Mackerel Tuna:
Most but not all have one to nine, and sometimes even more, small black spots or blotches between the pelvic fin and the pectoral fin. They can also be distinguished from the Frigate Mackerel by the lack of space between the two dorsal fins, the Mackerel Tunas first dorsal fin extends nearly all the way to the second dorsal fin where as the Frigate Mackerel's first dorsal fin is distinctly separate from the second dorsal fin.
Mackerel Tuna with 2 spots:
Frigate Mackerel:
Mackerel Tuna: