The bigeye trevally, Caranx sexfasciatus (also known as th e
big eye jack, great trevally, six-banded trevally and dusky jack),
is a species of widespread large marine fish classified in the jack
family Carangidae. The bigeye trevally is distributed throughout the
tropical waters of the Indian and Pacific Oceans, ranging from South
Africa in the west to California and Ecuador in the east, including
Australia to the south and Japan in the north. The bigeye trevally
is best distinguished by its colouration, having a dark second
dorsal fin with a white tip on the lobe, and also possessing a small
dark spot on the operculum. Other more detailed anatomical features
also set the species apart from other members of Caranx. The species
is known to grow to a length of 120 cm and 18 kg.
It is predominantly an inshore fish, inhabiting reefs down to
depths of around 100 m in both coastal zones and offshore islands,
often venturing into estuaries and sandy bays as juveniles. The big
eye trevally is commonly found in large slow moving schools during
the day, becoming active at night when it feeds, taking a variety of
fish, crustaceans, cephalopods and other invertebrates. The fish is
known to move from a more crustacean dominated diet as a juvenile to
a nearly completely fish dominated diet as an adult. Sexual maturity
is reached at 42 cm, with spawning occurring in large aggregations
occurring at different periods throughout its range, generally
between July and March. The bigeye trevally is of moderate
importance to fisheries throughout its range, and being of high
importance to some artisanal fisheries. It is taken by gill net,
purse seine, hook and line and other artisanal fishing methods. It
is rated as a fair to good table fish. The species is also
considered a good gamefish, taken by lure, bait and spear throughout
its range.
Description
The bigeye trevally is one of the larger members of Caranx, growing
to a maximum recorded size of 120 cm in length and 18.0 kg in
weight.It is similar to most other jacks in having a compressed,
oblong body, with the dorsal profile slightly more convex than the
ventral profile, particularly anteriorly. The snout is slightly
pointed, and is greater in length than the eye diameter. The dorsal
fin is in two distinct sections; the first consisting of 8 spine and
the second of 1 spine and 19 to 22 soft rays. The anal fin consists
of 2 anteriorly detached spines followed by 1 spine and 14 to 17
soft rays. The pelvic fins consists of 1 spine and 17 to 18 soft
rays, while the caudal fin is strongly forked and the pectoral fin
falcate.[8] The species lateral line is moderately arched anteriorly,
with 49 to 50 scales in this section, while the straight section
contains 0 to 3 scales and 27 to 36 strong scutes. The breast is
completely covered in scales. The species has well developed adipose
eyelids, while its dentition consists of an outer row of
widely-spaced canine teeth and an inner band of villiform teeth in
the upper jaw, with a row of widely-spaced conical teeth on the
lower jaw. The bigeye trevally has 21 to 25 gill rakers and 25
vertebrae.
The bigeye trevally shows a change in colour as it ages,
changing both overall colour and body patterns. Juveniles are a
silvery yellow to silvery brown in colour, and possess five to six
dark vertical bands on their sides,from which the specific epithet
sexfasciatus arose. As they mature, the bands fade and become
indistinct and the overall colour shifts to a silvery blue above and
whitish below. In adults, the bars are completely absent and the
dorsal colour is a silvery olive to blue green, fading to silvery
white below. In juveniles, the fins are pale grey to yellow with
darker edges, becoming darker overall in adulthood, with the anal
and caudal fins yellow to black and the second dorsal fin olive to
black. The tip of the second dorsal fin has a distinctive white tip.
The bigeye trevally also has a small dark opercular spot on the
upper margin.
The bigeye trevally predominantly live in inshore coastal
waters, although does occur in pelagic settings far offshore, and
around remote islands and seamounts. The species is known to reach
depths of around 100 m. It is mostly found over coral and rocky reef
complexes as adults, however often moves into more inshore areas in
sandy bays and lagoons in small numbers.Those that live offshore
often live on deeper seamounts or reefs around offshore islands. The
species moves with the tide in some regions, entering shallow
lagoonal areas as the tide rises, and moving back to the deeper
reefs as it retreats. Juveniles inhabit more inshore, shallower
waters around the coast, often venturing into lagoons, tidal flats,
mangrove zones and even estuaries.[16] Juvenile bigeye trevally have
been reported in rivers from several locations, and are known to
penetrate well into the upper reaches of rivers. As the fish grows,
it moves back to deeper waters over reefs. The species has been
reported in pelagic open ocean settings, milling around stationary
buoys, indicating the species may follow floating debris far out to
sea.
Biology
The bigeye trevally is a schools species, known to form aggregations
consisting of more than 1500 fish. The species is often seen in
these large schools either stationary or slowly moving around the
reef complexes they inhabit during the day. At night, these schools
dissolve, and the species become active, taking most of its prey
during the nocturnal period. This contrasts to most jacks, which are
generally diurnal hunters, although the species has been documented
hunting in shallower waters during the day, especially as juveniles.
The abundance of the species on the west coast of America has been
linked to the occurrence of El Nino or La Nina climate events, with
the abundance significantly dropping during El Nino events.
Diet and feeding
The bigeye trevally is a fast swimming predatory species that has
had several studies determine its diet in various places throughout
its distribution. As mentioned above, the species is mostly inactive
during the day, and feeds at dusk and through the night.The large
stationary aggregations break off as the fish move off to hunt in
small groups or individually. The species predominantly takes small
fish as prey, however supplements its diet with a varied array of
invertebrates. These include crustaceans such as shrimps, decapods,
copepods and stomatopods, cephalopods, gastropods, jellyfish,
sponges and even species of open ocean sea-skater insects. Bigeye
trevally were found to be the primary fish predators of these
sea-skates, and took far more of these than any other species of
carangid. Evidence from South African estuaries indicates there is a
shift in the species diet as it grows. Young fish below 200 mm in
length take mostly the juveniles of other fish species but still
have a significant intake of penaeid shrimps, while fish larger than
this take small fish nearly exclusively. The larger fish on reefs
tend to move between these reefs regularly, which is thought to be
due to prey availability. Bigeye trevally are also an important prey
for larger species, ranging from sharks and larger species of
carangid to sea lions and various birds.
Reproduction and growth
The bigeye trevally reaches sexual maturity at a length of around 42
cm, with both males and females reaching maturity at a similar
length and age. Spawning is known to occur between July and
September in the east Pacific and in November to March in South
Africa, indicating variation across the species range. There is also
evidence that spawning may occur during new moon periods. Fish
aggregate in large schools prior to spawning, with pairs breaking
off the main aggregation to commence spawning. The pair increase
their swimming speed to leave the school, with the fish underneath
instantly changing colour to a dark black, with this individual also
known to chase off any other individuals that approach the pair. The
trevally then press their ventral surfaces together to spawn, often
swimming almost horizontally, before returning to the school and
changing back to their normal silvery colour.
The larvae of bigeye trevally have been extensively described,
with defining features being a conspicuously pigmented
supraoccipital crest, relatively deep body and an anal fin ray count
of 15 to 17, the lowest of any east Pacific carangid.There has been
little research into the later stages of growth and their rates.
Juveniles are known to inhabit either inshore habitats such as
estuaries, with an influx of these small fish after spawning in
South Africa, while in other cases juveniles have been found living
pelagically around floating objects.
Relationship to humans
The bigeye trevally is of varying importance to both commercial and
recreational fisheries throughout its range, with the large school
sizes allowing for large catches in some regions. In many regions
the species is lumped with other jacks in fishery statistics, and
therefore worldwide catch is nearly impossible to estimate. Known
recent annual catches include those of Saudi Arabia (615-638 t),
Mexico (900 t), and Hawaii (297 lb). The species is especially
important in fisheries based around coral reefs, with one such
fishery in Papua New Guinea showing bigeye trevally are the most
frequently taken carangid, and one of the top five most important
species overall. It is also of high importance in small artisanal
and subsistence fisheries, for which statistics are not kept. Bigeye
trevally are taken by a number of methods including hook and line,
gill nets, purse seines and various other netting methods. Due to
their nocturnal hunting nature, a popular way to catch large numbers
involves illuminating the water surface at night, and using hook and
line to catch the fish attracted by the light.The species is sold
fresh, dried and salted at market.
The bigeye trevally is also popular with anglers, with the
species rated as an excellent gamefish in larger sizes. The species
is commonly taken by boat fishermen over reef complexes, however
schools often hang around jetties and wharves, allowing for frantic
fishing when the fish are on the bite, especially after dusk. The
species is also occasionally taken from beaches, but rarely in large
quantities. Common techniques for catching the fish include bait
fishing, which can involve both live or dead bit including fish,
squid or various crustaceans, or lure fishing. Bigeye trevally are
known to accept many lure types including bibbed lures, surface
poppers and metal slugs jigged in rapid retrieve from the ocean
floor.[Soft plastic lures are known to take the species, as are
saltwater fly patterns. In larger sizes, gear must be robust and
well maintained to land the fish.
The bigeye trevally's edibility is said to range from fair to
very good, with frying, steaming, grilling and even use in soup
popular in some South East Asian countries. Filipinos are said to
consider the fish of high quality, especially fish taken from the
volcanic Lake Taal, which are said to have a delicate flavour due to
the lake's sulphur content. Apart from fishing, the species is
popular with scuba divers which often photograph the huge schools
that mill around reefs during the day. Bigeye trevally have also
been held in large saltwater aquaria, however require large tank
volumes to survive. |
