The green jack (Caranx caballus), also known as the horse jack, is an abundant species of coastal marine fish in the jack family Carangidae.

DESCRIPTION

• The green jack is moderately large in size, growing to a maximum confirmed length of 55 cm and 2.81 kg in weight.

• The green jack is morphologically similar to a number of other carangids, having an elongated, moderately compressed fusiform body with dorsal and ventral profiles of approximately equal convexity and a slightly pointed snout.

• The posterior section of the eye is covered by a moderately well developed adipose eyelid, and the posterior extremity of the jaw is vertically under the center of the eye.

• The dorsal fin is in two parts, the first consisting of 8 spines and the second of 1 spine followed by 22 to 25 soft rays. The anal fin consists of 2 anteriorly detached spines followed by 1 spine and 16 to 21 soft rays.

• The lobes of both the second dorsal and anal fin are slightly elongated and almost entirely covered in small scales, but are still much shorter than the head length.

• The lateral line has a pronounced but short anterior arch, with the curved section intersecting the straight section below the spine of the second dorsal fin. The straight section contains 0 to 7 scales followed by 42 to 56 very strong scutes, and 43 to 47 scales overall.

• The chest is completely scaled. The upper jaw contains an irregular series of outer canines with an inner band of small, regularly spaced teeth, while the lower jaw contains a single band of small teeth. The species has 40 to 45 gill rakers in total; 10 to 15 on the upper limb and 27 to 30 on the lower limb, with this the only feature that differs between C. caballus and C. crysos. There are 25 vertebrae present.

• The green jack has a greenish blue colour overall, with an olive green to dark blue back and a golden to grey coloured belly, with a distinct back blotch on the outer edge of the operculum.

• Individuals in schools often have a very evident pearly white marking near this black blotch. Juveniles have 7 dark vertical stripes on their flanks which fade with age.

• The caudal fin is grey with dark tips, with all other fins being light grey to hyaline in colour.

DISTRIBUTION AND HABITAT

• The green jack is distributed throughout the coastal waters of the tropical and subtropical waters of the eastern Pacific Ocean.

• Its range extends along the coast of the Americas from Santa Cruz Island, California, south through Mexico and Central America, and down to Peru. The species also inhabits a number of offshore islands including the Galápagos Islands.

• The green jack inhabits a variety of continental shelf environments to a depth of at least 100 m, predominantly inshore reef systems, as well as shallow bays, estuaries and lagoons.

• It leads both a demersal and pelagic lifestyle, forming large schools which can move large distances offshore, allowing them to reach offshore island as well as deep, isolated seamounts.

 BEHAVIOUR

• The green jack is a gregarious species, forming moderately large schools of fish in the marine environment. Smaller shoals are formed by juveniles when entering shallower waters including bays and estuaries.

• The species is predatory in nature, taking small fish, cephalopods, crabs, shrimps and other crustaceans, either in midwater or on the sea floor.

• Green jack are also known to consume zooplankton, especially in deeper waters around islands and seamounts where the plankton is aggregated.

• It is an important part of the ecology, not only as a predator but as prey. It is known to be taken by larger fish such as marlin and sharks, as well a variety of other animals.

LIFE CYCLE

• Spawning appears to take place from May to October, with peaks in June and October.

• Larval growth has been extensive described, with the larvae identifiable by their pigmentation pattern and meristic count of 25 vertebrae.

• The body shape of early larvae resembles that of Chloroscombrus orquetu, however, differences in pigmentation, particularly on the dorsal body margin.

• The females and males grow at the same rate; 16.82 cm after the first year, 27.78 after the second, 34.66 after the third, with modelling suggesting the maximum length of 55 cm is reached at around 8 years of age.

• Most individuals reach sexual maturity before they reach 38 cm in length. Juveniles tend to move to protected waters such as estuaries, moving to deeper waters at adulthood.

RELATIONSHIP TO HUMANS

• The green jack is one of the most abundant species of fish within its range, and as such is quite important to fisheries throughout its range. It is taken by pelagic trawls, inshore gill nets, purse nets, and seines, as well as hook and line gear.