The thresher shark’s huge tail – often as long as its body – has enabled it to become one of the fastest and most enthralling of all the ocean’s predators.
With a well-earned reputation as a vigorous and exceptional hunter, the thresher puts its huge tail to good use when it comes to getting food. It traps its prey by swimming in ever-decreasing circles around a shoal of fish, thrashing its tail to herd them together before charging into the mass – whacking them as it goes. As the survivors disperse, the thresher is left to devour the dead or stunned fish.
The thresher feeds on a wide range of tured thresher – there is an unconfirmed report of a commercial fisherman having his head sliced clean off by one blow from this tail.
Starting out in life
The thresher is a live-bearing shark. Its eggs are fertilized internally and grow within the uterus of the female.
Usually only two young are born at a time – each up to 1.5m (5ft) long – but occasionally a litter may number as many as four pups. It seems that, as in several other live-bearing sharks such as the mako and the porbeagle, the pups eat the unfertilized eggs while in the uterus. Because of this, the newborn thresher has a swollen belly full of egg yolk – a good start for a young fish and one that means it is not immediately dependent on hunting for food. small fish and marine animals – there are even reports of it taking unsuspecting sea-birds roosting on the surface. In British waters its prey includes mackerel, herring, pilchards and garfish. In the open sea lantern fish, lancet fish, squid and octopus are all targets for a thresher thrashing.
With the exception of this enormous upper tail lobe – which sometimes reaches 2.8m (9ft) long – the thresher has all the regular features of most other sharks. The body is torpedo-shaped, the first dorsal fin is higher and much larger than the second, and it has a small anal fin, long, pointed pectoral fins and medium sized pelvic fins.
Its eyes, however are particularly small and distinguish it from the big-eyed thresher, which occurs only in warmer seas (mainly the Pacific) but also as far north as Spain and the Mediterranean.
The thresher is most abundant in tropical and warm temperate zones, but it penetrates northwards into cooler areas during the summertime. As a result it has been caught off British coasts and even as far north as Norwegian waters.
Rarely venturing into water shallower than 75m (250ft), the thresher is commonest in the open sea where some have been captured as deep as 370m (1200ft).
A few attacks on fishing vessels have been documented, including a report of a thresher leaping into a small boat. This is unlikely to have been a deliberate attack. It is more probable that the thresher landed in the boat after performing one of its huge out-of-water somersaults. One theory for the shark’s acrobatics is that it is trying to dislodge parasites from its dorsal fin. Another theory is that the thresher just likes jumping!
The thresher’s expertise in stunning smaller fishes with its tail suggests it can aim quite an accurate blow. It is wise for anglers to avoid going too close to a cap-