Greater amberjack

Appearance

The "greater amberjack" is a large predatory fish which has a body colouring which varies from brownish to bluish-grey on the dorsal surfaces contrasting with the silvery-white underparts. A diagonal sooty stripe starts at the snout and runs along the centre of the back dorsal fin; another dark stripe runs from the upper jaw, across the eye to in front of the first dorsal fin. Some fish may show a light yellow to reddish-brown stripe along the flanks. The fins are dusky in colour. The second dorsal and anal fins have a low anterior lobe. Small juveniles have clear fins and a series of five vertical bands along the body and a sixth band on the caudal peduncle. The shape of the body is elongated and fusiform and it is of moderate depth and laterally compressed, and has a covering of small cycloid scales. The largest fish have been measured at 190 cm in total length, but the more typical length found is 100 cm, while the largest published weight is 80.6 kg.

Naming

The greater amberjack was formally described as "Caranx dumerili" by French naturalist Antoine Risso in 1810, with the type locality given as Nice, France. The binomial consists of the generic name, which is the Italian name for this species, while the specific name is patronymic, the identity of the person honoured in the patronym was not identified by Risso, but is almost certainly French zoologist André Marie Constant Duméril, who was the father of another well-known French zoologist Auguste Duméril. It is the type species of the genus "Seriola"

Distribution

The greater amberjack has an almost circumglobal distribution in the world's subtropical and tropical seas and oceans. In the Indian Ocean, it is found along the African coast from South Africa east through the Persian Gulf to Western Australia and southern Japan, reaching the Hawaiian Islands and Micronesia in the Pacific Ocean. It reaches as far south as the island of Tasmania off the coast of Australia. In the western Atlantic Ocean, this species is found around Bermuda and on the North American coast as far north as Nova Scotia extending south as far as Brazil, including the Gulf of Mexico and the Caribbean Sea. In the eastern Atlantic Ocean, it has been recorded as a vagrant as far north as the British Isles and is found off in the Bay of Biscay south to Morocco and in the Mediterranean Sea. It may occur along the west African coast, but may be confused with the similar "Seriola carpenteri".

Habitat

The greater amberjack is found as solitary individuals or in small to moderate-sized schools, which are epibenthic and pelagic and occur in the vicinity of reefs, deep offshore caves, drop-offs, rocky outcrops, and deep seaward reefs. It sometimes enters coastal bays, while the juveniles are infrequently recorded quite far well out to sea where they shelter among floating algae, such as "Sargassum", and debris. It can also be found over wrecks. Smaller individuals, less than 3 kg, can be caught in shallow water, while the larger fish prefer deeper water, normally 18–72 m and have been recorded at depths of 360 m.

Reproduction

"S. dumerli" is gonochoric, meaning that the males and females are separate and determined at birth, and no sexual dimorphism exists other than size. The sexes begin to differentiate around 4–5 months of age when they attain a length of 24–26 cm. In the Mediterranean, they reach sexual maturity at 4 and 5 years of age when they attain a length around 109 and 113 centimetres in length in males and females. In western Atlantic populations, the males are mature after three years and 80 cm and the females after four years of age and attaining 83 cm. This species has been observed to show pair courtship off Belize where schools numbering around 120 individuals were found and pairing mainly occurred during when the Moon was either full or waning and between February and October. Half of the males are sexually mature at 64.6 cm fork length for males; half of the females are sexually mature on attaining 73.3 cm fork length. This is a highly productive fish; a single female may lay 18 to 59 million eggs in a single spawning season. Off the coast of Florida, spawning peaks in April and May, each female is estimated to spawn once every four to five days during the spawning season, lasting around 60 days. In the Mediterranean, spawning takes place in June and July. Spawning occurs over such habitats as reefs and shipwrecks, demonstrated by the abundance of juveniles in these habitats in the summer following the spring spawning in the western Atlantic.

The eggs are 1.9 mm in diameter, and after spawning, the embryos take around 40 hours to develop at 23 °C, and the newly hatched 2.9-millimetre larvae take 31–36 days to develop into juveniles. They can live up to 17 years.

Food

Greater amberjacks are opportunistic predators when they are adults, which prey on benthic and pelagic fishes, as well as cephalopods and crustaceans. Common fish prey species include the bigeye scad and sardines. The juvenile fish feed on plankton, including the larvae of decapods and other small invertebrates. The juveniles switch to feeding on larger prey such as larger benthic and nektonic organisms when they attain a length of 8–12 cm, and once they have grown larger than 12 cm, they restrict their prey choice to animals that swim on or over the substrate; when they attain 20 cm, they are mainly piscivorous and at this size, they move from open waters to more coastal areas.

Predators

Greater amberjack are preyed on by larger fishes including the yellowfin tuna and European hake, and seabirds including the brown noddy and sooty tern. Tapeworms are occasionally recorded as infesting this species, these worms being harmless to humans, albeit rather unappetising. The monogenean "Zeuxapta seriolae" has been documented as living as a parasite on the gills of these fish.