The common thresher shark (Alopias vulpinus) is the largest
species of thresher shark, family Alopiidae, reaching some 6 m (20
ft) in length. About half of its length consists of the elongated
upper lobe of its caudal fin. With a streamlined body, short pointed
snout, and modestly sized eyes, the common thresher resembles (and
has often been confused with) the pelagic thresher (A. pelagicus).
It can be distinguished from the latter species by the white of
its belly extending in a band over the bases of its pectoral fins.
The common thresher is distributed worldwide in tropical and temperate
waters, though it prefers cooler temperatures. It can be found both
close to shore and in the open ocean, from the surface to a depth
of 550 m (1,800 ft). It is seasonally migratory and spends summers
at higher latitudes.
The long tail of the common thresher, the source of many fanciful
tales through history, is used in a whip-like fashion to deliver
incapacitating blows to its prey. This species feeds mainly on small
schooling forage fishes such as herrings and anchovies. It is a
fast, strong swimmer that has been known to leap clear of the water,
and possesses physiological adaptations that allow it to maintain
an internal body temperature warmer than that of the surrounding
sea water. The common thresher gives birth to live young. The developing
embryos are oophagous, feeding on unfertilized eggs provided by
their mother. Females typically give birth to four pups at a time,
following a gestation period of nine months.
Despite its size, the common thresher poses little danger to
humans due to its relatively small teeth and timid disposition.
It is highly valued by commercial fishers for its meat, fins, hide,
and liver oil; large numbers are taken by longline and gillnet fisheries
throughout its range. This shark is also esteemed by recreational
anglers for the exceptional fight it offers on hook-and-line. The
common thresher has a low rate of reproduction and cannot withstand
heavy fishing pressure for long, which is exemplified by the rapid
collapse of the thresher shark fishery off California in the 1980s.
With commercial exploitation increasing in many parts of the world,
the International Union for Conservation of Nature (IUCN) has assessed
this species as Vulnerable.
Northern anchovies are the most important food source for common
threshers off California.
The common thresher is a rather specialized predator feeding
mostly on small schooling bony fishes of the open ocean, such as
anchovies, herring, and mackerel. It also eats larger pelagic fishes
such as lancetfish and bluefish, bottom-dwelling fishes such as
flounder, and invertebrates including squid, octopus, and swimming
crustaceans. In the California Current, the most important prey
species is the northern anchovy (Engraulis mordax); other common
prey are Pacific hake (Merluccius productus), Pacific sardine (Sardinops
sagax), Pacific mackerel (Scomber japonicus), market squid (Loligo
opalescens), and pelagic red crab (Pleuroncodes planipes). This
species concentrates on a few preferred prey species during cold
years, but becomes less discriminating during less productive, warmer
El Niño periods.
When hunting schooling prey, the common thresher first compacts
them into a tight ball by swimming in ever-smaller circles and splashing
the water with its tail, often in pairs or small groups. The shark
then stuns individual prey with blows from the long, flexible upper
lobe of its caudal fin. There are two general ways by which these
strikes are delivered. In the first, the shark rapidly undulates
the front of its body, sending a rippling motion backwards along
its length into its tail. In the second, the shark maneuvers parallel
to its target and whips its tail sideways; this method is used less
often than the first but has a higher success rate. Multiple strikes
may be made over a matter of seconds, and the prey can be hit by
almost any point along the caudal lobe from the base to the tip.
Many eyewitness accounts attest to the incredible power and
control with which the common thresher directs its tail strikes.
In the winter of 1865, Irish ichthyologist Harry Blake-Knox claimed
to have seen a thresher shark in Dublin Bay use its tail to strike
a wounded loon (probably a great northern diver, Gavia immer), which
it then swallowed. Blake-Knox's account was doubted by other ichthyologists
such as Charles Breder, who asserted that the thresher's tail is
not rigid or muscular enough to effect such a blow. In July 1914,
shark watcher Russell J. Coles reported seeing a thresher shark
off Cape Lookout, North Carolina use its tail to flip fish into
its mouth. In April 1923, oceanographer Winfred E. Allen observed
a common thresher pursuing a California smelt (Atherinopsis californiensis)
from a pier at the Scripps Institution of Oceanography in La Jolla,
California. The shark overtook the small fish and swung its tail
above the water twice "with very confusing speed", severely
injuring its target.
Embryos of the common thresher are nourished by eggs during development.
Like other mackerel sharks, common threshers are aplacental
viviparous. They give birth to litters of two to four (rarely six)
pups in the eastern Pacific, and three to seven pups in the eastern
Atlantic. They are believed to reproduce throughout their range;
one known nursery area is the Southern California Bight. Breeding
occurs in the summer, usually July or August, and parturition occurs
from March to June following a gestation period of nine months.
The developing embryos are oophagous, feeding on eggs ovulated by
the mother. The teeth of small embryos are peg-like and non-functional,
being covered by a sheath of soft tissues. As the embryos mature,
their series of teeth become progressively more like those of adults
in shape, though they remain depressed and hidden until shortly
Newborn pups usually measure 114160 cm (3.745.25
ft) long and weigh 56 kg (1113 lb), depending on the
size of the mother. The juveniles grow about 50 cm (1.6 ft) a year
while adults grow about 10 cm (0.33 ft) a year. The size at maturation
appears to vary between populations. In the eastern North Pacific
males mature at 3.3 m (11 ft) and five years old, and females at
around 2.64.5 m (8.514.8 ft) and seven years old. They
are known to live to at least 15 years of age and their maximum
lifespan has been estimated to be 4550 years.
While any large shark is capable of inflicting injury and thus merits
respect, the common thresher poses little danger to humans. Most
divers report that they are shy and difficult to approach underwater.
The International Shark Attack File lists a single provoked attack
by the thresher shark and four attacks on boats, which were probably
incidental from individuals fighting capture. There is an unsubstantiated
report of a common thresher acting aggressively towards a spearfisherman
off New Zealand.
Famed big-game angler Frank Mundus, in his book Sportsfishing
for Sharks, recounted a tale in which a longline fisherman off the
Carolinas leaned over the side of his boat to examine something
large that he had hooked, and was decapitated by the caudal fin
of a thresher shark estimated to be 5 m (16 ft) long. The head supposedly
fell into the water and was never recovered. This account is considered
highly improbable by most authors.