Butterfly Mackerel

The juveniles of the butterfly kingfish are characterized by enormous pelvic fins that are longer than the length of its head, and that become more proportional as the fish grows. At any size, the pelvic fin tucks into a deep ventral groove, in much the same way as the first dorsal spines do in all scombrids. This species has very large cycloid scales, below which is a thick layer of fat. The swim bladder has two anterior projections that extend into the back of the skull, near the inner ear. This fish lacks the median keel on the caudal peduncle – it only has the characteristic pair of small keels on each side of the base of the caudal fin, as do other scombrids. It has 21 precaudal vertebrae, plus 23 caudal vertebrae.

This fish can be found around the world in southern temperate waters of 8–15 °C (46–59 °F), but most commonly under 10 °C (50 °F), and at depths to 200 m (660 ft) in the open ocean. It grows to a length of 1.64 m (5.4 ft) with a weight of 50 kg (110 lb), possible up to 70 kg (150 lb). Most bony fishes are ectothermic, or cold-blooded, but this species, much like the related tunas, is endothermic and is able to raise its body temperature to achieve a degree of thermoregulation. It has a brain heater organ derived from the lateral rectus eye muscle, which is distinct from that of the billfishes, whose heater is derived from their superior rectus muscles.

The butterfly kingfish has a circumglobal distribution in southern temperate waters. It has caught as far north as Hawaii and seems to be common around Oceania. It prefers waters between 8 and 15 °C (46 and 59 °F).

Gasterochisma derives from the Ancient Greek: γαστήρ, romanized: (gaster), lit. 'stomach', and χίασμα (chiasma) "crossing; X-shaped; sign of the 'X'".