Dusky shark (Carcharhinus obscurus)

Description

Dusky shark fact file

• Description
• Biology
• Range
• Habitat
• Status
• Threats
• Conservation
• Find out more
• Glossary
• References
• Print factsheet

Dusky shark description

The dusky shark is a large, yet relatively slender-bodied species, with a distinctly rounded snout and a low ridge running between the front and rear dorsal fins. The upperparts of this species vary from bronzy-grey to blue-grey, while the underparts are white ⁽²⁾. Juveniles also have dusky tips on most fins, but these become inconspicuous in the adults. The mouth is roughly equal to or shorter than the width of the snout, and bears a set of formidable triangular, saw-edged upper teeth, and erect, more narrowly pointed and finely serrated lower teeth ⁽³⁾.

Also known as

bay shark, black whaler, Bronze whaler.

Size

Length: 3.6 m ⁽²⁾

Dusky shark biology

Although typically found feeding near to or on the sea-bed, the dusky shark will also readily hunt within the water column and at the surface, often following ships that dump food scraps into the water ^{(1) (3)}. This species takes a variety of prey, including fish, sharks and rays, crustaceans, octopi, cuttlefish, squid, starfish, barnacles, whale meat, and occasional garbage ⁽¹⁾. As a large and formidable predator, the mature dusky shark is rarely preyed upon by other animals, although smaller, younger specimens may fall victim to attacks by larger sharks, such as the bull shark ⁽³⁾.

Mating is believed to occur during the spring in the western Atlantic, while populations off the coast of Africa appear to be born throughout the year with a peak between April and June ⁽³⁾. Dusky shark embryos develop within the adult female’s uterus, receiving nourishment from a yolk sac and, later in development, from the mother via a structure similar to an umbilical cord and placenta ⁽⁴⁾. After an extensive gestation period, which may last as long as 16 months, a small litter of six to ten young are born, each measuring between 85 and 100 centimetres in length ^{(2) (3)}. This dusky shark is among the slowest and latest-maturing of known sharks, with males becoming sexually mature when 2.79 metres in length, at around 19 years old, and females at 2.84 metres in length, at around 21 years old ^{(1) (2)}. As a result of the long gestation period, dusky shark individuals usually reproduce only once every three years ⁽²⁾. After birth, the young congregate in very shallow coastal waters, where they are afforded some protection from larger sharks, while the adult females return to deeper waters ⁽³⁾.

In temperate and subtropical areas of the western Atlantic and eastern Pacific the dusky shark is highly migratory, travelling northwards during the warmer summer months and heading south again in the winter ^{(1) (3)}.

Dusky shark range

Present in most of the world’s major oceans, the dusky shark can be found in the western Atlantic off the coast of southern Massachusetts, south to the northern Gulf of Mexico, and from Nicaragua to southern Brazil. In the east Atlantic this species’ presence is uncertain, although it has been recorded around oceanic islands off western Africa. In the Indian Ocean, it has been recorded around South Africa, Mozambique and Madagascar in the west, and off the coast of western Australia in the east. In the Pacific, the dusky shark can be found around Japan, China, Vietnam, and eastern Australia in the west, while in the east it can be found from Southern California, south to the Gulf of California, the Revillagigedo Islands, and possibly Chile ^{(1) (2)}.

Dusky shark habitat

The dusky shark occupies coastal and adjacent oceanic, offshore waters in warm-temperate and tropical regions. It can be found from the surface to depths of 400 metres, and avoids estuaries and other areas with reduced salinity ⁽¹⁾.

Dusky shark status

Classified as Lower Risk / near threatened (LR/nt) on the IUCN Red List. The northwest Atlantic and Gulf of Mexico subpopulation is classified as Vulnerable (VU) ⁽¹⁾.

Dusky shark threats

As a result of the dusky shark’s low reproductive output and the lengthy time it takes to reach sexual maturity, it is extremely vulnerable to depletion by man ⁽¹⁾. The main threat to this species comes from recreational and commercial fishing, for which this species is both targeted directly, and is also caught in large numbers as bycatch, in particular during longlining for tuna and swordfish ^{(1) (3)}. As a result of this exploitation and accidental mortality, the north-western Atlantic and Gulf of Mexico subpopulation is now estimated to be at 15 to 20 percent of its abundance during the mid-1970s ⁽¹⁾. In other parts of its range, the impact of fishing on the dusky shark does not appear to have been as great, but nevertheless requires close monitoring ⁽¹⁾. Like many shark species, the fins and flesh of the dusky shark are commonly harvested for human consumption, along with its vitamin A-rich liver oil, and skin, which is used to make leather ⁽³⁾.

Dusky shark conservation

In response to the decline of the dusky shark in the western Atlantic, recreational and commercial fishing of this species in U.S.A. waters has been prohibited since 1998. Unfortunately, however, since that time, large numbers continue to be caught during recreational fishing ⁽²⁾.

In order to further aid the recovery of this species, waters off the coast of North Carolina to depths of 100 metres have been designated “The Mid-Atlantic Shark Closed Area”, in which all directed shark fishing between January and July is banned. This should help to preserve the smaller juvenile sharks, giving them a greater chance of reaching reproductive age ⁽⁵⁾.

View information on this species at the UNEP World Conservation Monitoring Centre.

Find out more

To learn more about the conservation of sharks and rays visit:

• Save Our Seas Foundation:
  www.saveourseas.com
• IUCN Shark Specialist Group:
  www.iucnssg.org
• Shark Research Institute:
  www.sharks.org
• Shark Trust:
  www.sharktrust.org

Authentication

Glossary

Bycatch

In the fishing industry, the part of the catch made up of non-target species.

Dorsal fins

The fins found on the back of the body of fish, or the raised structure on the back of most cetaceans.

Gestation

The state of being pregnant; the period from conception to birth.

References

1. IUCN Red List (May, 2009)
   http://www.iucnredlist.org
2. NOAA/NMFS. (2009) Dusky Shark. In: Species of Concern. NOAA National Marine Fisheries Service, Silver Spring. Available at:
   http://www.nmfs.noaa.gov/pr/pdfs/species/duskyshark_detailed.pdf
3. Florida Museum of Natural History (July, 2009)
   http://www.flmnh.ufl.edu/fish/gallery/Descript/DuskyShark/duskyshark.html
4. Bannister, K. (1993) The Book of the Shark. New Burlington Books, London.
5. Risenhoover, A.D. (2005) Atlantic highly migratory species; commercial shark management measures. Federal Register, 70: 24494 - 24495. Available at:
   http://www.gpoaccess.gov/fr/