the lateral line and below, these spots being a very diagnostic character. The fact that the membrane of the front one-third of its first dorsal fin is black (blue in the king, p. 348), whereas its rear part is greenish white, is an equally useful field mark. The second dorsal and pectoral fins are pale yellowish with dusky edges; the anal and the ventrals are white.
The maximum weight is about 9 or 10 pounds, maximum length about 36 inches, but the fish average less than 3 pounds as caught.
Both coasts of North America, north commonly as far as Chesapeake Bay in the Atlantic, and to Maine as a stray; south to Brazil.
The Spanish mackerel is a summer visitor all along the Atlantic coast of the United States, as far north as New York; less regularly along the southern coasts of New England, though a few are taken during most summers at Woods Hole. But it is only a stray in the colder waters of the Gulf of Maine, where occasional fish are taken in Cape Cod Bay every year or two. In 1896 the local catch rose to 37 fish (Provincetown and Truro traps), and there is record of it at Lynn, Mass. But Spanish mackerel are so rare north of this point that Monhegan Island is the only locality-record for Maine, and the most northerly known outpost for the species.
 A weight of 25 pounds is recorded by Smith (North Carolina Geol. Econ. Surv., vol. 2, 1907, p. 191) for a specimen observed in a Washington, D. C., fish market. If the identity was correct and this was not the closely related king mackerel Scomberomorus regalis it must be considered a case of giantism.