Giant Trevally | Species ID

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COMMON NAME:

Giant Trevally
 

SCIENTIFIC NAME:

Caranx ignobilis
 

LEGAL LENGTH & LIMIT

Check your local rules and regulations.
 

DESCRIPTION

 

The giant trevally is the largest member of the genus Caranx (Trevally), with a recorded maximum length of 170 cm and a weight of 80KG. Specimens this size are very rare, with the species only occasionally seen at lengths greater than 80cm. It appears the Hawaiian Islands contain the largest individuals, where indivIduals over 100 lbs are common. Elsewhere in the world, you will seldomly see Giant trevally at this size;

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, 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 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 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 cell 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.

HABITAT & FEEDING

 

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.^[20] 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.^[21][22] 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.^[23][24] Large individuals however do often return to these shallower waters as they patrol their range, often to hunt or reproduce.^[24] 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.^[25] 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.^[26]

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,^[27] Solomon Islands,^[28] Philippines,^[29] India,^[30] Taiwan,^[31] Thailand,^[32] northern Australia^[33] and Hawaii.^[21] In some of these locations, such as Australia, it is a common and relatively abundant inhabitant,^[28] while in others including South Africa and Hawaii, it is much rarer in estuaries.^[21] 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 ‰,^[34] with other studies also showing tolerance levels of less than 1 ‰.^[27] 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.^[21] 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.^[25] In the Philippines, a population of giant trevally inhabit (and were once common in) the landlocked fresh waters of 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 land locked maliputo is prominently featured on the reverse side of the newly redesigned Philippine 50 Peso bill.^[35]

 

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.^[20][21] 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,^[18] while off Zanzibar and Hong Kong it is said to be nocturnal in its habits.^[39][40] 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.^[9][41] 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.^[20] Off Africa the diet is similar, consisting mostly of fish including eels, with minor squid, octopus, mantis shrimps, lobsters and other crustaceans.^[39] 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.^[25] 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.^[21][42] There have also been reports of juvenile turtles and dolphins being found within the stomach contents of larger giant trevally.^[43] 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.^[34]

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.^[44] 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.^[45] 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.^[43] 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.^[46]

 

AGE & GROWTH

 

The giant trevally reaches sexual maturity at 54 to 61 cm in length and 3 to 4 years of age,^[39] although many authors narrow this down to 60 cm and 3 years of age.^[18] 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.^[20] 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;^[39] in the Philippines between December and January with a lesser peak during at June;^[47] and in Hawaii between April and November with a major peak during May to August.^[20] 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.^[24][36] Locations for spawning include reefs, the reef channels and offshore banks.^[40] Sampling of schools prior to spawning suggests the fish segregate into schools of only one sex, although the details of this are still unclear.^[39] 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.^[47] 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.^[47]

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.^[23] 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.^[23] 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.^[23] 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.^[20] 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,^[20] 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.^[22] 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.^[48]

FOOD QUALITY

 

The giant trevally has been utilised by humans since prehistoic 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.^[49] 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.^[11] 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, as being a predator it eats more than it is worth at market.^[49]

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.^[11] 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.^[50] FAO statistics of the Asian region record catches between 4000 and 10 000 tonnes between 1997 and 2007,^[51] 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 artisanal traps. The species has also successfully been bred for aquaculture purposes in Taiwan.^[52] It is sold at market fresh, frozen, salted, smoked, and as fishmeal and oil.^[10]

The giant trevally is considered one of the top gamefish of the Indo-Pacific region, having outstanding strength, speed and endurance once hooked.^[8] It can be taken by many methods, including baits 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.^[53] 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,^[8] with many charter operators based around this concept.^[43] 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.^[54] 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.^[54] 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.^[11]

HUNTING TECHNIQUES