Thresher shark (Alopias vulpinus)
|Also known as:||common thresher, fox shark, sea fox, swiveltail, thrasher|
|Size||Length: 600 cm (2)|
The thresher shark is classified as Vulnerable (VU) by the IUCN Red List (1).
With their enormous, curving upper lobe of the tail fin, which can be as long as the body, thresher sharks (Alopias vulpinus) are one of the most instantly recognisable of all sharks (2). This slender, whip-like tail can be a deadly weapon, used to herd, stun and kill the thresher shark’s prey (3). The body is blue-grey to dark grey or blackish on top, with silvery or coppery sides and white undersides (2). The pectoral fins are curved and taper to a point, and the snout is short and pointed (2). The eyes and jaws are relatively small (2), but the thresher shark’s sharp teeth are efficient at capturing slippery prey (3). This thresher shark can be distinguished from other threshers by the position of the first dorsal fin which has its leading edge situated above the trailing edge of the pectoral fins (3).
Occurs in oceanic and coastal areas in tropical to cold-temperate seas around the world, but the thresher shark is most common in temperate waters (2).
The thresher shark inhabits both coastal waters and oceans far from land, from the surface to depths of at least 366 metres (2). Young thresher sharks are often found close inshore and in shallow bays (2).
The thresher shark is an active strong-swimming fish that is occasionally seen leaping out of the water (2). It feeds primarily on small, schooling fish, but also fish that dwell on the ocean bottom, squids, octopi and, very rarely, seabirds (2). It uses its unique tail fin to herd fish together in tight shoals, and then stuns them with powerful swipes of the tail. Sometimes two threshers may cooperate in their attack, swimming round the school of fish in ever decreasing circles, then striking the shoal with their tails before turning to swallow their stunned or dead victims (2) (3).
Thresher sharks possess some remarkable physiological adaptations which explain their strength and endurance in a wide range of latitudes and depths. Along each of their flanks runs a strip of red muscle, which can contract powerfully for long periods, enabling the thresher shark to swim for long periods without fatigue (4). In addition, this red muscle contains a meshwork of tiny blood vessels which transfer heat back towards the body core. Retaining body heat enables the thresher shark to remain active and react quickly even in cold water (3) (4), and also results in much faster digestion, enabling it to feed again rapidly, should the opportunity arise (3).
Thresher sharks are ovoviviparous, meaning that the young develop inside a weakly formed shell within the female. The gestation period is reported to be nine months, with litters of two to seven pups born during the spring (2). Thresher sharks reach maturity at between three and eight years old, and are estimated to live for 45 to 50 years (2).
The thresher shark is an important economic species in many areas (1), such as the northwestern Indian Ocean, the western, central and eastern Pacific, and the North Atlantic (2). Its flesh is highly prized for human consumption, its fins are sought after for shark fin soup, its skin can be used for leather and the liver provides vitamin rich oil (2). The tendency of the thresher shark to fight strongly when caught also makes it a popular target for sports anglers (2). The International Union for Conservation of Nature (IUCN) considers there to be insufficient data on the thresher shark to determine the extent to which it may be threatened by these activities, although there is evidence that this shark is highly vulnerable to over-fishing (1). In California, a fishery started in 1977 for several species of shark, including the thresher. Catches peaked in 1982 and declined thereafter, showing that uncontrolled fishing can rapidly deplete a population. This fishery was outlawed in 1990 (5). Declines have also been observed in the Northwest Atlantic, where thresher shark numbers are estimated to have declined by over 75 percent between 1986 and 2000 (6). Here, and in many other areas, fishing of the thresher shark continues, often without any management measures in place (1).
The thresher shark is believed to be able recover from the impact of fisheries fairly quickly (2) (5); for example, since fishing of the thresher shark was banned in California in 1990 this population has begun to recover (5) (7). However, the brevity of the Californian fishery shows how uncontrolled fishing can decrease a population very quickly, indicating that harvesting of this species should be carried out with caution and with adequate management measures in place (1) (5). Unfortunately, unmanaged fisheries operating throughout the world might be over-exploiting the thresher shark, and reducing numbers to a level they may not always be able to recover from.
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- Dorsal fin: the unpaired fin 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.
- Ovoviviparous: method of reproduction whereby the egg shell is weakly formed and young hatch inside the female; they are nourished by their yolk sac and then ‘born’ live.
- Pectoral fins: in fish, the pair of fins that are found one on each side of the body just behind the gills. They are generally used for balancing and braking.
IUCN Red List (March, 2011)
- Compagno, L.J.V. (2001) Sharks of the World. An annotated and illustrated catalogue of shark species known to date. Vol. 2: Bullhead, mackerel and carpet sharks (Heterodontformes, Lamniformes and Orectolobiformes). Food and Agriculture Organization of the United Nations, Rome.
- Carwardine, M. and Watterson, K. (2002) The Shark Watcher’s Handbook. BBC Worldwide Ltd, London.
ReefQuest Centre for Shark Research (September, 2007)
FAO/SIDP Species Identification Sheets (September, 2007)
- Baum, J.K., Myers, R.A., Kehler, D.G., Worm, B., Harley, S.J. and Doherty, P.A. (2003) Collapse and conservation of shark populations in the Northwest Atlantic. Science, 299: 389 - 392.
Thresher shark Biological Profile, Ichthyology Department, Florida Museum of Natural History (September, 2007)