Crevalle jack

Appearance

The crevalle jack is one of the largest members of ''Caranx'', growing to a known maximum length of 124 cm and a weight of 32 kg, although are generally rare at lengths greater than 60 cm. Unverified reports of fish over 150 cm may also be attributable to this species. The crevalle jack is morphologically similar to a number of other deep bodied carangids, having an elongate, moderately compressed body with the dorsal profile more convex than the ventral profile, particularly anteriorly. The eye is covered by a well-developed adipose eyelid, and the posterior extremity of the jaw is vertically under or past the posterior margin of the eye. The dorsal fin is in two parts, the first consisting of 8 spines and the second of 1 spine followed by 19 to 21 soft rays. The anal fin consists of 2 anteriorly detached spines followed by 1 spine and 16 or 17 soft rays. The pelvic fins contain 1 spine and 5 soft rays while the pectoral fins contain 20 or 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 midway below the second dorsal fin. The straight section contains 23 to 35 very strong scutes, with bilateral keels present on the caudal peduncle. 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 teeth. The species has 35 to 42 gill rakers in total and there are 25 vertebrae present.

In 1972, a crevalle jack was caught by fishermen off South Carolina which displayed swollen, bulbous mandibles. These swellings were initially thought to be due to a copepod parasite, however radiographs and subsequent sectioning found them to be bony in nature. The cause of this calcified connective tissue is still unknown, and there remains only a single reported case of such an ailment in crevalle jack.

The crevalle jack's colour ranges from brassy green to blue or bluish black dorsally, becoming silvery white or golden ventrally. A dark spot is present on the pectoral fin, with a similar dark to dusky spot present on the upper margin of the operculum. Juveniles have around five dark vertical bands on their sides, with these fading at adulthood. The first dorsal fin, pectoral and pelvic fins range from white to dusky, occasionally with golden tinges throughout. The anal fin lobe is bright yellow, with the remainder of the fin ranging from golden to dusky, while the underside of the caudal peduncle often being yellow in adults. The caudal fin itself is also golden to dusky, with the lower lobe often brighter yellow than the upper, with both the lobes often having a black trailing edge.

Distribution

The crevalle jack inhabits the tropical and temperate waters of the Atlantic Ocean, ranging extensively along both the eastern American coastline and the western African and European coastlines. In the western Atlantic, the southernmost record comes from Uruguay, with the species ranging north along the central American coastline, and throughout the Caribbean and many of the numerous archipelagos within. The species is found throughout the Greater Antilles, however it is absent from the leeward Lesser Antilles, with its distribution being patchy throughout other Caribbean Archipelagos. From the Gulf of Mexico its distribution extends north along the U.S. coast and as far north as Nova Scotia in Canada, also taking in several north-west Atlantic islands. The crevalle jack is also known from Saint Helena Island in the southern Atlantic Ocean.

In the eastern Atlantic the southernmost record comes from Angola, with the species distributed north along the west African coastline up to West Sahara and Morocco, with its distribution also including much of the Mediterranean Sea. In the Mediterranean, the crevalle jack's range extends as far east as Libya in the south and Turkey in the north, and includes most of the northern Mediterranean including Greece, Italy and Spain. The species northernmost record in the eastern Atlantic comes from Portugal, with the species also known to inhabit many of the north-eastern Atlantic islands including Cape Verde, Madeira Island, and the Canary Islands.

Many older publications list the species range as from the eastern Pacific, which now are known to represent the Pacific crevalle jack and is considered a distinct species. There are also often mentions of the species erroneously having circumtropical and Indian Ocean distributions, with these records probably attributable to similar Indo-Pacific species, namely the blacktip trevally and giant trevally. The species distribution overlaps that of the similar longfin crevalle jack in the eastern Atlantic, with careful identification needed to distinguish the two. Within the Atlantic, confusion with both longfin crevalle jack and horse-eye jack, ''Caranx latus,'' have also led to erroneous records being made, with Smith-Vaniz and Carpenter suggesting this occurred in the Mediterranean, and the species may actually be absent from waters north of Mauritania.

Behavior

The crevalle jack is one of most abundant large carangids in the Atlantic Ocean, with at least two systematic studies placing it within the top five most abundant species of that region, namely lagoons in Nigeria and Chiapas, Mexico. Seasonal movements are known from both the American and African coastlines, with both juveniles and adults appearing to migrate. In North America, young individuals that are recruited to northern estuaries are known to move to warmer tropical waters at the onset of winter to escape possible hypothermia. At least one hypothermia driven mass mortality of 200 crevalle jacks has been reported from the Slocum River in Massachusetts, indicating that low temperature mortality is a major concern for north-ranging groups of the species, with temperatures below 9.0°C apparently being lethal to the fish. This applies not only to river dwelling fish, but marine migrants who linger too long in the temperate regions during winter. In Nigeria, and presumably other parts of Africa, it appears the species seasonally migrates, possibly to take advantage of prey, with the fish arriving in Nigeria during September to November. The species is more active during the day than the night, with larger catches in fisheries taken during the day also. The crevalle jack is a schooling species for most of its life, forming moderately large to very large fast moving schools. At larger sizes, the fish become more solitary and move to the deeper offshore reefs. Evidence from laboratory studies indicates crevalle jack are able to coordinate their feeding and spawning aggregations over coral reefs based on the release of dimethylsulfoniopropionate from the reef. DMSP is a naturally occurring chemical produced by marine algae and to a lesser extent corals and their symbiotic zooxanthellae. Field studies have also shown the species increases in abundance with increased levels of DMSP over coral reefs.

Habitat

The crevalle jack lives in both inshore and offshore habitats, with larger adults preferring deeper waters than juveniles. In the inshore environment, crevalle jack inhabit shallow flats, sandy bays, beaches, seagrass beds, shallow reef complexes and lagoons, which may be open or landocked, and may be composites of the aforementioned environments. The species is also known to enter brackish waters with some individuals known to penetrate far upstream, however like most euryhaline species they generally do not penetrate very far inland.
The salinities the species has been reported from range from 0% to 49%, indicating the species can adapt to a wide range of waters. Studies in West Africa found marked differences in the sex ratios of populations in brackish waters, with females very rarely seen in such environments once they are mature. Research in the coastal waters of Ghana suggests that the availability of food is the primary control on the species distribution in inshore waters.

Adults that move offshore generally do not leave continental shelf waters, however still penetrate to depths of 350 m, and possibly deeper. These individuals live on the outer shelf edges, sill reefs and upper slopes of the deep reef, and tend to be more solitary than juveniles. Adults have also been sighted around the large oil rig platforms throughout the Gulf of Mexico, where they use the man made structure like a reef to hunt prey. The larvae and young juveniles of the species live pelagically offshore along the continental shelf and slope, and are also known to congregate around oil platforms, as well as natural floating debris such as sargassum mats.The crevalle jack is one of most abundant large carangids in the Atlantic Ocean, with at least two systematic studies placing it within the top five most abundant species of that region, namely lagoons in Nigeria and Chiapas, Mexico. Seasonal movements are known from both the American and African coastlines, with both juveniles and adults appearing to migrate. In North America, young individuals that are recruited to northern estuaries are known to move to warmer tropical waters at the onset of winter to escape possible hypothermia. At least one hypothermia driven mass mortality of 200 crevalle jacks has been reported from the Slocum River in Massachusetts, indicating that low temperature mortality is a major concern for north-ranging groups of the species, with temperatures below 9.0°C apparently being lethal to the fish. This applies not only to river dwelling fish, but marine migrants who linger too long in the temperate regions during winter. In Nigeria, and presumably other parts of Africa, it appears the species seasonally migrates, possibly to take advantage of prey, with the fish arriving in Nigeria during September to November. The species is more active during the day than the night, with larger catches in fisheries taken during the day also. The crevalle jack is a schooling species for most of its life, forming moderately large to very large fast moving schools. At larger sizes, the fish become more solitary and move to the deeper offshore reefs. Evidence from laboratory studies indicates crevalle jack are able to coordinate their feeding and spawning aggregations over coral reefs based on the release of dimethylsulfoniopropionate from the reef. DMSP is a naturally occurring chemical produced by marine algae and to a lesser extent corals and their symbiotic zooxanthellae. Field studies have also shown the species increases in abundance with increased levels of DMSP over coral reefs.

Food

The crevalle jack is a powerful predatory fish which predominantly takes other small fishes as prey at all stages of its life, with various invertebrates generally being of secondary importance to its diet. Several studies conducted on the species diet over its range have found other aspects of its diet vary widely, including the specific types of prey the species takes and the change in diet with age. The most detailed of these studies was conducted in the southern states of the USA, which showed that the species diet comprised between 74% and 94% fish. The remainder of the diet was various prawns, shrimps, crabs, molluscs and stomatopods. The types of fish taken varied throughout the range, with members of Clupeidae, Sparidae, Carangidae and Trichiuridae all taken in variable amounts, usually with members of one family dominating the local diet. The percentage of various invertebrates also was highly variable, with penaeid shrimps, portunid crabs, stomatopods and squid being of importance to different populations. The study also indicated young crevalle jack predominantly take clupeids, adding sparids and later carangids to their diet as they grow larger. The larger individuals also took much higher amounts of invertebrates, and also small quantities of seagrass, indicating larger fish are more opportunistic. This general change in diet with age also seems spatially variable, with young crevalle jack in both Maryland and Puerto Rica consuming almost exclusively crustaceans including shrimps, crabs in Maryland and harpacticoid copepods in Puerto Rica before moving to fish dominated diets later in life. Research in Ghana shows a pattern somewhat intermediate to the previous two locations; adults take larger fish, predominantly ''Engraulis guineensis'' and ''Sardinella eba'', while juvenile fish take smaller fishes such as ''Epiplatys sexfasciatus'' or juvenile caridian and penaeid shrimps.

The widely variable diet of the species throughout its life stages led to authors in the 1950s and 1960s concluding the species was indiscriminate in its feeding habits, eating whatever was locally available. The diets of the populations in both the southern USA and Ghana also varied quite markedly by season and year, which led to the authors of both these studies agreeing with these earlier conclusions. Recent laboratory studies, however, have shown the species may have preferences for certain sizes of prey. In these experiments, the fish was presented with a range of size classes of the same prey species, ''Menidia beryllina'', with the results showing that crevalle jack prefer to take the smallest size class possible, which contrasts with more aggressive predators such as bluefish. Both adults and juveniles feed throughout the day, generally becoming inactive at night. During some feeding periods recorded in Ghana, digestion in the species was so rapid that food becomes unidentifiable within 4 to 5 hours of consumption. The crevalle jack is also an important prey species itself, taken by larger fish such as billfish and sharks, as well as seabirds. As well as being preyed on during its adult stage, the spawn of the crevalle jack is known to be eaten by planktivorous organisms, including whale sharks in the Caribbean.

Evolution

The crevalle jack reaches sexual maturity at different lengths in males and females, with estimates suggesting males reaching maturity at 55 cm and 4 to 5 years of age and females at 66 cm and 5 to 6 years of age. Reproduction is thought to occur year round in most areas, although there are peaks in activity. South of Florida, this period is between March and September, in Cuba it is April and May, while in Jamaica no definitive peak has yet been identified. The species also has a protracted spawning in Ghana, although there is a peak in activity between October and January. Juveniles are also present in lagoons year round in this location, indicating year round spawning and recruitment. The place of spawning also appears to be variable, with the act occurring offshore south of Florida, while in Colombia and Belize, they have been observed spawning over inshore reefs and bays. Large aggregations of crevalle jack form prior to spawning, with these schools containing upward of 1000 individuals. Pairs break off from the school to spawn, with one individual turning a much darker colour during this exchange. Once spawning has occurred, the pair rejoin the main school. Fecundity in the species has been estimated as up to one million eggs, with these being pelagic, and spherical in shape. They have a diameter of 0.7 to 0.9 mm, and contain a pigmented yolk and one yellow oil globule with dark pigments. The larvae have been extensively described in the scientific literature, although the sequence of fin formation is still not well known. Defining features of the larval crevalle jack include a relatively deep body, heavily pigmented head and body as well as more detailed meristic characteristics, with flexion occurring at 4 to 5 mm in length.

Otolith and vertebrae studies have proved useful in determining the age and thus growth patterns of the species, with other methods including scale and fin ray sectioning of lesser value. The species otoliths have been the subject of detailed x-ray diffraction studies, which have indicated that biomineralisation of the otoliths occurs predominantly in the aragonite phase. Females grow faster than males, reaching 266.5 mm after their first year of life, 364.4 after their second, 370.9 mm after the third and 546.7 after their fifth. A female of 676.6 mm was 9 years old. Males reach 252.4 mm in their first year, 336.2 mm in their second, 363.8 in the third and 510.3 in their fifth. A male of 554 mm was 8 years old. The oldest studied individual was a 934 mm individual of unspecified sex, which was 17 years old. The larvae are pelagic and are found over continental shelf waters and occasionally in the oceanic zone proximal to the continental slope. The larvae are present all year round in the Gulf of Mexico, with a peak in abundance during the summer months due to spawning peaks. While the young juveniles live in the exposed pelagic environment, they use a behaviour called 'piloting' to swim in very close proximity to both larger animals and floating objects such as sargassum mats, buoys and even boats. By the time juveniles make their way to shore, they may have been dispersed large distances from their initial spawning grounds and may face the challenge of migration to warmer climates during winter if they are to survive as outlined previously. Juveniles use estuaries and seagrass beds as their main nursery habitats.