Identification
Most important of the differences, anatomically, is the fact that the hardhead has a well-developed swim bladder attached with the esophagus, which the "true mackerels" in the Scomber genus lack. But it is not necessary to open the fish to identify it for there is a characteristic color difference between them, the Atlantic being silvery-sided below the mid line, whereas the lower part of the sides of the hardhead (otherwise colored somewhat like the Atlantic) are mottled with small dusky blotches, and the chub has a larger eye than the Atlantic. Less obvious differences are that the dorsal fins are closer together in the chub and that there are only 9 or 10 spines in its first dorsal fin instead of 11 or more, which is the usual count in the Atlantic mackerel. In most species the mackerel is known to travel in large schools.
Read more about this topic: Chub Mackerel