Tips for distinguishing fish: What are the main differences between mackerel and Spanish mackerel? Both mackerel and Spanish mackerel are seawater fish. The body shapes are basically similar, so they are often mistaken for the same fish, commonly known as mackerel. The main differences between the two are: Judging by body height: the mackerel has a taller body and is oval-shaped; the Spanish mackerel has a longer body and is round. Distinguish from the dorsal fin: the two dorsal fins of the mackerel are far apart; the two dorsal fins of the mackerel are close to each other. Identification from the tail fin: There are 5 small adipose fins on the upper and lower sides of the tail fin of the mackerel; there are 8-9 small adipose fins on the upper and lower sides of the tail fin of the mackerel. Identification by color: The back of the mackerel is blue-black with irregular dark blue markings, and the belly is light yellow; the back of the mackerel is black-blue with many black round spots, the belly is silver-gray, and the belly is off-white.