Dusky shark Conservation status Near Threatened (IUCN 2.3)[1] Scientific classification Kingdom: Animalia Phylum: Chordata Class: Chondrichthyes Subclass: Elasmobranchii Order: Carcharhiniformes Family: Carcharhinidae Genus: Carcharhinus Species: C. obscurus Binomial name Carcharhinus obscurus (Lesueur, 1818) Range of the dusky shark Synonyms Carcharhinus iranzae Fourmanoir, 1961 Carcharhinus obscurella Deng, Xiong & Zhan, 1981 Carcharias macrurus Ramsay & Ogilby, 1887 Carcharinus iranzae Fourmanoir, 1961 Galeolamna eblis Whitley, 1944 Galeolamna greyi* Owen, 1853 Prionodon obvelatus Valenciennes, 1844 Squalus obscurus Lesueur, 1818 *ambiguous synonym

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The dusky shark, Carcharhinus obscurus, is a species of requiem shark, family Carcharhinidae, occurring in tropical and warm-temperate continental seas worldwide. A generalist apex predator, the dusky shark can be found from the coast to the outer continental shelf and adjacent pelagic waters, and has been recorded from a depth of 400 m (1,300 ft). Populations migrate seasonally towards the poles in the summer and towards the equator in the winter, traveling hundreds to thousands of kilometers. One of the largest members of its genus, the dusky shark reaches 4.2 m (14 ft) in length and 347 kg (765 lb) in weight. It has a slender, streamlined body and can be identified by its short round snout, long sickle-shaped pectoral fins, ridge between the first and second dorsal fins, and faintly marked fins.

Adult dusky sharks have a broad and varied diet, consisting mostly of bony fishes, sharks and rays, and cephalopods, but also occasionally crustaceans, sea stars, bryozoans, sea turtles, marine mammals, carrion, and garbage. This species is viviparous with a three-year reproductive cycle; females bear litters of 3–14 young after a gestation period of 22–24 months, after which there is a year of rest before they become pregnant again. Females are capable of storing sperm for long periods, as their encounters with suitable mates may be few and far between due to their nomadic lifestyle and low overall abundance. Dusky sharks are one of the slowest-growing and latest-maturing sharks, not reaching adulthood until around 20 years of age.

Because of its slow reproductive rate, the dusky shark is very vulnerable to human-caused population depletion. This species is highly valued by commercial fisheries for its fins, used in shark fin soup, and for its meat, skin, and liver oil. It is also esteemed by recreational fishers. The International Union for Conservation of Nature (IUCN) has assessed this species as Near Threatened worldwide and Vulnerable off the eastern United States, where populations have dropped to 15–20% of 1970s levels. The dusky shark is regarded as potentially dangerous to humans due to its large size, but there are few attacks attributable to it.

[edit] Taxonomy and phylogeny

Illustration of a dusky shark and its tooth.

The dusky shark was first described by the French naturalist Charles Alexandre Lesueur in 1818 as Squalus obscurus, in the Journal of the Academy of Natural Sciences of Philadelphia. Later authors moved this species to the genus Carcharhinus.^[2] The specific epithet obscurus is Latin for "dark" or "dim", referring to this shark's coloration. In early literature, this species was referred to as C. lamiella. However, the type specimen of C. lamiella was later identified as a copper shark (C. brachyurus), so C. lamiella is not regarded as a synonym of C. obscurus.^[3] Other common names include bay-shark, black whaler, bronze whaler, brown common gray shark, brown dusky shark, brown shark, common whaler, dusky ground shark, dusky whaler, river whaler, shovelnose, and slender whaler shark. Some of these names are also used for other species.^[4]

In 1982, Jack Garrick placed the dusky shark and the Galapagos shark (C. galapagensis) at the center of the "obscurus group", one of two major groupings within Carcharhinus. This group consists of large, triangular-toothed sharks and is defined by the presence of a ridge between the two dorsal fins.^[5] This interpretation was corroborated by Gavin Naylor's 1992 allozyme analysis, which placed the dusky shark in a clade that also contains the Galapagos shark, oceanic whitetip shark (C. longimanus), silky shark (C. falciformis), sandbar shark (C. plumbeus), bignose shark (C. altimus), Caribbean reef shark (C. perezi), and the blue shark (Prionace glauca).^[6]

[edit] Distribution

The dusky shark occurs worldwide in tropical and warm-temperate waters. In the western Atlantic, it is known from Massachusetts to the northern Gulf of Mexico, and from Nicaragua to southern Brazil. In the eastern Atlantic, it is known from the western Mediterranean Sea, the Canary Islands, Cape Verde, Senegal, Sierra Leone, and possibly elsewhere. In the Indian Ocean, it occurs off South Africa, Mozambique, and Madagascar. In the western Pacific, it is found from Japan to Queensland and New South Wales, Australia. In the eastern Pacific, it occurs from southern California to the Sea of Cortez, and off Revillagigedo and possibly Chile.^[1][2] Records from insular waters may represent misidentifications of Galapagos sharks.^[7]

[edit] Habitat

Residing off continental coastlines from the surf zone to the outer continental shelf and adjacent oceanic waters, the dusky shark occupies an intermediate habitat that overlaps with its more specialized relatives such as the inshore sandbar shark, the oceanic silky shark, the deepwater bignose shark, and the islandic Galapagos shark. This species has been reported from the surface to as deep as 400 m (1,300 ft).^[2] They prefer temperatures of 19–23° C (66–73° F) and avoid areas of low salinity such as estuaries.^[3]

Dusky sharks are strongly migratory, undertaking movements of up to 3,800 km (2,360 mi). They shift closer to the poles in the summer and retreat towards the equator in the winter as temperatures fall, with adults swimming longer distances than juveniles.^[7] This shark is generally found in deeper water in cooler months. In the nursery area off KwaZulu-Natal, young sharks over 90 cm (3 ft) long disperse in different directions based on sex (with some overlap) – females northward and males southward – and join the adults several years later by a yet-unidentified route. Off South Africa, juveniles are found in the surf zone in spring and summer and offshore in fall and winter.^[2][8] Off Western Australia, both adults and juveniles migrate inshore in summer and fall.^[7]

[edit] Description

One of the distinguishing features of the dusky shark are its long pectoral fins.

The dusky shark has a long, streamlined body with a broadly rounded snout no longer than the width of the mouth, and medium-sized eyes. The flaps in front of the nares are poorly developed, and the furrows at the corners of the mouth are indistinct. There are 13–15 (usually 14) tooth rows in either half of both jaws. The upper teeth are broad, triangular, and slightly oblique with strongly serrated edges; the lower teeth are upright and broad-based, with narrower cusps and finer serrations.^[9]

The pectoral fins are large and falcate (sickle-shaped), measuring around one-fifth as long as the body. The first dorsal fin is of moderate size, with a pointed tip and curved rear margin; there is a low ridge running between it and the small second dorsal fin. The origin of the first dorsal fin is over the free rear tip of the pectoral fins. The dermal denticles are diamond-shaped and closely set, each bearing five horizontal ridges leading to posterior marginal teeth. Dusky sharks are bronzy to bluish gray above and white below, with a faint white band on the flanks. The tips of the fins are darker (but not black), which is more obvious in juveniles.^[2][9] One of the largest members of its genus, adults are typically 3.2 m (10 ft) long and weigh 160–180 kg (350–395 lb), with a maximum recorded length and weight of 4.2 m (14 ft) and 347 kg (765 lb).^[9][4] Females attain larger sizes than males.^[10]

[edit] Biology and ecology

Smaller dusky sharks are vulnerable to predators.

Dusky sharks are nomadic in nature and travel hundreds of kilometers over a single season, usually remaining at a depth of 10–80 m (33–260 ft) with occasional forays into deeper water.^[11] They are frequent followers of ships in offshore waters, such as in the Agulhas Current.^[8] Consistent with their high trophic level, dusky sharks are generally less common than other requiem sharks that share their range.^[2] However, they can be very abundant in particular areas.^[3]

Adult dusky sharks have no significant predators.^[9] Major predators of young dusky sharks include the sand tiger (Carcharias taurus), great white (Carcharodon carcharias), bull (C. leucas), and tiger sharks (Galeocerdo cuvier). Off KwaZulu-Natal, the use of protective nets around beaches has reduced the populations of these predators, leading to an increase in the numbers of juvenile dusky sharks (called predator release).^[2] Known parasites of this species include the copepod Pandarus sinuatus, which attaches to the skin.^[9]

[edit] Feeding

The dusky shark is an apex predator with a generalist diet that includes a wide variety of fishes, invertebrates, and marine mammals from all levels of the water column.^[12] Common prey include pelagic fishes such as mackerel and needlefish, benthic fishes such as flatfish and eels, reef fishes such as groupers and barracudas, cartilaginous fishes such as skates, stingrays, angel sharks, dogfish, houndsharks, and smaller requiem sharks, and cephalopods such as squid and octopus. Occasionally taken food items include crabs, lobsters, barnacles, sea stars, bryozoans, and in very rare cases mammalian carrion (e.g. whale meat) and human refuse.^[2][7]

In the northwest Atlantic, the dusky shark feeds on over ten families of bony fishes (with bluefish and summer flounder being particularly important), as well as on cartilaginous fishes (mainly skates and their egg capsules), crustaceans, molluscs, and sea turtles.^[12] Off South Africa, 75% of its diet is bony fishes, and it also attacks bottlenose dolphins (Tursiops truncatus).^[8][13] Newborns and juveniles feed primarily on small forage fishes such as sardines and anchovies, and squid.^[7] As the sharks age, their diet becomes more varied with larger bony fishes, sharks, and rays becoming more important.^[14] Young dusky sharks hunt for food in large groups.^[3] The bite force exerted by a dusky shark has been measured at 60 kgf (590 N; 130 lbf) at the tip of a single tooth, the highest known of any shark, though this also reflects the concentration of force in the small area of the tooth tip.^[15]

[edit] Life history

A newborn dusky shark; this species is one of the slowest-growing sharks.

Like other requiem sharks, the dusky shark is viviparous: the unborn young are initially sustained by a yolk sac, which is converted into a placental connection once the yolk supply is exhausted. Mating occurs in the spring in the western Atlantic, while there appears to be no reproductive seasonality in other regions such as off South Africa.^[2][3] The females are capable of storing masses of sperm, possibly from multiple males, within their nidamental glands (an organ that secretes egg cases) for months to years. This is likely an adaptation to their nomadic nature and low abundance, which make encounters with potential mates infrequent.^[16]

Female dusky sharks give birth once every three years; the gestation period has been estimated to be as long as 22–24 months, with a one-year interval between pregnancies. The litter size ranges from 3 to 14; the average litter size is larger in the southeastern Atlantic (6–14) than in the western Atlantic (6–10).^[9][7] Unlike other Carcharhinus species, there is no correlation between female size and number of young. Parturition occurs in shallow coastal nursery areas, which are separate from the ranges of the juveniles and adults. Known nursery areas are found off Baja California, KwaZulu-Natal, New Jersey and North Carolina, and southwestern Australia.^[3][7] The newborns measure 0.7–1.0 m (2.3–3.3 ft) long.^[9]

The dusky shark is one of the slowest-growing and latest-maturing shark species.^[7] Its growth rate for the first five years of life is 8–11 cm (3.1–4.3 in) per year.^[17] Juvenile mortality is relatively low, estimated at 27% annually off South Africa.^[18] Throughout their range, male dusky sharks mature at 2.3–2.4 m (7.5–7.9 ft) long, corresponding to an age of 17–22 years, and females mature at 2.2–2.5 m (7.2–8.2 ft) long, corresponding to an age of 20–23 years.^[10][19] The maximum lifespan is thought to be 40–50 years.^[7]

[edit] Human interactions

Dusky sharks are highly valued by commercial fisheries.

Because of its large size, the dusky shark is considered to be potentially dangerous, though little is known of its behavior towards humans underwater.^[2] As of 2008, the dusky shark has been implicated in six attacks listed on the International Shark Attack File, three of them unprovoked (one fatal).^[20] However, some attacks attributed to the dusky shark off Bermuda and other islands were probably caused by Galapagos sharks.^[2]

The dusky shark is one of the most sought-after species for shark fin trade, as its fins are large and contain a high number of internal rays (ceratotrichia).^[7] In addition, the meat is sold fresh, frozen, dried and salted, or smoked, the skin is made into leather, and the liver oil is processed for vitamins.^[4] Dusky sharks are taken by targeted commercial fisheries operating off eastern North America, southwestern Australia, and eastern South Africa using multi-species longlines and gillnets. The southwestern Australian fishery began in the 1940s and expanded in the 1970s to yield 500–600 tons per year. The fishery utilizes selective demersal gillnets that take almost exclusively young sharks under three years old, with 18–28% of all newborns captured in their first year. Demographic models suggest that the fishery is sustainable, provided that the mortality rate of sharks over 2 m (6.6 ft) long is under 4%.^[7]

In addition to commercial shark fisheries, dusky sharks are also caught as bycatch on longlines meant for tuna and swordfish (and usually kept for its valuable fins), and by recreational fishers. Large numbers of dusky sharks, mostly juveniles, are caught by sport fishers off South Africa and eastern Australia. This shark was once one of the most important species in the Florida trophy shark tournaments, before the population collapsed. Another source of mortality are shark nets placed around South African and Australian beaches to protect swimmers; from 1978–1999, an average of 256 dusky sharks were caught every year in mesh nets off KwaZulu-Natal.^[7]

[edit] Conservation status

The International Union for Conservation of Nature (IUCN) has assessed this species as Near Threatened worldwide and Vulnerable in the northwestern Atlantic and the Gulf of Mexico. The American Fisheries Society has also assessed North American dusky shark populations as Vulnerable.^[9] Its very low reproductive rate renders the dusky shark extremely susceptible to overfishing. Stocks off the eastern United States are severely overfished; a 2006 stock assessment survey by the U.S. National Marine Fisheries Service (NMFS) showed that its population had dropped to 15–20% of 1970s levels. In 1997, the dusky shark was identified as a "Species of Concern" by the NMFS, meaning that it warranted conservation concern but there was insufficient information for listing on the U.S. Endangered Species Act (ESA). Commercial and recreational retention of dusky sharks was prohibited in 1998, but this has been of limited effectiveness due to high bycatch mortality on multi-species gear. In addition, some 2,000 dusky sharks were caught by recreational fishers in 2003 despite the ban. In 2005, North Carolina implemented a time/area closure to reduce the impact of recreational fishing.^[21] To aid conservation efforts, molecular techniques using polymerase chain reaction (PCR) have been developed that can identify whether marketed shark parts (e.g. fins) are from prohibited species like the dusky shark, versus similar allowed species such as the sandbar shark.^[22]

[edit] References