The silvertip shark ,
Carcharhinus albimarginatus, is a large species of requiem shark found
in the tropical Indian and Pacific Oceans. This species is often
encountered around offshore islands and coral reefs, and has been known
to dive to a depth of 800 m (2,600 ft). The silvertip shark resembles a
larger, bulkier grey reef shark (C. amblyrhynchos), and can be easily
identified by the prominent white margins on its fins. It attains a
maximum length of 3 m (10 ft).
An aggressive, powerful apex predator, the silvertip shark feeds on a
wide variety of bony fishes, as well as eagle rays, smaller sharks, and
cephalopods. This species dominates other requiem sharks of equal size
when competing for food, and larger individuals are often heavily
scarred from conflicts with others of its species. As with other members
of its family, the silvertip shark is viviparous, with females giving
birth to 1–11 pups in the summer. Silvertip sharks are regarded as
potentially dangerous to humans, as they often approach divers quite
closely. This species is taken by commercial fisheries for its meat and
fins, though there is currently insufficient data for the International
Union for Conservation of Nature (IUCN) to assess its conservation
status.
The silvertip shark was originally described as Carcharias
albimarginatus by German naturalist Eduard Rüppell, in the 1837 Fische
des Rothen Meeres (Fishes of the Red Sea). The name was later changed to
the currently valid Carcharhinus albimarginatus.The specific epithet is
derived from the Latin albi meaning "white", and marginatus meaning "to
enclose with a border". In 1960, a 103 cm (3.4 ft) long immature male
caught off Ras Muhammad in the Red Sea was designated as the type
specimen. Based on similarities in morphology, tooth shape, and
vertebral characters, Garrick (1982) proposed the grey reef shark as the
closest relative of the silvertip shark. This interpretation was
corroborated by Lavery (1992), based on allozyme data.
Distribution and habitat
Silvertip sharks are most often found on or near coral reefs.
The silvertip shark is widely but non-continuously distributed in the
tropical Indian and Pacific Oceans. In the western Indian Ocean, this
species occurs from the Red Sea to South Africa, including Madagascar,
the Seychelles, the Aldabra Group, Mauritius, and the Chagos
Archipelago. In the western Pacific, it is known from off southern Japan
to northern Australia, including Taiwan, the Philippines, Indonesia, New
Caledonia, Guam, Palau, the Solomon Islands, the Marshall Islands, the
Phoenix Islands, and Tahiti. In the eastern Pacific, it occurs from
southern Baja California to Colombia, including the Cocos, Galapagos,
and Revillagigedo Islands. Its presence in the Gulf of Mexico and the
Caribbean Sea is unconfirmed.
Silvertip sharks are found over continental and insular shelves at a
depth of 30–800 m (100–2,600 ft), occupying all levels of the water
column. They are most common around isolated islands, coral banks, and
reef drop-offs. Juveniles frequent coastal shallows or lagoons while
adults occur in deeper water, with little overlap between the two age
groups.
[edit] Description
The silvertip shark can be recognized by its white-edged fins.
The silvertip shark is a robust and streamlined species with a
moderately long, broad snout and large round eyes. The five pairs of
gill slits are short. There are 12–14 tooth rows on each side of both
jaws, with 1–2 small teeth at the symphysis (middle of the jaws). The
upper teeth are broad with oblique triangular cusps and coarse
serrations near the base; the lower teeth have erect cusps with fine
serrations. The first dorsal fin is large and triangular, originating
above or slightly forward of the free pectoral fin tips. There is a
ridge between the first and second dorsal fins. The pectoral fins are
proportionately longer than in most requiem sharks and falcate
(sickle-like) in shape, with pointed tips.
The coloration is blue-gray above with a bronze sheen, and white
below. There is a subtle white band along the sides and distinctive
white tips and borders on all fins. Silvertip sharks can grow up to 3 m
(9.8 ft) long, but typically measure 2.0–2.5 m (6.6–8.2 ft) in length.
The maximum reported weight is 162.2 kg (358 lbs). Females are larger
than males.
Biology and ecology
Though silvertip sharks are quite mobile, they exhibit fidelity to
certain areas and there are reports of territorial behavior. They are
usually encountered alone or in pairs.Small groups of adult females have
been seen in deep water. Individual silvertip sharks behave very
aggressively towards one another, and many are heavily scarred. They are
also reported to dominate Galapagos sharks (C. galapagensis) and
blacktip sharks (C. limbatus) of equal size when competing for food.
This shark sometimes forms mixed-species aggregations with grey reef
sharks. Rainbow runners (Elagatis bipinnulata) have been observed
rubbing against silvertip sharks, using the sharks' rough skin to scrape
off parasites. They sometimes follow marine mammals such as bottlenose
dolphins (Tursiops sp.) in open water, and are themselves followed by
pilot fish (Naucrates ductor).Like the grey reef shark, silvertip sharks
sometimes perform a stereotyped threat display if pursued by divers,
warning that it is prepared to attack. The display begins with the shark
accelerating away to a distance of 15 m (50 ft), before turning and
charging towards the perceived threat. At a distance of two body
lengths, the shark brakes, turns broadside, drops its pectoral fins,
gapes its jaws, lowers the posterior two-thirds of its body, and
"shivers". The last two elements of this display are unique to this
species; the "shivering" may serve to emphasize its white fin markings.
If the diver persists, the shark may rapidly close in and slash with its
upper teeth.
Feeding
The diet of the silvertip shark consists primarily of bony fishes
such as grouper, mackerel, tuna, escolars, lanternfish, flyingfish,
wrasses, and soles. Eagle rays, smaller sharks, and octopus are
occasionally taken.[2] Larger sharks tend to be more sluggish and take
more benthic prey.[12] The differently shaped dentition in their upper
and lower jaws allows them to tackle large prey, gripping and sawing off
chunks of flesh with violent twists and turns.[7] Silvertip sharks have
been observed swimming around the periphery of groups of feeding sharks
of other species, occasionally dashing in to steal food.[2] This species
often approaches ships, as they are attracted to certain artificial,
low-frequency sounds.[15]
[edit] Life history
Like other requiem sharks, the silvertip shark is viviparous; once
the embryos exhaust their supply of yolk, the depleted yolk sac develops
into a placental connection through which the mother delivers
nourishment. In the southern hemisphere, mating and parturition both
occur in summer.[15] Courtship involves the male biting the female to
hold her for copulation; one female observed had the tip of her first
dorsal fin bitten off from such activity. Females bear litters of 1–11
(usually 5–6) young after a gestation period of about one year. The
newborns measure 63–68 cm (25–27 in) long.[2] The growth rate is highly
variable in the wild: Kato and Hernandez (1967) reported that juvenile
silvertip sharks grow an average of 3.8 cm (1.5 in), or 5.3% of their
body length, per year, with some individuals growing as much as 20.8 cm
(8.2 in, 30.1% of their body length) per year and others showing
negative "growth".Males are sexually mature at 1.6–1.8 m (5.2–5.9 ft)
long and females at 1.6–2.0 m (5.2–6.6 ft) long.
[edit] Human interactions
Silvertip sharks often behave boldly towards divers.
Inquisitive and bold, especially in the presence of food, the
silvertip shark is regarded as potentially dangerous to humans. Often
several silvertip sharks will rush up from deep water to inspect divers
when they first enter the water, which can be an intimidating experience
as the sharks may approach quite close. This species has also been known
to circle or pursue divers.[15] In one experiment involving bait, a
large silvertip shark tore the leg off of a dummy dressed in SCUBA gear,
demonstrating that this species is capable of inflicting lethal
injuries. As of 2008, the International Shark Attack File lists four
provoked attacks attributable to this species, none of them fatal.
The silvertip shark is caught by commercial and artisan fisheries
using longlines and gillnets, both intentionally and as bycatch. It is
susceptible to overfishing due to its slow reproductive rate and
tendency to stay in a certain area, and is becoming rare in many parts
of its range. The fins are highly valued for shark fin soup and the meat
is sold fresh or dried and salted. The skin and cartilage are also
utilized. Due to a paucity of fisheries information, the International
Union for Conservation of Nature (IUCN) deemed this species to be "Data
Deficient" worldwide. In Australia, where this species is not exploited
commercially, it has been assessed as "Least Concern". |
