Sandbar shark  (Carcharhinus plumbeus)

The most distinctive feature of this stocky, grey shark is its huge dorsal fin (3) (4), which increases its stability as it cruises the sea bottom in coastal areas. The sandbar shark belongs to the genus Carcharhinus, a word derived from the Greek “karcharos” meaning sharpen and “rhinos” meaning nose (3), although this species has a fairly rounded snout (2). The species name plumbeus comes for a Latin word meaning ‘of lead' (3), presumably referring to its colouration, which is bluish to brownish-grey on the back, and lighter, or whitish, on the underside (2) (5).

Occurs in temperate and tropical waters, in the Western Atlantic, from Massachusetts to southern Brazil; in the eastern Atlantic from Portugal to Zaire; and in the Indo-Pacific from South Africa to the Galapagos and from Vietnam to New Caledonia. It also occurs in the Red Sea and Mediterranean (2) (4).

The sandbar shark inhabits coastal waters, favouring water that is 20 to 65 metres deep (3), but occasionally venturing as down to depths of up to 1,800 metres (6). Only occasionally can its large dorsal fin be seen protruding from the water's surface, as the sandbar shark prefers remaining near the bottom (2). It commonly occurs in harbours, lagoons, muddy and sandy bays, and river mouths, but never moves into freshwater. It tends to avoid sandy beaches, the surf zone, coral reefs and other rough-bottom habitats (2).

The sandbar shark spends the majority of its time near the sea bottom, where it patrols continuously for prey (4), such as small bottom-dwelling fishes, molluscs and crustaceans (2). Whilst the diet of the adult consists primarily of fish, the pups appear to feed more on soft blue crabs (7). Despite its size and large triangular teeth, the sandbar shark has never been blamed for attacks on humans, preferring instead to stick to its live fish meals (2).

Like most other members of the Carcharhinidae family, the sandbar shark is viviparous, giving birth to between 1 and 14 pups in each litter (2). The size of the litter varies depending on the size of the mother, with large females giving birth to larger litters (2). Pregnancy is estimated to last from 8 to 12 months (2), and appears to differ between geographical locations (8) (9). The time of year in which the pups are born also varies slightly, but all females move inshore to shallow nursery areas to give birth (2) (8) (9). These nursery grounds are separated from the normal ranges of adults (9), and presumably offer the young a calm, food-rich environment in which to begin their lives. The females leave these coastal areas soon after giving birth, while the young remain in the nursery grounds until winter (9), when they move into warmer and deeper water (2).

A common feature of sharks is their slow growth rate and low reproductive output. Estimates of the age at which the sandbar shark matures range from eight to ten years in Hawaii (10), to between 12 and 15 years of age in the Northwest Atlantic Ocean (11). Females give birth every other year at most (2). Populations of sandbar sharks appear to segregate by age (6).

The slow growth, late sexual maturity and low reproductive output of the sandbar shark are all biological factors that make this shark vulnerable to overfishing. In most areas of its range, the sandbar shark is an important component of shark fisheries. The flesh is consumed by humans, the thick skins are prized for leather, vitamin-rich oil is extracted from the liver, and the fins are sold to Asian markets for use in shark fin soup (2). Although comprehensive catch data of this species is lacking, it is known to be severely overfished in the western North Atlantic (1), indicating that populations could be similarly impacted in other parts of its range. The inshore habitats which are important nursery grounds for the sandbar shark may also be impacted by the activities of humans which alter and degrade the natural environment (12).

A management plan for the sandbar shark, involving catch quotas and size restrictions, was implemented in U.S. waters in 1993. Subsequently, the depleted western North Atlantic population has ceased declining and is beginning to show signs of recovery (1). Further reductions and size restrictions have been proposed to enhance the chances for population recovery; as even if the fishery was completely banned, the sandbar shark would still take several decades to recover (12). The lack of data on populations and catches from other regions is worrying, as the sandbar shark could be edging towards a more threatened status without us even knowing.

For further information see:

• The Shark Trust:
  www.sharktrust.org/content.asp?did=26884
• Florida Museum of Natural History:
  www.flmnh.ufl.edu/fish/Gallery/Descript/Sandbarshark/sandbarshark.htm

• ProjectAware:
  www.projectaware.org

Authenticated (09/04/08) by Meaghen McCord, South African Shark Conservancy (SASC).
http://www.sharkconservancy.org