The giant trevally, Caranx ignobilis (also known as the giant kingfish, lowly trevally, barrier trevally, ulua or GT), is a species of large marine fish classified in the jack family, Carangidae. The giant trevally is distributed throughout the tropical waters of the Indo-Pacific region, with a range stretching from South Africa in the west to Hawaii in the east, including Japan in the north and Australia in the south. The giant trevally is distinguished by its steep head profile, strong tail scutes and a variety of other more detailed anatomical features. It is normally a silvery colour with occasional dark spots, however males may be black once they mature. It is the largest fish in the genus Caranx, growing to a maximum known size of 170 cm and a weight of 80 kg. The giant trevally inhabits a wide range of marine environments, from 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 prefer turbid waters.
The giant trevally is a powerful apex predator in most of its habitats, and is known to hunt individually and in schools. The species predominantly takes various fish as prey, although crustaceans, cephalopods and molluscs make up a considerable part of the diet in some regions. The species has some quite novel hunting strategies including following monk seals and stealing prey that is stirred up, as well as using sharks to ambush prey. The species reproduces in the warmer months, with peaks differing by region. Spawning occurs at specific stages of the lunar cycle, when large schools of giant trevally congregate to spawn over reefs and bays, with reproductive behaviour observed in the wild. The fish grows relatively fast, reaching sexual maturity at a length of around 60 cm at 3 years of age. The giant trevally is both an important species to commercial fisheries and a recognised gamefish, with the species taken by nets and lines by professionals and by bait and lures by anglers. Catch statistics in the Asian region show hauls of 4000-10 000 tonnes, while around 10 000 lbs of the species is taken in Hawaii each year. The species is considered poor to excellent table fare by different authors, although ciguatera poisoning is common in 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, a group of percoid fishes in the order Perciformes..
The giant trevally was first scientifically described by the Swedish naturalist Peter Forsskål in 1775 based on specimens taken from the Red Sea off both Yemen and Saudi Arabia, with one of these designated to be the holotype. He named the species Scomber ignobilis, with the specific epithet Latin for "unknown", "obscure" or "ignoble". It was assigned to the mackerel genus Scomber, where many carangids were placed before they were classified as a separate family. This later revision in classification saw the species moved to the genus Caranx, where it has remained. Even after its initial description, the giant trevally (and the bigeye trevally) were often confused with the Atlantic crevalle jack, Caranx hippos, due to their superficial similarity, which led to some authors claiming the crevalle jack had a circumtropical distribution. After Forsskål's initial description and naming, the species was independently renamed three times as Caranx lessonii, Caranx ekala and Carangus hippoides, all of which are now considered invalid junior synonyms. The latter of these names once again highlighted the similarity with the crevalle jack, with the epithet hippoides essentially meaning 'like Carangus hippos', which was the crevalle jack's Latin name at that point in time. Despite the resemblance with the crevalle jack, the two species have never been phylogenetically compared, either morphologically or genetically, to determine their relationship.
Caranx 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. 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. In Hawaii, the species is almost exclusively referred to as 'ulua', often in conjunction with the prefixes 'black', 'white', or 'giant'. Due to its wide distribution there are many other names for the species in different languages also.
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 and a weight of 80 kg. Specimens this size are very rare, with the species only occasionally seen at lengths greater than 80 cm. It appears the Hawaiian Islands contain the largest individuals, where indivduals over 100 lbs are common. Elsewhere in the world, only three individuals over 100 lbs have been reported to the IGFA.
The giant trevally is similar in shape to a number of other large jacks and trevallies, having an ovate, moderately compressed body with the dorsal profile more convex than the ventral profile, particularly anteriorly. The dorsal fin is in two parts, the first consisting of 8 spines and the second of 1 spine followed by 18 to 21 soft rays. The anal fin consists of 2 anteriorly detached spines followed by 1 spine and 15 to 17 soft rays. The pelvic fins contain 1 spine and 19 to 21 soft rays. The caudal fin is strongly forked, and the pectoral fins are falcate, being longer than the length of the head. The lateral line has a pronounced and moderately long anterior arch, with the curved section intersecting the straight section below the lobe of the second dorsal fin. The curved section of the lateral line contains 58-64 scales while the straight section contains 0 to 4 scales and 26 to 38 very strong scutes. The chest is devoid of scales with the exception of a small patch of scales in front of the pelvic fins. The upper jaw contains a series of strong outer canines with an inner band of smaller teeth, while the lower jaw contains a single row of conical teeth. The species has 20 to 24 gill rakers in total and there are 24 vertebrae present. 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. The eye of the giant trevally has a horizontal 'streak' in which ganglion and photoreceptor densities are markedly greater than the rest of the eye. It is believed this allows the fish to gain a panoramic view of its surroundings, removing the need to constantly move the eye. This in turn will allow easier of detection of prey or predator in that field of view.
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. Fish greater than 50 cm show sexual dimorphism in their colouration, with males having a dusky to jet black body, while females are a much lighter coloured silvery grey. Individuals with a darker dorsal colouration often also display striking silvery striations and markings on the upper part of their body, particularly their back. Black dots of a few millimetres in diameter may also be found scattered all over the body, although the coverage of these dots varies between widespread to none at all. All the fins are generally light grey to black, although fish taken from turbid waters often have yellowish fins, with the anal fin being the brightest. The leading edge and tips of the anal and dorsal fins are generally lighter in colour than the main fin. There is no black spot on the operculum. Traces of broad cross-bands on the fish's sides are occasionally seen after death.
The giant trevally is widely distributed throughout the tropical and subtropical waters of the Indian and Pacific Oceans, ranging along the coasts of three continents and many hundreds of smaller islands and archipelagos. In the Indian Ocean, the species easternmost range is the coast of continental Africa, being distributed from the southern tip of South Africa north along the east African coastline to the Red Sea and Persian Gulf. The giant trevally's range extends eastwards along the Asian coastline including Pakistan, India and into South East Asia, the Indonesian Archipelago and northern Australia. The southernmost record from the west coast of Australia comes from Rottnest Island, not far offshore from the capital city of 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.
The giant trevally is abundant in the central Indo-Pacific region, found throughout all the archipelagos and offshore islands including Indonesia, Philippines and Solomon Islands. Along continental Asia, the species has been recorded from Malaysia to Vietnam, but not China. Despite this, its offshore range does extend north to Hong Kong, Taiwan and southern Japan. In the south, the species reaches as far south as New South Wales in Australia and even to the northern tip of New Zealand in the southern Pacific. Its distribution continues throughout the western Pacific including Tonga, Western Samoa and Polynesia, with its westernmost limits known to be the Pitcairn and Hawaiian Islands.
The giant trevally inhabits a very wide range of offshore and inshore marine environments, with the species also known to tolerate the low salinity waters of estuaries and rivers. It is a semi-pelagic fish known to spend time throughout the water column, but is mostly demersal in nature. 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. 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. Large individuals however do often return to these shallower waters as they patrol their range, often to hunt or reproduce. In Hawaii, the juvenile to subadult giant trevally are the most common large carangids in the protected inshore waters, with all other species apparently preferring the outer, less protected reefs. 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.
Juvenile to sub-adult giant trevally are known to enter and inhabit estuaries, the upper reaches of rivers and coastal lakes in several locations including South Africa, Solomon Islands, Philippines, India, Taiwan, Thailand, northern Australia and Hawaii. In some of these locations, such as Australia, it is a common and relatively abundant inhabitant, while in others including South Africa and Hawaii, it is much rarer in estuaries. The species has a wide salinity tolerance, as evident from the ranges juvenile and subadult fish in South African estuaries have been recorded from; 0.5 to 38 ‰, with other studies also showing tolerance levels of less than 1 ‰. In these estuaries, the giant trevally is known from both highly turbid, dirty water to clean, high visibility waters, however in most cases the species prefers the turbid waters. It appears younger fish 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.
Biology and ecology
The giant trevally is a solitary fish once it reaches sexual maturity, only schooling for the purposes of reproduction and more rarely for feeding. Juveniles and sub-adults commonly school, both in marine and estuarine environments. Observations from South African estuaries indicate the schools of smaller juveniles tend not to intermingle with schools of other species, however larger sub-adults are known to form mixed species schools with the brassy trevally. As well as the movement between habitats as the species grows, there has also been research conducted on the movements of larger fish around their habitats to understand how marine reserves impact on the species. Adult giant trevally are known to range back and forth up to 9 km along a home range, with some evidence of diel and seasonal shifts in habitat use. In the Hawaiian Islands it has been shown giant trevally do not normally move between atolls, but have specific core areas they spent most their time. Within these core areas, habitat shifts during different times of the day have been recorded, with the fish being most active at dawn and dusk, and usually shifting location near sunrise or sunset. Furthermore, large seasonal migrations appear to occur for the purpose of aggregating for spawning, with this also known from the Solomon Islands. Despite not moving between atolls, they do make periodic atoll-wide journeys of up to 29 km. Long term studies show that juveniles can move up to 70 km away from their protected habitats to outer reefs and atolls. The giant trevally is one of the most important apex predators in its habitats, both as adults on reefs and as juveniles in estuaries. Observations in relatively untouched waters of the Northwestern Hawaiian Islands showed the giant trevally was of high ecological importance, comprising 71% of the apex predator biomass, and was the dominant apex predator. This number is considerably less in heavily fishes Hawaiian waters. At smaller sizes, the species is prey to sharks, and large individuals have been recorded as a host of the sharksucker, Echeneis naucrates, a fish which is normally seen attached to the underside of sharks.
Diet and feeding
The giant trevally is a powerful predatory fish, and is an apex predator in most environments; from the estuaries it inhabits as a juvenile to the outer reefs and atolls it patrols as an adult. Hunting appears to occur at different times of the day in different areas of its range; off South Africa it is most active during the day, especially at dawn and dusk, while off Zanzibar and Hong Kong it is said to be nocturnal in its habits. The species diet has been determined in several countries and habitats, with this research finding the diet generally varies slightly between locations and age. In all but one study (which was of juveniles), the giant trevally dominantly takes other fishes, with various crustaceans, cephalopods and occasionally molluscs making the remainder of the diet. In Hawaii the species has a predominantly fish based diet consisting of Scaridae and Labridae, with crustaceans, including lobsters, and cephalopods (squid and octopus) making up the remaining portion of the diet. The large number of reef fishes suggests that it spends much of its time foraging over shallow-water reef habitats, but the presence of squid and the schooling carangid Decapterus macarellus indicates exploitation of more open-water habitats as well. Off Africa the diet is similar, consisting mostly of fish including eels, with minor squid, octopus, mantis shrimps, lobsters and other crustaceans. Younger fish inside Kaneohe Bay, Hawaii showed the only instance when crustaceans were preferred over fish; stomatopods, shrimps and crabs were the most common prey taken at 89% of stomach content by volume, with fish, mostly of the family Blennidae, making up only 7% of the stomach contents. Estuarine fish in both Hawaii and Australia have mostly fish-based diets, with crustaceans such as prawns and amphipods also of importance, and are known to take more novel prey such as spiders and insects in these habitats. There have also been reports of juvenile turtles and dolphins being found within the stomach contents of larger giant trevally. Studies of different size classes of fish have found the diet does change with age in some locations, with the changes relating to an increased volume of fish taken.
Studies in controlled environments on the giant trevally's feeding strategies have found that hunting in schools increases their capture efficiency, however is not necessary for an individual's survival. When a school is formed during feeding, one individual will take a leading position, with others trailing behind it. Several individuals will attack the prey school, striking and stunning the prey, with the leading individual generally being more successful. Some individuals act individually and opportunistically within the school if one of the 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. Another hunting strategy of the giant trevally is to 'escort' Monk Seals, a behavior which has been observed in 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. 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. 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 in 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 its in a defensive stance.
The giant trevally reaches sexual maturity at 54 to 61 cm in length and 3 to 4 years of age, although many authors narrow this down to 60 cm and 3 years of age. Sex ratio estimates from the Hawaiian Islands suggest that the population is slightly skewed toward females, with the male:female ratio being 1:1.39. Spawning occurs during the warmer months in most locations, although the exact dates differ by location. In southern Africa this occurs between July to March, with a peak between November and March; in the Philippines between December and January with a lesser peak during at June; and in Hawaii between April and November with a major peak during May to August. Lunar cycles are also known to control the spawning events, with large schools forming in certain locations at specific phases of the moon in Hawaii and the Solomon Islands. Locations for spawning include reefs, the reef channels and offshore banks. Sampling of schools prior to spawning suggests the fish segregate into schools of only one sex, although the details of this are still unclear. Observations in the natural habitat found spawning occurred during the day immediately after and just before the change of tide when there were no currents. Giant trevally gathered in schools of over 100 individuals, although ripe individuals occurred slightly deeper; around 2 to 3 m above the seabed in groups of 3 to 4, with one silver female being chased by several black males. Eventually a pair would sink down to a sandy bottom, where eggs and sperm were released. The fish then diverged and swum away. It appears that that each individual spawns more than once in each period, with only part of the gonads ripe in spawners. Fecundity is not known, although females are known to release several thousand eggs on capture during the spawning process. Eggs are described as pelagic and transparent in nature.
The giant trevally's early larval stages and their behaviour have been extensively described, with all fins having formed by at least 8 mm of age, with larvae and sub-juveniles being silver with six dark vertical bars. Laboratory populations of fish show a significant variability in the length at a certain age, with the average range being around 6.5 mm. Growth rates in larvae between 8.0 and 16.5 mm are on average 0.36 mm per day. The speed at which larvae swim increases with age from 12 cm/s at 8 mm in length to 40 cm/s at 16.5 mm, with size rather than age a better predictor of this parameter. Size is also a better predictor of endurance in larvae than age. These observations suggest that the species becomes an effective swimmer (is able to swim against a current) at around 7–14 mm. No obvious relationships with age and either swimming depth or trajectory have been found. Larvae appear to also opportunistically feed on small zooplankton while swimming. The larvae actively avoid other large fish, and jellyfish are occasionally used as temporary cover. Larvae have no association with reefs, and appear to prefer to live pelagically. Daily growth is estimated at between 3.82 g/day and 20.87 g/day, with larger fish growing at a more rapid rate. Age at 1 year old is 18 cm, age at 2 years is 35 cm and by 3 years, the fish is around 50 to 60 cm. The use of von Bertalanffy growth curves fitted to observed otolith data shows an individual of around 1 m in length is approximately 8 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, however 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.
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, however was later rejected when it was discovered to be a hybrid. Initial evidence of hybridisation was morphological characteristics intermediate to the two species, with later genetic tests confirmed that it was indeed a hybrid. It is known the two species school together, including at spawning time, which was considered to be the reason for hybridisation.
Relationship to humans
The giant trevally has been utilised by humans since prehistory, 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 the fish to a fine man and strong warrior, which was the cause of a ban on women eating the species in antiquity. The species was often used in Hawaiian religious rites, and took 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. The Hawaiians considered the fish be of excellent quality, with white firm flesh. Despite this, intrusions of giant trevally into modern day fishponds used by Hawaiians for rearing fish is unwelcome, as being a predator it eats more than it is worth at market.
The giant trevally is of high importance to modern day 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, with the 1998 catch consisted of 10 194 pounds of giant trevally worth around 12 000 US dollars. Historically the species has been taken in far greater numbers, and have been important food, market and game fish since the early 1900s. However their exploitation has seen the landings of the species decrease by over 84% since the turn of the century, declining from 725 000 lb catches to the 10 000 lb catches of recent. FAO statistics of the Asian region record catches between 4000 and 10 000 tonnes between 1997 and 2007, although this excludes most fisheries which are not monitored or don't discriminate between trevally species. The giant trevally is commercially caught by a number of methods including hook and line, including handlines, gill nets and other types of artisinal traps. The species has also successfully been bred for aquaculture purposes in Taiwan. It is sold at market fresh, frozen, salted, smoked, and as fishmeal and oil.
The giant trevally is considered one of the top gamefish of the Indo-Pacific region, having outstanding strength, speed and endurance once hooked. It can be taken by many methods, including baits comprising of cut or live fish and squid, as well as a wide array of lures. The species is commonly taken on bibbed plugs, minnows, spoons, jigs and poppers as well as soft plastic lures and saltwater flies. In recent years, the development of both jigging and surface-popping techniques have seen the giant trevally become an extremely popular candidate for catch and release fishing, with many charter operators based around this concept. The species is also popular with spearfishermen throughout its range. The species edibility has been rated from poor to excellent by different authors, although there are numerous reports of ciguatera poisoning from the species. Detailed tests on a large (1m) specimen taken from Palmyra Island showed the toxicity of the fish's flesh, liver and washed intestinal tract produced no or weakly positive symptoms to laboratory mice, however the digested contents of the intestine were lethal. The authors argued that 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 that an accumulative effect occurs with repeated consumption; and also that tests like the one outlined above are not reliable, as the toxin appears to be distributed haphazardly throughout each fish. 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.
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 biomass study in the Hawaiian Islands indicates that the main Hawaiian Islands are heavily depleted in the species, which in untouched ecosystems comprises 71% of the apex predator biomass. In contrast, it comprises less than 0.03% of the apex biomass in exploited habitats, with only a single fish observed during the course of this research study. Prior to this, a 1993 report suggested the population around the main Hawaiian islands were not stressed, even though several biological indicators suggested to the contrary. This was due to the highly size-selective nature of the fishery, which theoretically should prevent a decrease in numbers. Despite this, populations have decreased, and in light of their continued falling abundance in Hawaii, several recommendations including banning the commercial take of the species, increasing minimum lengths and decreasing bag limits for anglers, as well as reassessment of protected areas for the species have been proposed by officials. The species (nor any other carangid) has not been assessed by the IUCN.
Some recreational fishing groups are also promoting a catch and release practise for the giant trevally, with this becoming an increasingly popular option for charter boat operators, who have also begun to tag giant trevally for scientific purposes. At large sizes, the species is more likely to be ciguatoxic, and therefore if the fish is kept it must be disposed of or sent to a taxidermist if it is a trophy fish. A catch and release approach has also been adopted by operators outside of Hawaii, with Australian operators who target the species by popping and jigging rarely keeping any fish. 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.
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