Dusky shark

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Dusky shark
A large gray shark swimming over sand and scattered rocks, being shadowed by a smaller fish with a horizontal stripe and a forked tail
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Subdivision: Selachimorpha
Order: Carcharhiniformes
Family: Carcharhinidae
Genus: Carcharhinus
Species:
C. obscurus
Binomial name
Carcharhinus obscurus
(Lesueur, 1818)
World map with dark blue shading from off New England to southern Brazil, in spots along northwestern African and in the Indian Ocean, around southern Africa, off Japan and China, all around Australia, and around Baja California, and light blue shading from Spain to southern Africa, including the western Mediterranean, and off northern Chila
Confirmed (dark blue) and suspected (light blue) range of the dusky shark[2]
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

The dusky shark (Carcharhinus obscurus) is a

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

gestation period of 22–24 months, after which there is a year of rest before they become pregnant again. This shark, tied with the Spiny dogfish as a result is the animal with the longest gestation period. 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 Endangered 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.

Taxonomy

French

type specimen, though he was presumably working from a shark caught in North American waters.[5]

Many early sources gave the scientific name of the dusky shark as Carcharias (later Carcharhinus) lamiella, which originated from an 1882 account by David Starr Jordan and Charles Henry Gilbert. Although Jordan and Gilbert referred to a set of jaws that came from a dusky shark, the type specimen they designated was later discovered to be a copper shark (C. brachyurus). Therefore, C. lamiella is not considered a synonym of C. obscurus but rather of C. brachyurus.[4][6] Other common names for this species include bay shark, black whaler, brown common gray shark, brown dusky shark, brown shark, common whaler, dusky ground shark, dusky whaler, river whaler, shovelnose, and slender whaler shark.[7]

Phylogeny and evolution

Carcharhinus altimus

Carcharhinus plumbeus

Carcharhinus falciformis

Carcharhinus perezi

Carcharhinus galapagensis

Carcharhinus obscurus

Carcharhinus longimanus

Prionace glauca

Phylogenetic relationships of the dusky shark, based on allozyme sequences.[8]

Teeth belonging to the dusky shark are fairly well represented in the

Late Pliocene (c. 3.5 Ma), and the other in mud dating to the Pleistocene-Holocene (c. 12,000 years ago).[14]

In 1982,

derived clade.[8]

Distribution and habitat

A grey shark swimming in shallow, sun-dappled waters, with a large school of smaller fish in the background
A female dusky shark on display at Sea World, Queensland; this species is found throughout Australian waters.

The range of the dusky shark extends worldwide, albeit discontinuously, in tropical and warm-temperate waters. In the western Atlantic Ocean, it is found from Massachusetts and the

microsatellite evidence suggest that Indonesian and Australian sharks represent distinct populations.[17]

Residing off continental coastlines from the

estuaries.[4][18]

The dusky shark is nomadic and strongly migratory, undertaking recorded movements of up to 3,800 km (2,400 mi); adults generally move longer distances than juveniles. Sharks along both coasts of North America shift northward with warmer summer temperatures, and retreat back towards the equator in winter.[1] Off South Africa, young males and females over 0.9 m (3.0 ft) long disperse southward and northward respectively (with some overlap) from the nursery area off KwaZulu-Natal; they join the adults several years later by a yet-unidentified route. In addition, juveniles spend spring and summer in the surf zone and fall and winter in offshore waters, and as they approach 2.2 m (7.2 ft) in length begin to conduct a north-south migration between KwaZulu-Natal in the winter and the Western Cape in summer. Still-larger sharks, over 2.8 m (9.2 ft) long, migrate as far as southern Mozambique.[1][5][19] Off Western Australia, adult and juvenile dusky sharks migrate towards the coast in summer and fall, though not to the inshore nurseries occupied by newborns.[1]

Description

The dusky shark can be identified by its sickle-shaped first dorsal and pectoral fins, with the former positioned over the rear tips of the latter
Upper and lower teeth

One of the largest members of its genus, the dusky shark commonly reaches a length of 3.2 m (10 ft) and a weight of 160–180 kg (350–400 lb); the maximum recorded length and weight are 4.2 m (14 ft) and 372 kg (820 lb) respectively.[7][20][21] However, the maximum reported size of the species is 4.5 m (15 ft), while the maximum weight is reported to reach up to 500 kg (1,100 lb).[22] Females grow larger than males.[23] This shark has a slender, streamlined body with a broadly rounded snout no longer than the width of the mouth. The nostrils are preceded by barely developed flaps of skin. The medium-sized, circular eyes are equipped with nictitating membranes (protective third eyelids). The mouth has very short, subtle furrows at the corners and contains 13-15 (typically 14) tooth rows on either side of both jaws. The upper teeth are distinctively broad, triangular, and slightly oblique with strong, coarse serrations, while the lower teeth are narrower and upright, with finer serrations. The five pairs of gill slits are fairly long.[20]

The large

dermal denticles are diamond-shaped and closely set, each bearing five horizontal ridges leading to teeth on the posterior margin.[20] This species is bronzy to bluish gray above and white below, which extends onto the flanks as a faint lighter stripe. The fins, particularly the underside of the pectoral fins and the lower caudal fin lobe) darken towards the tips; this is more obvious in juveniles.[25]
Dusky sharks can be found at Redondo Beach, southern California to the Gulf of California, and to Ecuador. But sometimes rarely off southern California; common in tropics. Dusky sharks have a total length of at least 3.6 m (11.8 ft) or possibly to 4.2 m (13.8 ft). At birth, dusky sharks are about a length of 70–100 cm (27.6-39.3 in). In the surf zone, dusky sharks swim to a depth of 573 m (1,879 ft). Dusky sharks have a color of Gray or beige.

Biology and ecology

As an

isopods,[37] and the sea lamprey (Petromyzon marinus).[38]

Full-grown dusky sharks have no significant natural predators.

bony fishes, with negative consequences for the biodiversity of the local ecosystem.[5][39]

Feeding

A silvery, spindle-shaped ray-finned fish with a forked tail
The bluefish is a major prey species of dusky sharks in the northwestern Atlantic.

The dusky shark is a generalist that takes a wide variety of prey from all levels of the water column, though it favors hunting near the bottom.[25][40] A large individual can consume over a tenth of its body weight at a single sitting.[41] The bite force exerted by a 2 m (6.6 ft) long dusky shark has been measured at 60 kg (130 lb) over the 2 mm2 (0.0031 in2) area at the tip of a tooth. This is the highest figure thus far measured from any shark, though it also reflects the concentration of force at the tooth tip.[42] Dense aggregations of young sharks, forming in response to feeding opportunities, have been documented in the Indian Ocean.[1]

The known diet of the dusky shark encompasses

sea stars. Very rarely, the largest dusky sharks may also consume sea turtles, marine mammals (mainly as carrion), and human refuse.[4][5][40][43][44]

In the northwestern Atlantic, around 60% of the dusky shark's diet consists of bony fishes, from over ten families with

southern African pilchard (Sardinops sagax), occurring off the eastern coast of South Africa every winter, is attended by medium and large-sized dusky sharks. Pregnant and post-partum females do not join, possibly because the energy cost of gestation leaves them unable to pursue such swift prey.[41] One South African study reported that 0.2% of the sharks examined had preyed upon bottlenose dolphins (Tursiops truncatus).[47]

Life history

Like other requiem sharks, the dusky shark is

viviparous: the developing embryos are initially nourished by a yolk sac, which is converted into a placental connection to the mother once the yolk supply is exhausted. Mating occurs during spring in the northwestern Atlantic, while there appears to be no reproductive seasonality in other regions such as off South Africa.[4][5] Females are capable of storing masses of sperm, possibly from multiple males, for months to years within their nidamental glands (an organ that secretes egg cases). This would be advantageous given the sharks' itinerant natures and low natural abundance, which would make encounters with suitable mates infrequent and unpredictable.[48]

With a

gestation period estimated at up to 22–24 months and a one-year resting period between pregnancies, female dusky sharks bear at most one litter of young every three years.[1] Litter size ranges from 3 to 16, with 6 to 12 being typical, and does not correlate with female size.[4][41] Sharks in the western Atlantic tend to produce slightly smaller litters than those from the southeastern Atlantic (averaging 8 versus 10 pups per litter).[20] Depending on region, birthing may occur throughout the year or over a span of several months: newborn sharks have been reported from late winter to summer in the northwestern Atlantic, in summer and fall off Western Australia, and throughout the year with a peak in fall off southern Africa.[5][25] Females move into shallow inshore habitats such as lagoons to give birth, as such areas offer their pups rich food supplies and shelter from predation (including from their own species), and leave immediately afterward.[4][46] These nursery areas are known along the coasts of KwaZulu-Natal, southwestern Australia, western Baja California, and the eastern United States from New Jersey to North Carolina.[1][4]

Life history parameters of the dusky shark
Region Male length and age at maturity Female length and age at maturity
Northwestern Atlantic 2.80 m (9.2 ft), 19 years[23] 2.84 m (9.3 ft), 21 years[23]
Eastern South Africa 2.80 m (9.2 ft), 19–21 years[4][49][50] 2.60–3.00 m (8.53–9.84 ft), 17–24 years[4][49]
Indonesia 2.80–3.00 m (9.19–9.84 ft), age unknown[51] 2.80 m (9.2 ft), age unknown[51]
Western Australia 2.65–2.80 m (8.7–9.2 ft), 18–23 years[25][52] 2.95–3.10 m (9.7–10.2 ft), 27–32 years[25][53]

Newborn dusky sharks measure 0.7–1.0 m (2.3–3.3 ft) long;

maximum lifespan is believed to be 40–50 years or more.[54]

Human interactions

Dusky shark tooth on a Gilbertese weapon.

Danger to humans

The dusky shark is considered to be potentially dangerous to humans because of its large size, though little is known of how it behaves towards people underwater.[5] As of 2009, the International Shark Attack File lists it as responsible for six attacks on people and boats, three of them unprovoked and one fatal.[56] However, attacks attributed to this species off Bermuda and other islands were probably in reality caused by Galapagos sharks.[5]

Shark nets

Shark nets used to protect beaches in South Africa and Australia entangle adult and larger juvenile dusky sharks in some numbers. From 1978 to 1999, an average of 256 individuals were caught annually in nets off KwaZulu-Natal; species-specific data is not available for nets off Australia.[54]

In aquariums

Young dusky sharks adapt well to display in public aquariums.[5]

Fishing

A fishery worker standing on a ship, holding a small shark in his gloved hands
Dusky sharks are highly valued by commercial fisheries.

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).[54] 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.[7] 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 under 2 m (6.6 ft) long is under 4%.[54]

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.[54]

Conservation

The International Union for Conservation of Nature (IUCN) has assessed this species as Endangered worldwide. The American Fisheries Society has assessed North American dusky shark populations as Vulnerable.[20] 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

Endangered Species Act (ESA).[57] 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.[58] 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.[59]

The New Zealand Department of Conservation has classified the dusky shark as "Migrant" with the qualifier "Secure Overseas" under the New Zealand Threat Classification System.[60]

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