The silky shark (Carcharhinus falciformis) is a species of requiem shark, family Carcharhinidae, named for the smooth texture of its skin. It is one of the most abundant sharks in the pelagic zone, and can be found around the world in tropical waters. Highly mobile and migratory, this shark is most often found over the edge of the continental shelf down to a depth of 50 m (164 ft).
The silky shark has a slender, streamlined body and typically grows to a length of 2.5 m (8 ft 2 in). It can be distinguished from other large requiem sharks by its relatively small first dorsal fin with a curving rear margin, its tiny second dorsal fin with a long free rear tip, and its long, sickle-shapedpectoral fins. It is a deep, metallic bronze-gray above and white below.
With prey often scarce in its oceanic environment, the silky shark is a swift, inquisitive, and persistent hunter. It feeds mainly on bony fishes and cephalopods, and has been known to drive them into compacted schools before launching open-mouthed, slashing attacks. This species often trails schools of tuna, a favored prey. Its sense of hearing is extremely acute, allowing it to localize the low-frequency noises generated by other feeding animals, and, by extension, sources of food. The silky shark is viviparous, meaning that the developing embryos are sustained by aplacental connection to their mother. There is significant geographical variation in its life historydetails. Reproduction occurs year-round except in the Gulf of Mexico, where it follows a seasonal cycle. Females give birth to litters of up to 16 pups annually or biennially. The newborn sharks spend their first months in relatively sheltered reef nurseries on the outer continental shelf, growing substantially before moving into the open ocean.
The large size and cutting teeth of the silky shark make it potentially dangerous, and it has behaved aggressively towards divers. However, attacks are rare as few humans enter its oceanic habitat. Silky sharks are valued for their fins, and to a lesser extent their meat, hide, liver oil, and jaws. Because of their abundance, they form a major component of commercial and artisanal shark fisheries in many countries. Furthermore, their association with tuna results in many sharks being taken as bycatch in tuna fisheries. Although slow-reproducing like most other sharks, the wide distribution and large population size of the silky shark was once thought to buffer the species against these fishing pressures. However, data now suggest that silky shark numbers are declining around the world, which prompted the International Union for Conservation of Nature(IUCN) to reassess its conservation status from Least Concern to Near Threatened in 2007.
Slim and streamlined, the silky shark has a fairly long, rounded snout with barely developed flaps of skin in front of the nostrils. The circular, medium-sized eyes are equipped with nictitating membranes (protective third eyelids). Short, shallow furrows are present at the corners of the mouth. There are 14–16 and 13–17 tooth rows on either side of the upper and lower jaws respectively (typically 15 for both). The upper teeth are triangular and strongly serrated, with a notch in the posterior edge; they are erect at the center and become more oblique towards the sides. The lower teeth are narrow, erect, and smooth-edged. The five pairs of gill slits are moderate in length.
The dorsal and pectoral fins are distinctive and help to distinguish the silky shark from similar species. The first dorsal fin is relatively small, measuring less than a tenth as high as the shark is long, and originates behind the free rear tips of the pectoral fins. It has a rounded apex, an “S”-shaped rear margin, and a free rear tip about half as long as the fin is tall. The second dorsal fin is tiny, smaller than the anal fin, with a drawn-out free rear tip up to three times as long as the fin is tall. There is a narrow dorsal ridge running between the dorsal fins. The pectoral fins are narrow and falcate (sickle-shaped), and particularly long in adults. The anal fin originates slightly ahead of the second dorsal fin and has a deep notch in the posterior margin. The caudal fin is fairly high with a well-developed lower lobe.
The skin is densely covered by minute, overlapping dermal denticles. Each dermal denticle is diamond-shaped and bears horizontal ridges leading to posterior marginal teeth, which increase in number as the shark grows. The back is metallic golden-brown to dark gray and the belly is snowy white, which extends onto the flank as a faint lighter stripe. The fins (except for the first dorsal) darken at the tips; this is more obvious in young sharks. The coloration quickly fades to a dull gray after death. One of the larger members of its genus, the silky shark commonly reaches a length of 2.5 m (8.2 ft), with a maximum recorded length and weight of 3.5 m (11 ft) and 346 kg (760 lb) respectively. Females grow larger than males.
Biology and ecology
The silky shark is one of the three most common pelagic sharks along with the blue and oceanic whitetip sharks, and counts among the most numerous large oceanic animals in the world with a population of at least tens of millions. Compared to the other two species, it is less strictly pelagic with the greatest numbers found in offshore waters associated with land, where food is more readily obtained than farther out in the truly open ocean. The silky shark is an active, inquisitive, and aggressive predator, though it will defer to the slower but more powerful oceanic whitetip shark in competitive situations. When approaching something of interest it may seem inattentive, sedately circling and sometimes swinging its head from side to side. However, it can respond with startling swiftness to any shift in its immediate surroundings. This shark is often found around floating objects such as logs or tethered naval buoys.
Younger silky sharks are known to form large, loosely organized aggregations, possibly for mutual defense. During migrations, over a thousand individuals may gather. These groups are generally segregated by size, and in the Pacific perhaps also by sex. Silky sharks within a group have been observed to “tilt”, presenting their full lateral profile towards each other, as well as gape their jaws or puff out their gills. On occasion, sharks have also been seen suddenly charging straight up, veering away just before reaching the surface and gliding back down to deeper water. The significance of these behaviors is unknown. When confronted, the silky shark may perform a threat display, in which it arches its back, drops its tail and pectoral fins, and elevates its head. The shark then proceeds to swim in tight loops with a stiff, jerky motion, often turning broadside towards the perceived threat.
Potential predators of the silky shark include larger sharks and killer whales (Orcinus orca). Known parasites of this shark include the isopodGnathia trimaculata, the copepod Kroeyerina cortezensis, and the tapeworms Dasyrhynchus variouncinatus and Phyllobothrium sp.Silky sharks frequently intermingle with schools of scalloped hammerheads (Sphyrna lewini), and have been known to follow marine mammals. One account from the Red Sea describes 25 silky sharks following a large pod of bottlenose dolphins (Tursiops sp.), along with 25 grey reef sharks (C. amblyrhynchos) and a lone silvertip shark (C. albimarginatus). Silky sharks are themselves accompanied by juvenile pilot fish(Naucrates ductor), who “ride” the pressure wave ahead of the shark, as well as by jacks, who snatch scraps of food and rub against the shark’s skin to scrape off parasites.
The silky shark is an opportunistic predator feeding mainly on bony fishes from all levels of thewater column, including tuna, mackerel, sardines, mullets, groupers, snappers, mackerel scads,sea chubs, sea catfish, eels, lanternfishes, filefishes, triggerfishes, and porcupinefishes. It may also take squid, paper nautilus, and swimming crabs, and there is fossil evidence of it scavenging on whale carcasses. Good feeding opportunities can draw silky sharks in large numbers; one such feeding aggregation in the Pacific has been documented “herding” a school of small fishes into a compact mass (a bait ball) and trapping it against the surface, whereupon the sharks consumed the entire school. When attacking tightly packed fish, silky sharks charge through the ball and slash open-mouthed, catching the prey fish at the corners of their jaws. Although multiple individuals may feed at once, each launches its attack independently.
Studies conducted off the Florida coast and the Bahamas have shown that silky sharks are highly sensitive to sound, in particular low-frequency (10–20 Hz), irregular pulses. Experiments in which these sounds were played underwater attracted sharks from hundreds of meters away. Silky sharks likely orient to these sounds because they are similar to the noise generated by feeding animals such as birds or dolphins, and thus indicate promising sources of food. These studies have also demonstrated that a silky shark attracted by one sound will quickly withdraw if that sound abruptly changes in amplitude or character; this change need not be to a sound produced by a predator to evoke the reaction. Over repeated exposures, silky sharks will habituate to the sound change and stop withdrawing, though it takes them much longer to do so compared to the bolder oceanic whitetip shark.
The bite force of a 2 m (6.6 ft) long silky shark has been measured at 890 newtons (200 lbf). There is a well-established association between this species and tuna: off Ghana, almost every tuna school has silky sharks trailing behind, and in the eastern Pacific these sharks inflict such damage to tuna fishing gear and catches that fishery workers haven given them the moniker “net-eating shark”. Silky sharks and bottlenose dolphins compete with one another when both species target the same school of fish; the amount eaten by the dolphins decreases relative to the number of sharks present. If there are a large number of sharks, they tend to remain inside the prey school while the dolphins consign themselves to the periphery, possibly to avoid incidental injury from the sharks’ slashing attacks. Conversely, if a large enough group of dolphins gathers, they become able to chase the sharks away from the prey school. Regardless of which one dominates, the two predators do not engage in any overtly aggressive behavior against each other.
Silky sharks in most parts of the world are thought to reproduce year-round, whereas mating and birthing in the Gulf of Mexico take place in late spring or early summer (May to August).However, in some cases the presence of reproductive seasonality may have been obscured by biases in data collection. Females give birth after a gestation period of 12 months, either every year or every other year. The litter size ranges from 1 to 16 and increases with female size, with 6–12 being typical. The pups are born in reef nursery areas on the outer continental shelf, where there are ample food supplies and protection from large pelagic sharks. The risk of predation has selected for fast growth in young sharks, which add 25–30 cm (9.8–12 in) to their length within their first year of life. After a few months (or by the first winter in the Gulf of Mexico), the now-subadult sharks migrate out from the nursery into the open ocean.
|Region||Length at birth||Male length at maturity||Female length at maturity|
|Northwestern Atlantic||68–84 cm (27–33 in)||2.15–2.25 m (7.1–7.4 ft)||2.32–2.46 m (7.6–8.1 ft)|
|Eastern Atlantic||?||2.20 m (7.2 ft)||2.38–2.50 m (7.8–8.2 ft)|
|Indian||56–87 cm (22–34 in)||2.39–2.40 m (7.8–7.9 ft)||2.16–2.60 m (7.1–8.5 ft)|
|Western Pacific||?||2.10–2.14 m (6.9–7.0 ft)||2.02–2.20 m (6.6–7.2 ft)|
|Central Pacific||65–81 cm (26–32 in)||1.86 m (6.1 ft)||2.00–2.18 m (6.56–7.15 ft)|
|Eastern Pacific||70 cm (28 in)||1.80–1.82 m (5.9–6.0 ft)||1.80–1.82 m (5.9–6.0 ft)|
The life history characteristics of the silky shark differ across its range (see table). Northwestern Atlantic sharks tend to be larger than those in the western-central Pacific at all ages, while eastern Pacific sharks tend to be smaller than sharks in other regions. Eastern Atlantic and Indian Ocean sharks seem to match or exceed the size of northwestern Atlantic sharks, but the figures are based on relatively few individuals and more data is needed.
The overall growth rate of the silky shark is moderate compared to other shark species and similar for both sexes, though it varies significantly between individuals. One central Pacific study has found females growing much slower than males, but the results may have been skewed by missing data from large females. The highest reported growth rates are from sharks in the northern Gulf of Mexico, and the lowest from sharks off northeastern Taiwan. Males and females reach sexual maturity at ages of 6–10 years and 7–12+ years respectively. Sharks from more temperate waters may grow slower and mature later than those in warmer regions. The maximum lifespan is at least 22 years.
As one of the most abundant and widely distributed sharks on Earth, the silky shark was once thought to be mostly immune to depletion despite heavy fishing mortality. In 1989 alone, some 900,000 individuals were taken as bycatch in the southern and central Pacific tuna longline fishery, seemingly without effect on the total population. Fishery data on this shark is often confounded by under-reporting, lack of species-level separation, and problematic identification. Nevertheless, there is mounting evidence that the silky shark has in fact declined substantially worldwide, a consequence of its modest reproductive rate which is unable to sustain such high levels of exploitation. The total annual catch reported to the Food and Agricultural Organization (FAO) fell steadily from 11,680 tons in 2000 to 4,358 tons in 2004. Regional assessments have found similar trends, estimating declines of some 90% in the central Pacific from the 1950s to the 1990s, 60% off Costa Rica from 1991 to 2000, 91% in the Gulf of Mexico from the 1950s to the 1990s, and 85% (for all large requiem sharks) in the northwestern Atlantic from 1986 to 2005. The silky shark fishery off Sri Lanka reported a drop from a peak catch of 25,400 tons in 1994 to only 1,960 tons in 2006, indicative of a local stockcollapse. On the other hand, Japanese fisheries in the Pacific and Indian Oceans have recorded no change in catch rate between the 1970s and the 1990s, and the validity of the methodologies used to assess declines in the Gulf of Mexico and the northwestern Atlantic have come under much debate.
In light of recent findings, in 2007 the International Union for Conservation of Nature (IUCN) reassessed the silky shark from Least Concern to Near Threatened worldwide. Regionally, it is listed as Near Threatened in the southwestern Atlantic, Indian Ocean and western central Pacific, and asVulnerable in the eastern central and southeastern Pacific and the northwestern and western central Atlantic. The silky shark is listed on Annex I, Highly Migratory Species, of the United Nations Convention on the Law of the Sea, though this has yet to result in any management schemes. The species should benefit from bans on shark finning, which are being increasingly implemented by nations and supranational entities, including the United States, Australia, and the European Union. Organizations such as the International Commission for the Conservation of Atlantic Tunas (ICCAT) and the Inter-American Tropical Tuna Commission (IATTC) have also taken steps to improve fishery monitoring, with the ultimate goal of reducing shark bycatch. However, given the highly migratory nature of the silky shark and its association with tuna, there is no simple way to reduce bycatch without also affecting the economics of the fishery.