Thresher Sharks, Alopias vulpinus
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Description & Behavior
Thresher sharks, Alopias vulpinus (Bonnaterre, 1788), aka Atlantic threshers, big-eye threshers, common threshers, fox sharks, grayfishes, green threshers, sea foxes, slashers, swingletails, swiveltails, thintail threshers, thrashers, tresher sharks, whip-tailed sharks, and Zorro thresher sharks, are easily recognizable because of their long caudal (tail) fins which equals about half the total length of their body. This awesome shark also has a very characteristic dorsal fin and pelvic fins. These sharks' teeth are small, curved, smooth and razor sharp. Their teeth are similarly shaped in both the upper and lower jaws. Thresher sharks' color varies from metallic brown to blue on their dorsal (upper) sides and white on their ventral (under) sides. They range from 2.5-7.6 m in length (7.6 m max length for males, 5.5 m max length for females). Their max published weight is 348 kg.
A similar species, bigeye thresher sharks, Alopias superciliosus, are named for their enormous eyes that are much larger than those of common threshers. Bigeye threshers are thought to navigate deep waters, however little data is yet available about the distribution of this species.
There is also a smaller third species of thresher shark called pelagic or fine-tooth threshers, Alopias pelagicus.
World Range & Habitat
Juvenile threshers are often found close inshore and in shallow bays.
Feeding Behavior (Ecology)
Threshers feed on squid, octopuses, crustaceans and small schooling fish such as bluefish, needlefish, lancetfish, lanternfish, menhaden, shad, mackerel, and others. They are also thought to stun prey with blows from their powerful tails.
Thresher sharks are ovoviviparous. Males reach sexual maturity at 2.7 m, females at 3 m. Litters usually consist of 4-6 pups measuring between 1.37-1.55 m in length and weighing between 5-6 kgs at birth. Pups have a fast growth rate and are born in open water.
Ovoviviparous: eggs are retained within the body of the female in a brood chamber where the embryo develops, receiving nourishment from a yolk sac. This is the method of reproduction for the "live-bearing" fishes where pups hatch from egg capsules inside the mother's uterus and are born soon afterward. Also known as aplacental viviparous.
Conservation Status & Comments
As with many shark breeds, threshers often become entangled in fishing nets (bycatch). They are considered harmless to humans. Though uncommon in US fish markets, threshers are consumed in other parts of the world (valued for their meat, liver, hide, and fins; utilized fresh, dried-salted, smoked, and frozen). They are also a sought after gamefish.
Thresher shark abundance in US Atlantic waters has apparently decreased by about 67%, whereas for US Pacific waters, some mid-size sharks are again being reported from samples of wholesale markets, probably as a positive effect of very restrictive fishing regulations.
The common thresher shark, Alopias vulpinus, bigeye thresher shark, Alopias superciliosus and the pelagic thresher shark, Alopias superciliosus are all listed as Vulnerable (VU A2bd, A2d+4d and A2bd+3bd+4bd, respectively) on the IUCN Red List of Threatened Species due to:
"All members of genus Alopias, the thresher sharks, are listed as Vulnerable globally because of their declining populations. These downward trends are the result of a combination of slow life history characteristics, hence low capacity to recover from moderate levels of exploitation, and high levels of largely unmanaged and unreported mortality in target and bycatch fisheries.
The Common Thresher Shark (Alopias vulpinus) is virtually circumglobal, with a noted tolerance for cold waters. This species is especially vulnerable to fisheries exploitation (target and by-catch) because its epipelagic habitat occurs within the range of many largely unregulated and under-reported gillnet and longline fisheries, in which it is readily caught. It is an important economic species in many areas and is valued highly for its meat and large fins. Its life-history characteristics (2–4 pups per litter; 8–14 year generation period) and high value in both target and bycatch fisheries make it vulnerable to rapid depletion. Serious declines have occurred where this species has been heavily fished, for example in the 1980s eastern central Pacific drift gillnet fishery, where reported landings collapsed to 27% of peak levels between 1982 and the late 1980s. Analyses of pelagic longline CPUE data from logbook reports covering the species’ entire range in the northwest and western central Atlantic vary according to the time period, but suggest thresher shark stocks declined by between 63–80% during 1986–2000. There is evidence that thresher sharks are being increasingly targeted by pelagic fisheries for swordfish and tuna (e.g., in the Mediterranean Sea) in attempts to sustain catches, and exploitation is increasing in these areas. The high value of the species and its exploitation by unmanaged fisheries combined with its biological vulnerability, indicate that at least some, if not most, subpopulations in other parts of the world are likely to be equally, or more seriously at risk than those for which data are available and, unlike the Californian stock, are not the subject of management, enabling stocks to rebuild.
In addition to the Vulnerable global assessment, a number of regional assessments have also been designated for this species as follows: Near Threatened in the eastern central Pacific; Vulnerable (VU A2bd) in the northwest Atlantic and western central Atlantic; Vulnerable (VU A3bd) in the Mediterranean Sea; and Data Deficient in the Indo-west Pacific.
The Bigeye Thresher Shark (Alopias superciliosus) apparently is a highly migratory, oceanic and coastal species found virtually circumglobally in tropical and temperate seas. It has low fecundity (2-4 pups/litter) and an exceptionally low (0.002) potential annual rate of population increase, compared with other thresher sharks. This species is especially vulnerable to fisheries exploitation (target and bycatch) as its epipelagic habitat occurs within the range of many largely unregulated gillnet and longline fisheries in which it is readily caught, and it has been fished throughout its range. Significant reductions in thresher CPUE have been reported in pelagic longline fisheries in the northwest Atlantic and the eastern tropical Pacific, and declines are also suspected to have occurred in other areas. Although data are lacking for many parts of its range, it is evident that this Vulnerable species, with such low productivity, faces major threats throughout most of its range, where fishing pressure is unlikely to cease or decrease anytime in the immediate future. However, this may underestimate the extent of global decline and there is an urgent need for global review of all available data throughout its range.
In addition to the Vulnerable global assessment, a number of regional assessments have also been designated for the Bigeye Thresher Shark as follows: Vulnerable (VU A2bd) in the eastern central Pacific; Endangered (EN A2bd) in the northwest Atlantic and western central Atlantic; Near Threatened in the southwest Atlantic; Data Deficient in the Mediterranean Sea; and Vulnerable (VU A2d) in the Indo-west Pacific.
The Pelagic Thresher Shark (Alopias pelagicus) is a large, wide-ranging Indo-Pacific Ocean pelagic shark, apparently highly migratory, with low fecundity (two pups/litter) and a low (2-4%) annual rate of population increase. This species is especially vulnerable to fisheries exploitation (target and by-catch) because its epipelagic habitat occurs within the range of many largely unregulated and under-reported gillnet and longline fisheries, in which it is readily caught. Although this species is reportedly relatively common in some coastal localities, current levels of exploitation in some areas are considered to be unsustainable. Overall, it is considered highly likely that serious depletion of the global population has occurred."
References & Further Research
The Thresher Shark Research & Conservation Project
Thresher Sharks - Alopias sp. - The Pelagic Shark Research Foundation - The Sharks of the Monterey Bay - Pelagic Sharks
Thresher Shark - Shark Foundation
THRESHER SHARK - Florida Museum of Natural History
Castro, Jose I. 1993. Sharks of North American Waters.
Research Alopias vulpinus » Barcode of Life ~ BioOne ~ Biodiversity Heritage Library ~ CITES ~ Cornell Macaulay Library [audio / video] ~ Encyclopedia of Life (EOL) ~ ESA Online Journals ~ FishBase ~ Florida Museum of Natural History Ichthyology Department ~ GBIF ~ Google Scholar ~ ITIS ~ IUCN RedList (Threatened Status) ~ Marine Species Identification Portal ~ NCBI (PubMed, GenBank, etc.) ~ Ocean Biogeographic Information System ~ PLOS ~ SIRIS ~ Tree of Life Web Project ~ UNEP-WCMC Species Database ~ WoRMS
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