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Biological Profiles




DUSKY SHARK
Order: Carcharhiniformes
Family: Carcharhinidae
Genus: Carcharhinus
Species: obscurus
Carcharhinus obscurus

Above: ex Kato, Springer & Wagner (1967) Bur. Comm. Fish. Circ. 271
Below: Photo by George Burgess©

Taxonomy

Lessueur first described the dusky shark in 1818 and classified it as Squalus obscurus, later renaming it the currently valid name Carcharhinus obscurus. The genus name Carcharhinus is derived from the Greek "karcharos" = sharpen and "rhinos" = nose. Obscurus can be translated from Latin as "dark, indistinct." Since its original description, this shark has appeared in the literature under several different names, including Geleolamna greyi Owen 1853, Carcharias macrurus Ramsay & Ogilby 1887, Galeolamna (Galeolamnoides) eblis Whitley 1944, Carcharhinus iranzae Fourmanoir 1961, and Carcharhinus obscurella Deng, Xiong & Zhan 1981.


Common Names

Common English language names for this shark include bay-shark, black whaler, bronze whaler, brown dusky shark, brown shark, common whaler, dusky ground shark, dusky shark, shark, and shovelnose. Other names include arenero (Spanish), blauhai (German), caçăo fidalgo (Portuguese), cazón (Spanish), donkerhaai (Afrikaans), dotabuka (Japanese), estrela (Portuguese), karcharynos skotinochromos (Greek), köpek baligi (Turkish), lamia (Spanish), marracho areneiro (Portuguese), requiem de sable (French), schemerhaai (Dutch), squalo scuro (Italian), sumuhai (Finnish), tiburón arenero (Spanish), and zarlacz ciemnoskóry (Polish).


Geographical Distribution

The dusky shark is a cosmopolitan species that occurs along continental coastlines in tropical and temperate waters. It ranges from Nova Scotia to Cuba (including the northern Gulf of Mexico) and from Nicaragua to southern Brazil in the western Atlantic and from southern California to the Gulf of California in the eastern Pacific. It is also found in the Mediterranean, Indian and western Pacific, including Madagascar and Australia.

World distribution map for the dusky shark
World distribution map for the dusky shark


Habitat

C. obscurus occurs along continental shorelines where it ranges from shallow inshore waters to the outer reaches of the continental shelf and adjacent oceanic waters. Although generally a bottom feeder, it can be found from the surface to a depth of 400 m (1240 ft). Adults of this species tend to avoid areas of low salinity and rarely enter estuaries. The young congregate in very shallow coastal water (nurseries) in estuaries and bays from New Jersey to Cape Hattaras.

This species is known to be highly migratory in the western Atlantic and eastern Pacific, moving north during the summer months and south in the winter. Males and females undertake these seasonal migrations separately.

Along the coast of South Africa, immature dusky sharks are known to undergo sex-segregated migrations with females moving north and males moving south. However, this phenomenon is complicated by the occurrence of temperature-regulated seasonal migration with sharks traveling southward during summer and northward in winter. This, in turn, is further complicated by the tendency for some sharks to move into deeper water during summer months.

In comparison, dusky sharks in western Australia undergo distinct seasonal migrations with both adolescents and adults moving inshore during summer and autumn.


Biology

ˇ Distinctive Features
This species is characterized by a snout that is slightly shorter than or as long as the width of the mouth, origin of the first dorsal fin over the free rear tip of moderately large falcate pectoral fins, and a low interdorsal ridge. The dusky shark is sometimes confused with the sandbar shark (Carcharhinus plumbeus) but can easily be distinguished by its smaller and more posterior first dorsal fin.




ˇ Coloration
Typical of many of the carcharhinids, the dusky shark is bluish gray above and white below. Tips of most fins dusky in younger sharks but inconspicuous in adults.


ˇDentition
Right upper and lower teeth of Carcharhinus obscurus, ex Garrick (1982) NOAA Tech. Rep. NMFS Circ. 445
Upper and lower teeth of the dusky shark


Upper teeth are triangular and slightly oblique with serrated edges. Lower teeth are erect, narrowly cusped and more finely serrated. Both sets of teeth become increasingly concave as they move distally in the jaw.


ˇ Denticles
A) Top view (magnified) of dermal denticles of the silky shark and B) rear view of a single denticle, ex Radcliffe (1916) Bull. Bur. Fish. Circ. 822
Dermal denticles of the dusky shark
The dermal denticles for C. obscurus are large, closely imbricated, and show five keels with five distinct pointed lobes. The denticles rest on a large rhomboidal base plate.


ˇSize, Age, and Growth
A large shark, C. obscurus can attain a length of about 400 cm (12.5 ft). Average size and weight are 320 cm (10 ft) and 160-180 kg (350-395 lbs), respectively. Males mature at about 280 cm (8.5 ft) and females the same or slightly larger. Size at birth ranges from 70-100 cm (33-39 in). Dusky sharks are very slow growing, mature at about 20 years and may live as long as 45 years.


ˇ Food Habits
C. obscurus preys on a wide array of bony and cartilaginous fishes as well as a variety of invertebrates. Food items include herring, eels, mullet, groupers, grunts, croakers, bluefish, mackerel, tunas, various flatfish, a variety of sharks, skates and rays, crabs, octopuses, squid, starfish and sometimes human refuse.


ˇ Reproduction
In the western Atlantic, mating occurs in the spring. Due to the presence of two class sizes of young found in pregnant females off the coast of Florida, it is believed that females of this species only mate every second year. These different class sizes suggest alternating birth seasons every two years with a gestation period of about 8 months or a single season with a longer gestation period of about 16 months.
Dusky shark embryo showing placental connection, photo by José Castro©
Dusky shark embryo


As with other carcharhinids, developing embryos are nourished via a pseudo-placental sac, a reproductive strategy known as viviparity. In the western Atlantic, the number of young per liter ranges from 6-10 with an average of 8, whereas in the southeastern Atlantic, numbers are slightly higher (range 6-14, average 10). In both cases, both sexes are represented in a 1:1 ratio.

Populations off the coast of Africa show no distinct breeding season although it appears that the young are born throughout the year with an increase in births from April to June. In the western North Atlantic, females give birth in the shallow bays and estuaries along the southeastern United States leave the area shortly after. This shallow water habitat provides a 'nursery' area for young sharks where they are afforded protection from larger sharks.


ˇ Predators
Young dusky sharks may fall prey to larger sharks including the bull shark. However, mature dusky sharks have few if any predators.


ˇ Parasites
Pandarus sinuatus, a copepod, is parasitic on the body surface of the dusky shark.



Importance to Humans
Dusky shark caught on a commerical longline set, photo by George Burgess©
Carcharhinus obscurus


The dusky shark is commonly harvested in the western Atlantic where its fins are sold overseas for shark-fin soup base. It is also regularly taken on commercial longlines as a bycatch in the swordfish/tuna fishery. Its flesh is marketed for human consumption, its skin is used for leather and its rich liver oil yields important vitamins. In this region, catch rates of all sizes of dusky sharks are greatly reduced; large individuals are now a rarity in recreational catches and their occurrence on commercial gear has declined. Because of its large size and tenacity, the dusky shark is regularly sought after by anglers.

In southwestern Australia, juvenile and newborn dusky sharks are targeted in the directed gill-net fishery.




Danger to Humans

Although it has been associated with few attacks, the dusky shark is considered potentially harmful. This is due in large part to its large size and tendency to inhabit shallow coastal waters.


Conservation

On a global scale, dusky shark populations are considered at-risk, with the World Conservation Union (IUCN) assessing the species as "Near Threatened". However, an ongoing decline in numbers indicated by low catch rates in the western North Atlantic has prompted a ban on the harvesting of dusky sharks by U.S. commercial fishermen and has led to this regional population being placed on the 2000 IUCN's Redlist of threatened species. Presently, the dusky shark in the western North Atlantic and Gulf of Mexico is listed as "Vulnerable" due to a population reduction of 20% over the last ten years. In a recent assessment of fish stocks at risk of extinction by the AFS (American Fisheries Society), populations of dusky sharks in the western Atlantic and eastern Pacific are both considered vulnerable (not endangered or threatened severely but at possible risk of falling into one of these categories in the near future).


Prepared by:
Craig Knickle