Japanese jack mackerel
(Trachurus japonicus)

The Japanese jack mackerel (Trachurus japonicus), also known as the Japanese horse mackerel or Japanese scad, is a species named after its resemblance to mackerel but which is in the family Carangidae, the jacks, pompanos, trevallies and scads.

Their maximum reported length is 50 cm (20 in) with a common length of 35 cm (14 in). They have a maximum reported weight of 0.66 kg (1.5 lb) and a maximum reported age of 12 years.

They are found around the coast of Japan, apart from Okinawa Island, usually on sandy bottoms of 50–275 m (164–902 ft) deep. They feed mainly on small crustaceans such as copepods, and shrimps and small fish. They are similar to the yellowtail horse mackerel around New Zealand and Australia, apart from having more gill rakers and larger eyes.

It is greyish blue on its back, while having a silver color on the underside of the fish with a black blotch the gills. The dorsal and pectoral fins are a dusky color whereas the pelvic and anal fins are pale. One of the main distinguishing features of the Japanese Jack Mackerel is its bright yellow caudal fin which stands out compared to the rest of the species coloration.

Its scales are common among other species in its family, being dense in a horizontal and coarse in a vertical direction with a bend in the midpoint of the scale to follow the curvature of the animal. Scales on anterior half of the species have this curve, while those on the posterior half are straight, with any scales in between these two sections being pointed.

The spine of the species consists of 14 vertebrae connected to the dorsal fin, with another 10 vertebrae on the underside of the animal all with fine ridges lining them, common with other species in the family Trachurus.

The dorsal fin is split into two sections, the front set consisting of eight bony spines, and the rear set having one bony spine and another 25 – 27 soft rays, depending on the size of the individual. All these fins are controlled by 3 muscles and many ligaments. The caudal fin on the side of jack mackerel is a much more complex structure of about 12 cm with a series of 6 bones, more than 20 soft rays projecting outwards and 6 muscles controlling its movement. The anal fin is made up of a collection of 3 spines with 26 – 30 soft rays coming from them.

The jaw of the Japanese Jack Mackerel has a projected lower section and is lined with 4 separate segments of small conical teeth with medially bent tips which become larger moving towards the back of the jaw.