Giant trevally

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Giant trevally

Least Concern  (IUCN 3.1)[1]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Carangiformes
Family: Carangidae
Genus: Caranx
Species:
C. ignobilis
Binomial name
Caranx ignobilis
(Forsskål, 1775)
Approximate range of the giant trevally: dark blue (typical range), light blue (two known specimens)
Synonyms
  • Scomber ignobilis
    Forsskål, 1775
  • Caranx lessonii
    Lesson, 1831
  • Caranx ekala
    Cuvier, 1833
  • Carangus hippoides
    Jenkins, 1903
  • Caranx sansun
    (Forsskål, 1775)

The giant trevally (Caranx ignobilis), also known as the lowly trevally, barrier trevally, ronin jack, giant kingfish, or ulua, is a

Galápagos), but whether the species will become established there remains to be seen.[2]

The giant trevally is distinguished by its steep head profile, strong tail

estuaries, shallow bays, and lagoons as a juvenile to deeper reefs, offshore atolls, and large embayments as an adult. Juveniles of the species are known to live in waters of very low salinity such as coastal lakes and upper reaches of rivers, and tend to prefer turbid
waters.

The giant trevally is an 

molluscs make up a considerable part of their diets in some regions. Their novel hunting strategies include shadowing monk seals to pick off escaping prey, using sharks
to ambush prey, and leaping to catch birds.

The giant trevally reproduces in the warmer months, with peaks differing by region.

lunar cycle, when large schools congregate to spawn over reefs and bays, with reproductive behaviour observed in the wild. The fish grow relatively fast, reaching sexual maturity
at a length around 60 cm at three years of age.

The giant trevally is both an important species to commercial

ciguatera
poisoning is common from eating the fish. Dwindling numbers around the main Hawaiian Islands have also led to several proposals to reduce the catch of fish in this region.

Taxonomy and phylogeny

The giant trevally is classified within the genus Caranx, one of a number of groups known as the jacks or trevallies. Caranx itself is part of the larger jack and horse mackerel family Carangidae, which in turn is part of the order Carangiformes.[3]

The giant trevally was first

junior synonyms.[8] The latter of these names once again highlighted the similarity with the crevalle jack, with the epithet hippoides essentially meaning "like Carangus hippos",[9] which was the crevalle jack's Latin name at that time. Despite the resemblance with the crevalle jack, the two species have never been phylogenetically compared, either morphologically or genetically, to determine their relationship.[citation needed
]

C. ignobilis is most commonly referred to as the giant trevally (or giant kingfish) due to its large maximum size, with this often abbreviated to simply GT by many anglers.[10] Other names occasionally used include lowly trevally, barrier trevally, yellowfin jack (not to be confused with Hemicaranx leucurus), Forsskål's Indo-Pacific jack fish and Goyan fish.[6] In Hawaii, the species is almost exclusively referred to as ulua, often in conjunction with the prefixes black, white, or giant.[11] Due to its wide distribution, many other names for the species in different languages are also used.[6] In the Philippines, the species is referred to as talakitok. Some success has been achieved in raising giant trevally commercially in small fish farms there, typically to an age of seven months.[citation needed]

Description

Profile of an adult giant trevally

The giant trevally is the largest member of the genus Caranx, and the fifth-largest member of the family Carangidae (exceeded by the yellowtail amberjack, greater amberjack, leerfish, and rainbow runner), with a recorded maximum length of 170 cm (67 in) and a weight of 80 kg (180 lb).[6] Specimens of 50 kg and above are not uncommon in sportfishing. The Hallaniyat Islands in southern Oman are the most prolific places for these large fish, with fish of 100 lb being caught almost daily. Unfortunately in recent times, the fishery has dwindled with commercial pressure and fish that size are becoming ever rarer throughout the islands.

The giant trevally is similar in shape to a number of other large jacks and trevallies, having an ovate, moderately compressed body with the

vertebrae are present.[15]
The eye is covered by a moderately well-developed adipose eyelid, and the posterior extremity of the jaw is vertically under or just past the posterior margin of the pupil.[15] The eye of the giant trevally has a horizontal streak in which ganglion and photoreceptor cell densities are markedly greater than the rest of the eye. This is believed to allow the fish to gain a panoramic view of its surroundings, removing the need to constantly move the eye, which in turn will allow easier of detection of prey or predators in that field of view.[16]

At sizes less than 50 cm, the giant trevally is a silvery-grey fish, with the head and upper body slightly darker in both sexes.

turbid waters often have yellowish fins, with the anal fin being the brightest.[15] The leading edges and tips of the anal and dorsal fins are generally lighter in colour than the main part of the fins. There is no black spot on the operculum.[12] Traces of broad cross-bands on the fish's sides are occasionally seen after death. The fishes have been known to prey and eat on the dead fish.[18]

Distribution

A mature male giant trevally showing the black colouration common in these older fish

The giant trevally is widely distributed throughout the

Perth. Elsewhere in the Indian Ocean, the species has been recorded from hundreds of small island groups, including the Maldives, Seychelles, Madagascar and the Cocos (Keeling) Islands.[6]

The giant trevally is abundant in the central Indo-Pacific region, found throughout all the archipelagos and offshore islands including Indonesia, the

Pitcairn and Hawaiian Islands.[11][14]

Habitat

A giant trevally patrolling a reef in Hawaii

The giant trevally inhabits a very wide range of

demersal in nature.[21]
The species is most common in shallow coastal waters in a number of environments, including coral and rocky reefs and shorefaces, lagoons, embayments, tidal flats and channels. They commonly move between reef patches, often over large expanses of deeper sand and mud bottoms between the reefs.[22][23] Older individuals tend to move to deeper seaward reefs, bomboras and drop-offs away from the protection of fringing reefs, often to depths greater than 80 m.[24][25] Large individuals, however, often return to these shallower waters as they patrol their ranges, often to hunt or reproduce.[25] In Hawaii, the juvenile to subadult giant trevally is the most common large carangid in the protected inshore waters, with all other species apparently preferring the outer, less protected reefs.[26] It is also easily attracted to artificial reefs, where studies have found it to be one of the predominant species around these structures in Taiwan.[27]

Juvenile to subadult giant trevally are known to enter and inhabit estuaries, the upper reaches of rivers and coastal lakes in several locations, including South Africa,

turbid, dirty water to clean, high visibility waters, but in most cases, the species prefers the turbid waters.[22] Younger fish apparently actively seek out these turbid waters, and when no estuaries are present, they live in the turbid inshore waters of bays and beaches. These young fish eventually move to inshore reefs as they mature, before again moving to deeper outer reefs.[26]

In the Philippines, a population of giant trevally inhabit (and were once common in) the landlocked fresh waters of the formerly saltwater Taal Lake, and are referred to as maliputo to distinguish them from the marine variant (locally named talakitok). Along with Taal Volcano and Taal Lake, the maliputo is prominently featured on the reverse side of the newly redesigned Philippine 50 peso bill.[36]

Biology and ecology

The giant trevally is a solitary fish once it reaches

sharksucker, Echeneis naucrates, a fish which is normally seen attached to the undersides of sharks.[41]

Diet and feeding

Frontal view of a giant trevally illustrating the compressed form of the species

The giant trevally is a powerful

amphipods also of importance, and they are known to take more novel prey, such as spiders and insects, in these habitats.[22][45] Juvenile turtles and dolphins were reported being found within the stomach contents of larger giant trevally.[46] Studies of different size classes of fish have found their diets change with age in some locations, with the changes relating to an increased volume of fish taken.[35]

Giant trevally also feed on fledgling sooty terns on Farquhar Atoll snatching them from the water surface and even jumping acrobatically to catch them in the air as seen on the BBC documentary Blue Planet II, episode 1.[47] So far this behaviour has not been observed elsewhere.[citation needed]

Studies in controlled environments on the giant trevally's feeding strategies have found

prey fish becomes isolated, with the main advantage of schooling appearing to be the ability to further break up and isolate prey schools. The only time hunting in schools is a disadvantage is when only isolated prey are present, such as close to a reef; here, an individual hunter has a greater probability of capturing it than if a group is present.[48] Another hunting strategy of the giant trevally is to 'escort' monk seals, a behavior which has been observed near the Hawaiian Islands. The trevally swim close by the seal, and when the seal stops to forage, the trevally positions its mouth inches away from the seal. If a prey item is disturbed, the trevally will attempt to steal the prey from the seal, which routinely does occur. The seal does not appear to gain any benefit from this relationship, and it is thought juvenile seals being followed in this way may be outcompeted by the larger fish.[49] A similar strategy has been employed by fish in the presence of large reef sharks, as they use the larger animal as a tool to ambush prey.[46] The opportunistic nature of giant trevally has also been made evident by studies on the mortality rate of undersized or egg-bearing lobsters released from traps at the water's surface of the Hawaiian Islands. The fish are efficient predators of these crustaceans, with individuals often seizing a lobster before it could sink to the seafloor after being released, or attacking before the lobster moves into a defensive position. Some bolder, large individuals are even known to eat the lobster head first when it is in a defensive stance.[50]

Life history

A school of subadult giant trevally in a sandy bay, Hawaii

The giant trevally reaches

pelagic and transparent in nature.[51]

The giant trevally's early

von Bertalanffy growth curves fitted to observed otolith data show an individual of around 1 m in length is about eight years old, while a 1.7 m fish would be around 24 years old. The maximum theoretical length of the species predicted by the growth curves is 1.84 m,[21] but the largest reported individual was 1.7 m long. As previously mentioned, as the giant trevally grows, it shifts from turbid inshore waters or estuaries to reefs and lagoons in bays, moving finally to outer reefs and atolls.[23]
A hybrid of C. ignobilis and C. melampygus (bluefin trevally) has been recorded from Hawaii. The specimen was initially thought to be a bluefin trevally of world-record size, but was later rejected when it was discovered to be a hybrid. Initial evidence of hybridisation was morphological characteristics intermediate to the two species; later genetic tests confirmed it was indeed a hybrid. The two species are known to school together, including at spawning time, which was considered to be the reason for hybridisation.[52]

Relationship to humans

A giant trevally taken by a soft plastic lure

The giant trevally has been used by humans since prehistoric times, with the oldest known records of the capture of this species by Hawaiians, whose culture held the fish in high regard. The ulua, as the fish is known to Hawaiians, was likened to a fine man and strong warrior, which was the cause of a ban on women eating the species in antiquity.[53] The species was often used in Hawaiian religious rites, and took the place of a human sacrifice when none was available. Culturally, the fish was seen as a god, and treated as gamefish which commoners could not hunt. There are many mentions of ulua in Hawaiian proverbs, all generally relating to the strength and warrior-like qualities of the fish.[54] The Hawaiians considered the fish to be of excellent quality, with white, firm flesh. Despite this, intrusions of giant trevally into modern-day fishponds used by Hawaiians for rearing fish are unwelcome; being a predator, it eats more than it is worth at market.[53]

The giant trevally is of high importance to modern fisheries throughout its range, although quantifying the amounts taken is very difficult due to the lack of fishery statistics kept in most of these countries. Hawaii has the best-kept statistics, where the 1998 catch consisted of 10,194 pounds of giant trevally worth around US$12,000.

fishmeal and oil.[15]

The giant trevally is considered one of the top

intestinal tract produced no or weakly positive symptoms to laboratory mice, but the digested contents of the intestine were lethal. The authors argued, based on this test, the flesh of giant trevally was safe to consume. However, analysis of case studies in which ciguatera poisoning was reported after eating the fish suggested an accumulative effect occurs with repeated consumption; and tests like the one outlined above are not reliable, as the toxin appears to be distributed haphazardly throughout each fish.[59] Since 1990, giant trevally taken from the main Hawaiian islands have been blocked from sale by auction internationally due to concerns over liability from ciguatera poisoning.[54]

Conservation

Scuba diver interacting with a giant trevally at Guam's Gab Gab II reef

A decline in giant trevally numbers around inhabited regions has been well documented in Hawaii, with both catch data as presented above and ecological studies showing this decrease in numbers. A

IUCN.[citation needed
]

Some recreational fishing groups are also promoting a

taxidermist if it is a trophy fish. A catch and release approach has also been adopted by operators outside Hawaii, with Australian operators who target the species by popping and jigging rarely keeping any fish.[10] Careful fish handling techniques have also been implemented by anglers so as not to damage the fish; such techniques include supporting the fish's weight, using barbless single, rather than treble, hooks and restricting the time the fish spends out the water to a minimum.[46]

References

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