Island trevally

Appearance

The island trevally is a moderately large fish, growing to a maximum recorded length of 75 cm and 6.61 kg in weight. The species is quite similar to ''C. ferdau'', although the prominent yellow spots serve as a quick identifying feature.

The island trevally has a compressed oblong shaped body with the dorsal profile more convex than the ventral profile and the snout usually being slightly rounded. The dorsal fin is in two sections, the first consisting of eight spines and the second of one spine and 28 to 31 soft rays, with the lobe of the second dorsal fin being slightly falcate in younger individuals, but is always shorter than the head length.

The anal fin has two detached spines followed by one spine attached to 24 to 26 soft rays, while the pelvic fin has one spine attached to 21 or 22 soft rays. The lateral line has a very slight anterior arch, with the intersection of the straight and curved sections between the 15th and 19th soft rays of the dorsal fin. The curved section contains 96 to 106 scales, while the straight section contains 20 to 30 scales and 21 to 28 scutes.

The breast is scaleless ventrally until the origin of pelvic fins, often with a small patch of prepelvic scales. Laterally, this naked area of the breast is separated from the naked base of the pectoral fins by a moderate band of scales. The lips are notably fleshy in adults, with both jaws containing narrow bands of villiform teeth which become obsolescent with age. There are 28 to 32 gill rakers in total and 24 vertebrae.

The island trevally is a pale blue-green above, becoming more silvery below, with adults having several quite large, elliptical, yellow to brassy spots scattered on their bodies close to the midline. Nine 9 or 10 dark vertical bars may be on the body from the head to the caudal peduncle. The soft dorsal, anal, and caudal fins are a pale brownish- to brilliant-blue, with all other fins being pale green to hyaline in colour....hieroglyph snipped...

Naming

The island trevally is classified within the genus ''Carangoides'', a group of fish commonly called jacks and trevallies. ''Carangoides'' falls into the jack and horse mackerel family Carangidae, the Carangidae are part of the order Carangiformes.

The species was first scientifically described by the American ichthyologists Jordan and Gilbert in 1881 based on a specimen taken from the Revillagigedo Islands, which was designated to be the holotype. They named this new species ''Caranx orthogrammus'', placing the species in a closely related genus of jacks, but this was later changed to ''Carangoides'' after further revision of the family had occurred.

The species was independently redescribed a number of times, with the fish placed in subspecies status twice. John Treadwell Nichols considered his ''C. ferdau jordani'' to be separate from ''C. orthogrammus'', or possibly a subspecies of the blue trevally, ''C. ferdau'', and was later treansferred to ''C. jordani''. It was also considered a subspecies or a synonym of the bludger, ''C. gymnostethoides'', and was also renamed as ''C. nitidus''.

The fish is now considered a separate species, with the subspecies names and the later names rendered invalid under the ICZN naming rules. The species is commonly known as island trevally in reference to its preferred offshore habitat, with the names thicklip trevally and false bluefin trevally also used as descriptive names. The specific epithet means "straight lined" in Greek.

Distribution

The island trevally is distributed throughout the offshore waters of the tropical and subtropical regions of the Indian and Pacific Oceans. In the Indian Ocean, it ranges from Mozambique on the east coast of Africa north to the Gulf of Aden, but has not been recorded further north until India and Sri Lanka. Its range extends throughout Southeast Asia, Indonesia, and northern Australia in the eastern Indian Ocean. In the Pacific, the species has been recorded from Taiwan, Japan, and Hawaii to the Revillagigedo Islands off Mexico in the eastern Pacific, as well as many Pacific islands.

As the species common name partly suggests, the island trevally very rarely is found on the continental shelves, instead living around offshore islands formed by volcanic activity or isolated slivers of continental material remaining from continental drifting. These islands are often quite large, and have most of the geographical features of the continental environment, including beaches, reefs, lagoons, and even estuaries. The island trevally is known from all of these habitats, as well as slightly deeper seaward reefs up to 50 m deep.
In Hawaii, the species is less frequently found inside protected bays, and more often slightly further offshore presumably due to competition with other species....hieroglyph snipped...
The island trevally is one of a number of carangid species known to be attracted to man-made fish-attracting devices, which are installed to aggregate fish for anglers....hieroglyph snipped...

Behavior

The island trevally moves both as a solitary individual or in small schools through its habitat, with the species often accounting for large proportions of an areas immediate biomass when moving in schools.

The species' diet consists of small fish species and crustaceans, with studies on their exact diet composition finding these fish take different species and different ratios of prey throughout their range.

Reproduction

Reproduction and growth are poorly studied in the species, with only a general estimate of spawning timing of March in the Solomon Islands.