Golden trevally

Appearance

The golden trevally is a relatively large fish, growing to a maximum recorded size of 120 cm in length and 15.0 kg in weight. It is similar to most other trevallies and jacks in having a compressed, oblong body, with the dorsal profile slightly more convex than the ventral profile, particularly anteriorly. The species' mouth is one of its defining features; the mouth is highly protractile and fleshy, with specimens greater than 90 mm having no teeth on the jaws, vomer or tongue. Smaller individuals have a series of small villiform teeth in both jaws. The dorsal fin is in two parts, the first with 7 spines, the second with 1 spine and 18 to 20 soft rays. The anal fin has 2 detached spines followed by 1 spine and 15 to 17 soft rays, while the pelvic fin consists of 1 spine and 19 to 20 soft rays. The curved part of the lateral line is moderately arched; containing 62 to 73 scales, and approximately equal in length to the straight section containing 15 to 27 scales and 18 to 25 scutes. The breast is completely scaled. There are 27 to 30 gill rakers and 24 vertebrae in total.

The golden trevally's colour is the species most prominent distinguishing feature, and for which it acquired its common names. Juveniles are a bright golden yellow colour over their entire body and all fins, with 7 to 11 black vertical crossbars all over their body. These bars generally alternate between broad and narrow. The caudal fin lobes have dark tips and there is a prominent black edge to the operculum. As the fish grows, the body becomes more silver to silvery golden and the cross bars fade or disappear, often replaced by dark blotches. The fins remain yellow, often with greenish tinges. The dark edge of the operculum also fades with age.

Distribution

The golden trevally is widely distributed throughout the tropical and subtropical waters of the Indian and Pacific Oceans. In the Indian Ocean, the species is distributed from South Africa along the east African coastline, including the Red Sea and Persian Gulf. Its distribution extends east along the Indian and South East Asian coastlines, and south through Indonesia and northern Australia. Golden trevally are recorded from many Indian Ocean islands including Madagascar, The Seychelles and The Maldives. In the Pacific, the species is spread throughout the South East Asian and Indonesian archipelago north mainland China and Japan and south to eastern Australia and New Zealand. Golden trevally have been recorded from many central Pacific Islands, including Hawaii, with their distribution extending to central America. Here its range extends from the Gulf of California in the north to Colombia in the south.

Behavior

The golden trevally is found either as a solitary individual or in small schools as an adult. Juveniles tend to form larger schools which tend to congregate and follow larger fish such as groupers, sharks, and even jellyfish. This behaviour mimics that of the related pilot fish, "Naucrates ductor", with their maneuverability protecting them from their hosts, which in turn provides them with protection from predation from other fish. This behaviour extends to scuba divers, with one diver reporting a single young individual obsessively stationing itself in front of his face plate.

The golden trevally is a diurnal foraging carnivore which, unlike other carangids, does not normally seek out individual prey items. The highly protractile mouth possessed by the species is used to form a tube to suck prey out of both reef- and algae-dominated habitats, as well as filtering organisms out of sandy substrates. In the latter case, both sand and any prey items are taken into the mouth and filtered through the gill rakers; sand is expelled, while small organisms are trapped and swallowed. The species takes a variety of prey including crustaceans such as shrimp, crabs, and amphipods, as well as molluscs and small fish. Golden trevally found inhabiting a mangrove swamp were found to have fed exclusively on fish, suggesting the species also actively hunts down fleeing prey. A laboratory study using only four individuals being fed found that one fish will take the 'lead' position in this situation while another will attack the other fish in the school, apparently in competition for food.

A variety of parasites have been recorded from the species, including copepods, flatworms, and a nematode which inhabits the fish's swimbladder.

Reproduction in the golden trevally has been studied in both the Indian and Pacific Oceans. In Hawaii, spawning occurred from late February to early October, with a peak from late April to early September. Five distinct peaks during this time were correlated with the first and third quarters of the moon. Spawning occurred from the early evening into the night. In the Indian Ocean, in the southern Persian Gulf, spawning occurred in April and May, with defined peaks in recruitment of juveniles into the local fishery during September and October....hieroglyph snipped... The male:female sex ratio in this population was 1:1.01, close to parity. Growth rates were also studied using otoliths in this population, with an increase in growth rate during the winter. Von Bertalanffy growth curves were also calculated for the species.

Habitat

The golden trevally predominantly occupies inshore waters of varying substrate, although is known to occur on deeper continental shelf reefs in Australia. In coastal areas the species inhabits rocky and coral reefs as well as open sand flats where it forages for food. A systematic study in northern Australia indicated it to be one of the only species to be approximately equally distributed in both reef and soft-bottom habitats. Golden trevally appear to prefer clear water to turbid waters, and thus is only encountered rarely in low turbidity estuarine environments. One known exception to this was the capture of several individuals in a shallow mangrove swamp in Baja California which appeared to be foraging for prey.The golden trevally is found either as a solitary individual or in small schools as an adult. Juveniles tend to form larger schools which tend to congregate and follow larger fish such as groupers, sharks, and even jellyfish. This behaviour mimics that of the related pilot fish, "Naucrates ductor", with their maneuverability protecting them from their hosts, which in turn provides them with protection from predation from other fish. This behaviour extends to scuba divers, with one diver reporting a single young individual obsessively stationing itself in front of his face plate.

The golden trevally is a diurnal foraging carnivore which, unlike other carangids, does not normally seek out individual prey items. The highly protractile mouth possessed by the species is used to form a tube to suck prey out of both reef- and algae-dominated habitats, as well as filtering organisms out of sandy substrates. In the latter case, both sand and any prey items are taken into the mouth and filtered through the gill rakers; sand is expelled, while small organisms are trapped and swallowed. The species takes a variety of prey including crustaceans such as shrimp, crabs, and amphipods, as well as molluscs and small fish. Golden trevally found inhabiting a mangrove swamp were found to have fed exclusively on fish, suggesting the species also actively hunts down fleeing prey. A laboratory study using only four individuals being fed found that one fish will take the 'lead' position in this situation while another will attack the other fish in the school, apparently in competition for food.

A variety of parasites have been recorded from the species, including copepods, flatworms, and a nematode which inhabits the fish's swimbladder.

Reproduction in the golden trevally has been studied in both the Indian and Pacific Oceans. In Hawaii, spawning occurred from late February to early October, with a peak from late April to early September. Five distinct peaks during this time were correlated with the first and third quarters of the moon. Spawning occurred from the early evening into the night. In the Indian Ocean, in the southern Persian Gulf, spawning occurred in April and May, with defined peaks in recruitment of juveniles into the local fishery during September and October....hieroglyph snipped... The male:female sex ratio in this population was 1:1.01, close to parity. Growth rates were also studied using otoliths in this population, with an increase in growth rate during the winter. Von Bertalanffy growth curves were also calculated for the species.