The koheru, Decapterus koheru, is a jack of the genus Decapterus, found only between the North Cape and the East Cape of the North Island of New Zealand, in mid-waters of reef and offshore areas. Its length is between 25 and 50 cm.
The koheru is a streamlined cylindrical-shaped schooling fish, with pelagic keels on each side of the thin caudal peduncle edged with a row of sharp scutes. The mouth has minute teeth in the flexible jaws allowing the mouth to form a plankton sucking tube.
Koheru are electric blue on the back tinged with green, and silver-white on the belly with a prominent yellow stripe running along the back and on to the caudal peduncle. The stripe can be suppressed by the fish at will. When a koheru school is threatened by a large predator it often dives and uses the reef for protection - unusual behaviour for schooling fish.