| Jacks swim in the open water, feeding on animal plankton or small fishes. They form schools. They swim fast. They grow very old. Four species are common in NZ. |
The fishes in this family, which contains over 200 species worldwide (Eocene to present), are all moderate to large pelagic forms. Their lateral lines are usually strongly arched towards their tails. Most of them are silvery or pale green-blue in colour and are markedly countershaded, being darker on top. With their streamlined bodies, they superficially resemble mackerel and tuna, having a small V-shaped tail and distinctive thin tail stock but they lack the finlets behind the dorsal and anal fins. They have long, sickle-shaped breast fins. Because they have hard spines, which mackerels lack, they are also called spiny mackerels.
Many
jacks have rough, keeled scales along their lateral lines. The body is
covered in small, thin scales, which gives the fish the required flexibility
for fast swimming. Jacks are identified by having two retractable anal
spines that are not connected to the anal fin.
Jacks form schools that live in open water, close to the surface, usually over the inner continental shelf. Many of the jacks are fished commercially, being valued for their fine reddish flesh but in NZ jacks are often used for bait only. Some have been the subject of recent biological studies. These have indicated that many carangids are extremely long lived and that the seemingly large schools of adults do not re-establish themselves easily after they have been fished using modern purse seining techniques. There are seven species of carangid recorded from New Zealand, six of which are illustrated here.
The carangidae (caranx) were known to the Romans and Greek from the Mediterranean Sea.
| The trevally lives in dense schools that can foam the water's surface. They feed on small planktonic shrimp. Their bodies are very flat. They swim fast. They grow old. They often stay in one area. |
The jack that resembles the toothed jack: (pseudo=resembling), (caranx=jack), (dentex=toothed), (georgianus=named in honour of King George), (trevally= an Australian name for jacks, origin unknown)
The
trevally is a deep bodied pelagic fish that normally averages between 30
and 60cm in length although occasional specimens approaching 1m are seen
in the far north of NZ. Like most carangids they have two dorsal fins,
the first high and triangular and the second lower and longer, a long scythe-like
pectoral fin and a deeply forked tail. In common with many other pelagic
fishes and sharks there is a sharp strengthening keel on each side of the
caudal peduncle that in the trevally is covered with bony scutes that are
a part of the lateral line system.
This species changes considerably in shape and colour throughout its life and some of these stages have previously been regarded as separate species. Up to 10 or 15cm in length they are moderately deep bodied, silver in colour. with a series of distinct dark vertical stripes. At this stage trevally feed on small planktonic animals and can often be seen picking parasites off other fishes to supplement their diet. These small fishes are usually solitary and are frequently seen swimming in the company of larger fishes in the same fashion as striped pilotfish swim with sharks.
From 15 to about 40cm trevally are more slender with a pointed snout and a uniform silver-grey body colour, sometimes with faint yellow tinges on the fins and a black spot on the gill cover. Fishes of this size are often seen schooling in coastal waters and feed on small invertebrates grubbed from sand or mud bottoms, as well as on planktonic animals.
Over
40cm in size trevally become deeper bodied and develop a pronounced hump
on their foreheads. They become dark green-blue above, often with a yellow
sheen, and have bright yellow tinged fins. These adult fishes form large
schools, sometimes covering many hectares of sea surface. Around the offshore
islands and pinnacles of eastern North Island they are often seen feeding
intensively on the surface, gulping down large euphausid crustaceans (krill)
with their humped foreheads out of the water. When disturbed by a boat
or a large predatory fish these schools dive for deeper water in a welter
of foam.
Preliminary studies have been made on the biology and habits of the trevally and have shown that they are slow growing and long lived. At a year old they are less than 10cm long, at three vears 3lcm, and at 10 years have only reached a length of about 40cm. A 30 year old fish is about 45cm long and some larger specimens have been aged at over 45 years, using the annual rings laid down on the ear bones or otoliths. These sizes are only the average and other fish the same age may be considerably larger or smaller.
Tagging of adult fishes has revealed that the large offshore schools do not normally move far and tend to remain associated with one group of islands for some time. If these schools of old resident fishes are removed by purse seine fishing the trevally numbers in that area will be drastically depleted for a considerable time.
Fishing:
When schooling and feeding on plankton shrimps, trevally are not keen to
take the bait but small trevally who usually feed on the bottom, can easily
be caught by baited lines. In Australia the great trevally (Caranx sexfasciatus),
which grows to 1.2m and 40Kg, is a spectacular catch that fights savagely.
Trevally have delicious soft, delicate pink flesh which is best eaten rare as sashimi or cured in salt, like gravalax (cured salmon). Its flesh and that of kingfish is highly prized in Japan.
Trevally are caught commercially around the North island and about 6500 tonnes are landed annually, usually destined to be canned or used as bait. Traditionally trevally have been caught by trawling (at depths of less than 80m), but between 1960 and 1980, purse seiners have almost completely wiped the entire trevally stock.
The meat is processed and sold as frozen fillets and canned, often as smoked fillets. It is also reprocessed in fish fingers.
Fins: D VIII + 25; A II+I 23; P I 5; LL scutes 25. Size to 60cm.
Life history: Trevally can live for about 40 years. They grow at a moderate rate for the first four years, taking about 5 years to reach the size of sexual maturity (320-370mm long). Afterwards, growth slows markedly. A 60cm trevally is at maximum size, weighing about 5 Kg. The bulk in commercial trawls is 45cm and 1.4Kg.
Spawning
has not yet been observed, however, examination of the gonads indicates
that trevally are serial spawners (spawning several times in the season)
and that spawning occurs over the summer and is usually completed by March.
These fish do not appear to have defined spawning grounds, nor do they
school to spawn.
Trevally eggs are pelagic. They hatch about 28 hours after fertilisation
and the larvae settle after 2-3 months in the plankton. Juvenile fish (about
40mm long) are found in inshore areas, including estuaries and harbours.
Juveniles smaller than this size are often found sheltering among the tentacles
of jellyfish such as Plagusia noctiluca.
Distribution: Trevally are most abundant around the North Island
but occasionally stray as far south as Banks Peninsula in the summer. The
same species is also found in southern Australia, Lord Howe, Norfolk and
Kermadec Islands. Juveniles (50-200mm long) are usually found in midwater
groups in shallow coastal waters.
| Divers' tales
The memories I have of trevally are those of happy fish, always inquisitive and always actively moving about. Feeding frenzy
(19971117 Floor Anthoni) |
| Little cleaners
When 4-10cm in size, trevally advertise themselves as cleanerfish. Their sharp snouts and tiny teeth appear to offer a good grip on sea lice that bury themselves half under a scale and the other half into the flesh of anything that swims in the ocean. Being sideways compressed, allows trevally to give a good side swipe to pull the bothersome critters out. Along our dirty coasts, the traditional cleaners such as the crimson cleaner and combfish stand out by their absence. So a baby trevally sets up shop in their place. It is a tricky practice, fish of that size being a hearty one-bite meal for most. But they seem to get away with it. What amazes me time and again is how the needs of two or more entirely different species are met by a common activity, and that the signals going with it are sufficiently universal that unlikely cleaners such as spotties and even crested blennies can act as clearnerfish. It is usually the patient who recognises the shiny trevally, taking the initiative by assuming an unusual posture, facing the sick spot up. Sometimes a breast fin is used to point at the itch. When the trevally comes to inspect, it almost invariably starts somewhere else at less urgent spots that visually need attention. When the patient's patience runs out, it wil cautiously move out of position, in order to present the itchy spot more clearly. This works as long as the cleaner can see what is wrong. Sometimes there's an itch without a sealouse. More patient still are all peers of lesser rank who have parked themselves a small distance away. Sociable fish such as parore, silver drummer and even blue maomao, wait patiently but where no ranking order exists such as with demoiselles, the cleanerfish simply gets mobbed for attention. All jacks have a nasty trick up their sleeves for rubbing their parasites off in mid water. Jacks have very fine, almost slippery scales and cannot usefully rub against one another but a passing snapper or big-scaled wrasse is fair game. Approaching the big fish from behind, the jacks rub themselves on its back and sides, while overtaking at the same time. In this manner the recipient's scales rub the pesky parasites off. This is of course not appreciated but jacks being so playfully fast, get away with it. I have observed how big fish such as snapper and parore appreciate the attentions and nitpicking of their little trevally friends so much that they take pride in having one privately. They even sleep together at night. Being flat, trevallies can travel as fast as their protectors because they travel inside an eddy alongside the bigger fish. Pilotfish have been seen riding the bow wave off a shark's nose, appearing to 'guide' it. 19971117 Floor Anthoni. |
| Koheru is a NZ species, living in schools along the north-east coast. It can be brightly coloured blue-yellow but it can also hide its colours. It is hunted by kingfish and dolphins. |
(koheru= maori name for this fish), (decapterus=????) (capere, captare=to catch) (de-=completely)
The
koheru is an elongate fish, rounded in cross-section. There are typical
pelagic keels on each side of the thin caudal peduncle, edged with a row
of sharp scutes as in the trevally. Minute teeth line the flexible jaws
and the mouth can be protruded to form a plankton sucking tube. The koheru
can be distinguished from the jackmackerel by its firmer and rounder body,
and a less pronounced lateral line without rough scales, which also curves
more gradually than the sharp zigzag of the jackmackerel.
Koheru can change colour rapidly, particularly their yellow stripe. Otherwise, they are electric blue on the back, sometimes tinged with green, and silver-white on the belly, with a prominent yellow stripe running along the back and on to the caudal peduncle. Juveniles are usually green-yellow in colour.
Large
schools of small individuals are usually seen swimming above the bottom
near sea cliffs and rocky reefs, whereas the larger fish form fast swimming
schools further from the shore. Watched from above these rapid moving schools
show occasional flashes of white as an individual extends its mouth tube
and gulps down a planktonic crustacean. They sometimes sweep in close to
the rock faces on the offshore islands and pose briefly, bodies twitching,
for the parasite-picking crimson cleaners, milling around for a few minutes
before streaming back out into open water.
Fishing: Koheru are not fished commercially but they do take the bait. Anglers use koheru as bait although its flesh tastes good (like kingfish) and it has plenty of meat on the carcass.
Fins: D VI+I 29: A III 28: P I 5; very small scales. size to 40cm
Description: Blue-green above, with golden stripe along back; silver below; yellow tail; black spot on gill cover. Pectoral fin short (does not reach second dorsal fin). Lateral line dips gently downwards below second dorsal fin. Length 50 cm. Differs from jack mackerel in length of pectoral fin, shape of lateral line and in lacking large bony scales along lateral line.
Life
history: Koheru live to a maximum age of 10 years. Initial growth is
rapid: 1year=20cm, 2years=32cm, 3 years=37cm but then growth slows down.
Age and size at maturity are not known. Group spawning has been reported
in mid water during summer (Dec to Feb).
Koheru feed on planktonic crustaceans such as swimming shrimps, and larvae. In the food chain, Koheru play an important role, since they are eaten by many predators, including dolphins.
Distribution: Koheru are found between North Cape and East Cape
on the east coast of the North Island and they occur only in New Zealand
waters. Koheru prefer the clean waters of outer islands where they are
common, whereas jackmackerels prefer the coastal waters.
| Dying schools
In the beginning of June 1995 dead pilchards were seen washed ashore and floating around Little Barrier Island and further inside the Hauraki Gulf. Preliminary observations showed that a large part of the pilchard stock was affected. Deaths were also reported for koheru, jack mackerel and yellow-eyed mullet.
Later I would learn that these fish had been ill and that many were dying, leaving their bodies scattered over the seafloor, too many to be cleaned up by the then satiated scavengers. 19971117, Floor Anthoni |
| The jack mackerel is a true oceanic fish. It is found everywhere in the world from tropical seas to subtropical seas. It swims fast and grows old for a fish so small. Many predators depend on it for a living. It tolerates polluted waters. |
The NZ fish with the rough tail: (trachurus=rough tail, Gk trakhus=rough). The name yellowtail is not such a good choice for this fish, since koheru, kingfish and trevally all have yellow tails.
The
jack mackerel is an elongate fish similar in shape and appearance to the
koheru but growing to a larger size, averaging 35 to 50cm in length. It
is easily distinguished from the koheru by having a blunter snout, larger
eyes, and a line of large and conspicuous bony scutes along the entire
lateral line. Jack mackerel are pale blue-green on the back, faintly barred
with brown, and silver-white beneath. They are a schooling pelagic fish
common all around New Zealand, near the bottom and in mid-water rather
than on the surface, from near the shore out to depths of about 300m on
the upper continental slope. Smaller individuals are more brown-yellow
in colour and are generally found in shallower water closer to shore. Like
other carangids they are relatively slow growing and large specimens have
been aged at over 30 years. The other New Zealand mackerel Trachurus novaezelandiae
which is indistinguishable from the jack mackerel unless examined closely,
has been reported from south-east Asia and Japan as well.
Fishing:
Jack mackerel are an important commercial species with the main fishing
grounds off the west coast of the North Island. Total annual catch in the
New Zealand area is 1000 tonnes by local boats and an estimated 20,000
tonnes by foreign fishing vessels. The same species is also found in southern
Australia.
Description: Yellow-green with faint brown or grey bands above, silver or white below; yellow tail; black spot on gill cover. Juveniles mostly silver. A row of large bony scales along lateral line drops sharply below second dorsal fin. Pectoral fin long, reaching beyond front of second dorsal fin. Length 45 cm. Differs from koheru in length of pectoral fin, shape of lateral line, and size of lateral line scales.
Fins: D VIII+I 33; A III 29;P I 5; LL 80. Size 35-50cm.
Life
history: Jack mackerel eat plankton and small pelagic fishes. They
spawn in groups during summer, and schools of the mirrorlike juveniles
are seen close to reefs during autumn. Juveniles are found sheltering within
the stinging tentacles of jellyfish. They grow to 25 cm in 3-4 years, and
may live up to 25 years.
Distribution: North Cape to Foveaux Strait. Australia, Japan,
Asia.
The lure of the wharf
 Jack
mackerels are versatile fish, found inside estuaries and also in the wide
open ocean. Obviously these fish possess some secret survival skills because
they grow very old as well. The problem with pelagic fish is that an encounter
under water is possible only by a very lucky set of circumstances. But
underneath wharves, the situation is entirely different. Wharves with their
platforms perching over the water, act as manmade archways, attracting
fish by the thousands, a fact well known by little school children. Houhora
wharf in the far north is my favourite. Underneath it one finds the densest
aggregation of fish I have ever seen: jack mackerel, koheru and yellow-eyed
mullet, three schools mixing reluctantly. The recent extension of this
wharf has been welcomed by our fishy friends but is not nearly enough to
accommodate them all. Snorkelling here on the high tide - with care and
respect for the commercial fishing boats - must be rated as a super event.
During mid day, the sunlight peeps in between the moored ships, to form
curtains of light under water, through which these fish swim. Unfortunately
conditions are a bit too dark for taking photos.
19971117 Floor Anthoni |
Trachurus declivis - ocean jack mackerel
The
fish with the rough down-sloping lateral line: (declivis=sloping down,
L clivus=slope), (trachurus=rough tail, Gk trakhus=rough).
A second species of mackerel Trachurus declivis, very simi!ar to the jack mackerel Trachurus novaezelandiae, is also found in New Zealand but is not illustrated here. It is most easily distinguished from the common jack mackerel by having between76 and 82 lateral line scutes, compared with 68 to73 for the jack mackerel, and by having more anal fin rays (26-29 compared with 24-28).
Trachurus declivis is difficult to distinguish, and it tends to occur further offshore and is rarely seen by divers.
| Kingfish are perhaps the most impressive fish around NZ. They can grow as big as our small dolphins. They are beautifully efficient, even awesome. They hunt in small groups. They swim very fast. They fight savagely when caught and their flesh which has few bones, tastes good. They roam vast distances and are hard to protect inside marine reserves. |
The imposing, solemn looking fish: (seriola=serious, L serius=serious), (grandis= great, L grandis=fully grown). The name seriola was already used in the Mediterranean Sea for similar looking fish, even today .
Kingfish
are impressive encounters under water. They swarm around the diver, with
interest that soon wanes. After a few close passes (during which they come
close enough to be speared), they carry on with what they were doing and
disappear out of sight. Their round bodies are extremely well streamlined
which allows them to cruise effortlessly at high speed. With a few flicks
of their tails they can accelerate quickly to a high speed. But their round,
streamlined bodies do not allow them to dodge quickly. When attacking schoolfish
such as young koheru, jack mackerel and pilchard, they work co-operatively
as a group, creating confusion in the attacked school and making quick
passes in and out of the scattering school. In this respect their hunting
resembles that of dolphins. Meeting a big one of nearly 2m is a breathtaking
experience. Also being buzzed by some twenty smaller ones of 1.2m is quite
awesome, even frightening.
The kingfish is a large carangid that is almost legendary for its abilities as a game fish. Although only averaging 80cm to 1.2m in length it occasionally reaches a far larger size in the offshore waters of northern New Zealand, and specimens of almost 2m length have been sighted by divers. The record weight for this species is just over 50Kg but fishes up to 70 or 80Kg probably occur. They are solid bodied with the typical carangid fin arrangement, but with a very low first dorsal fin.
Kingfish
are a beautiful dark blue-green on the back and silver-white on the belly,
the two colours being separated by a broad yellow longitudinal stripe.
The pelvic fins are blue-white and the rest of the fins including the tail
are yellow.
These fishes are fast swimming active carnivores, moving either singly or in schools that may contain anything up to several thousand individuals. They are often found in association with schools of smaller pelagic fishes such as trevally and koheru, circling on the outskirts in search of a weak or unwary member of the other school. Bands of minute grasping teeth line the jaws and are used to hold prey fishes while they are swallowed whole.
Fishing:
Kingfish are a sought-after game fish because of their size, fine flesh
and fighting abilities. They can be caught on a lure while trolling or
jigging but they also take the bait. Kingfish fight strongly when speared
or hooked. Little is known of their habits or biology, although they are
probably slow growing. Preliminary evidence indicates that some schools
are resident in one area for some time.
Kingfish is not caught commercially although numbers obtained from bycatches are substantial.
Description: Long, streamlined body, green above and white below; green-gold stripe from snout through eye to yellow tail. Length up to 150 cm.
Fins: D VI+I 33;A II 21; P I 5; LL 180. size 100-120cm.
Life
history: Little is known. Group spawning activity has been observed
over mid summer but the exact duration of the spawning season is unknown.
Kingfish can grow to 150cm in length at 50 Kg but their maximum age is
not known.
Distribution: Kingfish are found in inshore waters throughout the North Island, and around the north of the South Island during the summer months. They are larger and more abundant around the offshore islands of Northland and the Bay of Plenty and a big school of these fishes swimming in the clear oceanic waters is an awe inspiring sight.
They are found at the Kermadec Islands and from Three Kings Islands
to Foveaux Strait, but are uncommon south of Cook Strait, penetrating farthest
South in summer and autumn. This species is also found around Norfolk Island,
Lord Howe Island, and off southern and eastern Australia.
| Dodging a kingfish
Making a quiet dive at the Poor Knights in 1995, studying the return of small schooling fish, I was observing a school of 5cm small koheru which were feeding on invisible plankton shrimps. While doing so, the school was scattered over an area of 4 by 4 m. Then out of nowhere, a small kingfish of about 70cm attacked. The small school immediately concentrated into a dense ball and swam away from it, its many yellow-white facets shimmering in the sunlight. The speed with which they swam came as a complete surprise. They outswam the fast attacker and effortlessly dodged it with fast sidesteps. The kingfish made several passes but was not able to dislodge the group. It then swam away, only to return after some time. It repeated the attack six times, then gave up. Each time peace returned quickly, the little koheru scattering again and feeding as if nothing had happened. The big surprise of this event was how fast little fish can swim when packed into a tight formation. Their speed would have been over 5m/s, about 20Km/hr, much faster than each on its own could swim. It appears as if a tight formation adds an unknown synergy in the form of either less friction or less turbulence or something else. The single 'motor' formed by little whipping tails close to another and spaced at mathematical distances, may be of superb design, multiplying the power of each little fin while dividing the friction of the whole. Have our engineers overlooked something?
Floor Anthoni, Leigh. 19971117 |
| Samson fish are not normally found in NZ but may arrive here during warm summers. They are big fish, flatter than the kingfish but longer and higher. They are seen in small schools, sometimes inside harbours. |
The serious horse fish: (hippos=horse, Gk&L hippos=horse), (Samson= a person of great strength, Gk Sampson=Simson). The name seriola has been in use by Romans and Greek for a similar looking fish.
The
Samson fish is very similar in shape and colour to a large trevally but
has a low first dorsal fin and a shorter pectoral fin. It grows considerably
larger than the trevally and adult specimens range in size from 80cm to
over l.5m. Dark blue-green above and silver-white below, they have conspicuous
golden yellow flanks and fins, and a large individual is an impressive
sight when seen underwater or when freshly caught. Samson fish are fast
swimming pelagic carnivores with similar habits to the kingfish.
In Australia and perhaps here too, the amber jack (Seriola purpurascens) is often mistaken for samson fish. The amberjack grows bigger still to 180cm!
Fishing: not normally caught here. In Australis its meat is claimed to be 'only fair and dry'.
Fins: Size 80-150cm; D VIII+I 24; A III 16; P I 5; very small scales
Life history:
Distribution: They are common in southern and eastern Australia
and a few stragglers have been reported from eastern Northland at the end
of exceptionally warm summers. It occurs frequently in western Australia.
| A rare sight
In the summer of 1994 (?) an oyster farmer near Warkworth observed large fish amongst the oyster racks at high tide, something he had never seen before. They had bright yellow flanks and looked a little like kingfish. They were swimming with their dorsal fins poking out of the water. It couldn't be determined what they were doing. These fish were later identified as Samson fish. The following year around the same time, they returned to visit the same oyster farm. |
The
pilotfish is similar in shape to a kingfish but is considerably smaller
in size, rarely reaching 60cm in length. They are pale blue-grey in colour
with five or six darker vertical bands that extend on to the fins. When
they are small pilotfish are often found in groups beneath the protective
umbrella of a large jellyfish. Larger individuals swim with sharks or manta
rays or are associated with floating logs and seaweed. These fishes were
named pilotfish because of old tales that told of them guiding their supposedly
poor sighted shark hosts towards suitable prey. In fact sharks can see
very well and pilotfish, like remoras, merely feed on the scraps they leave
or on small animals they disturb. These fishes are widespread in the tropical
Indo-Pacific and are sometimes found in offshore waters of northern New
Zealand during the summer.
Fins: D IV+I 27; A III 16; P I 5;very small scales. Size 30-60cm.
The
dolphinfish, not to be confused with the true dolphin which is an air breathing
mammal and not a fish, is a spectacularly coloured blue ocean pelagic species.
Although it reaches a maximum length of about 1.8m the specimens found
in New Zealand are generally much smaller, rarely exceeding 80cm. It is
a streamlined fleet-finned swimmer, fast enough to run down flying fish
and other fast pelagics on which it feeds.
When experiments were done with aggregation buoys in NE waters, dolphinfish were often found sheltering underneath. To encounter a live dolphinfish under water is an amazing spectacle. This fish looks so bright and golden and is able to change its colours so rapidly and completely that perhaps no other creature in the world can match it.
Fishing: On a line, it is a fast and bold fighting fish, dashing about wildly on the surface and usually putting up some amazing leaps. In common with other very fast fish, such as the tunas, some of its fins can be folded into recesses in the body to give improved streamlining. In this case the rather large ventral fins, which are unusually close together, fold away into an abdominal groove. In its form it presents a number of unusual features. It has a tightly compressed, deep body, and its steep frontal slope and surmounting bony crest give it a high front elevation which becomes more pronounced with age. Its unusually long dorsal fin, which runs from the top of the head back to the butt of the tail, contains no hard spines. Its very wide tail fin is so deeply forked as to leave the lobes extraordinarily long and narrow.
Its colours, and the colour changes it undergoes when removed from the water, are more remarkable still. The eyes are emerald green with black pupils. The head is olive green, the dorsal fin dark purplish-blue. The back and upper body brilliant green-blue merging to yellower silver on the belly and the tail fin greenish olive gold. There are yellow areas under the throat and jaws and numerous blue-green spots on the head and body. The whole is overshot with varying tones of gold and purple. On removal from the water its colours fade and brighten again repeatedly as it extends and folds its big dorsal fin, while flushes of pink and gold come and go on its skin. On death the superb colours fade away to a dull leaden-grey.
Dolphinfish flesh commands good prices by sports fishermen. It cooks greyish-white but is of excellent flavour.
Description: These fishes have a large blunt snouted head and a long tapering body that ends in a huge deeply forked tail. The front portion of the long dorsal fin is raised into a rounded peak behind the head. Large males develop a steep bony crest on the forehead but the females and smaller males found in New Zealand have a more gently rounded head. The colours of the dolphinfish are almost as famous as their fighting ability when hooked and have even been praised in glowing terms by otherwise staid fish scientists. A bright green-blue back grades into a golden belly with small black spots on the sides, but a play of other iridescent colours is seen on freshly caught specimens. The dorsal fin is electric blue and the other fins are yellow or orange. These incredible glowing colours fade within a few minutes of death.
Fins: D 62; A 27; P I 5; small scales. Length 100-180cm. Weight up to 35 Kg.
Life history: very litle is known about the life histories of truely oceanic fishes, the dolphinfish being no exception.
Distribution: All around the Pacific Ocean, from Japan to New Zealand.
Meeting
a school of blunt-nosed old trevally is an incredible sight, particularly
when they are in a feeding frenzy. Once such encounters were possible at
the Poor Knights Islands and the Cavalli Islands. But the technique of
purse-seining with bowl-shaped nets, together with the absence of a quota,
allowed a well-known fishing company to almost annihilate this species.
The whole diving fraternity was stunned but had confidence in them returning
in due time. But this never happened. As fish stocks were rebuilding and
surface-schooling was present again (1990,1991), suddenly El Niño
struck and all schooling fish disappeared at the end of 1992. Only at the
time of writing are fish returning, although not even one tenth of what
was there.
Planktonic
shrimp of many types have fine sieves on some appendages with which they
can catch the minute plant plankton. Although these shrimp swim energetically,
they nonetheless grow rapidly, explosively even. They somehow manage to
aggregate in densities adequate to stimulate a feeding frenzy. Large trevally
of 70cm need big (3cm) shrimps and when these are found, patience is in
short supply. The trevally attack, driving the shrimps to the surface,
where the birds are waiting for them. More trevally arrive from the deep
and more shrimps are herded to the surface, where the fish gobble shrimps
mixed with water and air while turning the sea into a humming noise. Then
as by some surreptitious cue, the fish suddenly dive under, leaving a white
foam cushion behind with a loud swish. The sea birds pause and wait for
the next attack.
Trying
to be in the middle of it all, as a diver, is impossible because the schools
move fast. Even as a snorkeldiver you would stand a poor chance. But with
the help of a boat, an assistant, and a few hours of persistence, one may
finally become accepted into the middle of it all. That is an incredible
experience, inviting a fierce rush of adrenaline. These fish are so fast
and with their grim looks, appear to mean serious business. During one
of these encounters, being completely surrounded by them, I could for the
first time hear and feel fish swim. Despite all efforts, I have been let
down by the camera. Perhaps because everything happens so quickly, a good
shot remains elusive.
Little
trevally have an entirely different story to tell. You can find the 1-2cm
postage stamps hanging under the stinging tentacles of the Plagusia jellyfish
or even underneath the open bowl of the non-stinging earlobe jellyfish.
When snorkelling, you may find yourself accompanied like pilot fish accompany
a big shark But most endearing is the cleaning service offered by these
little entrepreneurs.
On
8 June 1995, I encountered a huge school (perhaps 5000 fish) of koheru
quietly parked alongside the vertical wall off Panetiki Island, outside
Leigh Harbour. It was very unusual that these fish seemed to ignore my
presence completely. They just hung there in a loose formation, very slowly
swimming against the current while lazily dodging my rising bubbles. Normally
such fish would be very lively and dart around you in ever changing formations,
swimming around you in opposite directions as well. This time they were
just lethargic, letting me take photographs from nearby.
On
one dive there, a long time ago, I was filming the (now disappeared) mussel
community at 12 m depth, when the sky suddenly darkened and a huge object
moved over me. I was suitably frightened, since sharks are not uncommon
here, only to discover that the body was that of a dense school. As it
moved further, the night suddenly fell, leaving me in almost complete darkness,
only with the movie lights bouncing off thousands of mirroring fish bodies.
I had never realised that a school of fish could form a shadow so complete
as to leave complete darkness underneath.
The
story coninues with a similar situation a few years later, when a small
kingfish tried in vain to attack a mixed school of blue maomao and trevally.
After apparently giving up, it reappeared hiding underneath the belly of
a stingray. Its plan would have been perfect, but the stingray could not
burrow in the white sand, with a kingfish underneath. So it made tight
passes over the stalked kelp to rub it off, and this foiled the little
kingfish's plans. Suffice to say that the target prey fish had no idea
of what went on.