Thresher Shark

Alopias vulpinus

Thresher shark (Alopias vulpinus).

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Distribution: The thresher is distributed circumglobally in the Atlantic, Pacific and Indian Oceans and in the Mediterranean. In the eastern Pacific it occurs from Goose Bay, British Columbia south to off Baja California; also off Panama and Chile, occurring in temperate and warm oceans and penetrating into tropical waters, being most abundant over continental and insular shelves and slopes. This species is often associated with areas characterized by high biological productivity the presence of strong frontal zones separating regions of upwelling and adjacent waters, and strong horizontal and vertical mixing of surface and subsurface waters-habitats conducive to production and maintenance of schooling pelagic prey upon which it feeds. Young often occur close to shore and can enter bays.

A thresher shark satellite pop-up tagging study has shown southern California thresher shark tagging in spring can travel to off Baja California Mexico by the following October and 540 miles southwest of La Paz, Mexico by January. Other indirect evidence, including patterns in catches, suggest a seasonal north-south migration between San Diego/Baja California Mexico and Oregon and Washington. It has been proposed that large adult common thresher sharks pass through southern California waters in early spring of the year, remaining in offshore waters from one to two months during which time pupping occurs. Pups are then thought to move into shallow coastal waters. The adults then continue to follow warming water and perhaps schools of bait northward, and by late summer, arrive off Oregon and Washington. Subadult members of the stock appear to arrive in southern California waters during early summer, and as summer progresses move up the coast as far north as San Francisco, with some moving as far north as the Columbia River area. In fall, these subadults are thought to move south again, arriving in the Channel Islands area. Little is known about the presumed southward migration of the large adults, which do not appear along the coast until the following spring. Genetic analyses have indicated that the Pacific U.S. -Mexico common thresher shark is stock is a single homogenous West Coast population.

Growth and Development: Maximum size to 610 cm TL (20 ft); off the U.S. West Coast the largest reported is 550 cm (18 ft ). Size at first maturity has been variously estimated and interpreted and needs re-examination. Female size and age at first maturity is likely between 260-270 cm TL (8 ?-9 ft long) and about 4 or 5 years old; male size and age of first maturity is between 246-333 cm TL (8-11 ft) and 3 to 6 years old. Size at birth varies considerably, ranging from 115 cm to 156 cm TL (45-61 in), with only slight variation among geographical regions around the world. The species has been variously estimated to reach a maximum age of from 19 to 50 years.

Live-bearing, normal brood size appears to be 2-4 fetuses although broods of up to 7 fetuses have been recorded off Spain. The developing fetuses feed on yolk-filled egg capsules produced by the female and consumed in utero. Mating presumably takes place in midsummer along U.S. West Coast EEZ with a gestation period of about 9 months; birth is thought to occur in the spring months off California, judging from the cluster of postpartum-sized pups taken in the catch at this time.

Feeding: Prey items from California and other waters include anchovy, Pacific sardine, herring, mackerel, Pacific hake, lancetfish, lanternfishes, Pacific salmon, squids, octopus, pelagic red crab, and shrimp. Preliminary results of a recent food habit study of samples collected during the warm-water period of 1998 from the CA/OR drift gill net fishery, has revealed a varied diet of primarily anchovy, sardine, Pacific hake, Pacific mackerel, jack mackerel, shortbelly rockfish, and market squid; and secondarily, grunion and other atherinids, louvar, white croaker, queenfish, Pacific sanddab and pelagic red crab. Thresher sharks have been observed to use their long caudal fin to bunch up, disorient and stun prey at or near the surface and are often caught tailhooked on longlines.

Conservation and Management: Thresher sharks have been observed to use their long caudal fin to bunch up, disorient and stun prey at or near the surface and consequently, they are often caught tailhooked on longlines. Due to their wide distribution and abundance, threshers are often caught as bycatch in fisheries worldwide.

Last modified: 12/8/2015