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Thresher Sharks


Thresher sharks are very much the reason behind Malapascua’s success as a diving destination. In Latin the species are known as alopex, meaning fox. This has translated into many languages, English as well, as thresher sharks are sometimes called fox sharks. The common name thresher shark is derived from the distinctive caudal fin or thresher-like tail, which can be as long as the body of the shark itself.

There are three thresher shark species. (The existence of a fourth was once speculated, but the theory is now widely disregarded)

The three species are the

  1. -Bigeye thresher, Alopias superciliosus

  2. -Common thresher, Alopias vulpinus

  3. -Pelagic thresher, Alopias pelagicus

Throughout the world thresher sharks are only occasionally sighted in shallow, inshore waters, with the only well known exceptions being Malapascua and Brother Island in the Red Sea, where they appear much more frequently.



Appearance


By far the largest of the three species is the common thresher shark, that can reach a length of 6.1 metres and a weight of around half a ton. The bigeye thresher is next in size, reaching a length of 4.9 m; at just three meters, the pelagic thresher shark is the smallest. Only the pelagic thresher shark can be seen while diving at Monad Shoal. Unsubstantiated reports by local divemasters of bigeye sigtings surface from time to time, but no photographic evidence support this. In any case, if they do appear, it a very rare indeed.

Apart from the Bigeye thresher, these sharks have relatively small eyes positioned to the forward of the head. Apart from the eyes, the three kinds can often be told apart by the colour of the dorsal surface of the body. Common threshers are dark green, bigeye threshers are brown and pelagic threshers are generally blue. Lighting conditions and water clarity can of course affect how a shark appears, but the color test is generally supported when other features are considered.



Behaviour


Thresher sharks are one of the few shark species known to jump fully out of the water (making turns rather like dolphins), this behaviour is called breaching. This is often observed at Monad Shoal, but noone really knows for sure why it’s done, although there’s several plausible theories. 


Impacts of humans

Like all large sharks, thresher sharks give birth to relatively few pups and they in turn take many years to mature. (For more info read the textbox “baby cannibals” to the left) They are therefore highly vulnerable to overfishing. Other than for its meat, the sharks are hunted for their liver oil, skin (for leather), and their fins, for use in shark-fin soup.

They do not appear to be a threat to humans, although reports exist of divers who have been hit by the upper tail lobe. Any theorising as to why this has happened is highly speculative, as there as so few reports, and the circumstances are not very clear at all. Some say there is no indication that the shark did so deliberately, and assume that the divers “just got in the way or startled the shark.” It is, however, pure conjecture. To the best of our knowledge no attack has ever been reported to have occured at Malapascua.

At monad shoal off Malapascua the threshers are very shy, and therefore the use of flash on underwater cameras is forbidden or discouraged by most of the dive shops. Also all divers kneel or lie on the bottom so as not to disturb the thresher sharks as they are being cleaned - this practise raises some environmental concerns, as it obviously damages the strata. It is however currently (as of April 2010) how all diveshop organize their diving there. 


Conservation

All three thresher shark species have been recently listed as vulnerable to extinction. You can make a difference by discouraging shark finning, game fishing for thresher sharks and by refusing to eat in restaurants serving shark fin soup! On Malapascua the Thresher Sharks Research and Conservation Project are doing a big job educating the locals about the vulnerability of sharks.




We’d like to thank Simon Oliver of the Thresher Shark Research and Conservation Project who corrected numerous initial mistakes. Any remaining mistakes are entirely our fault.

 

BABY CANNIBALS

Unlike most other fish, thresher sharks fertilisation and embryonic development occur within the female shark. This live-bearing way of reproduction results in a small litter (usually 2 to 4) of large well-developed pups, each up to a meter and half at birth. The young fish exhaust their yolk sacs while still inside the mother, and that is when they begin feeding on the mother's unfertilised eggs! In nature, anything that can give you a leg up is fair game. 


Thresher sharks are slow to mature, males won’t reach sexual maturity before they are between 7 and 13 years of age, females a bit older. It’s because of the small litter and necessary time to reach maturity that thresher sharks are so vulnerable to overfishing. Thresher sharks may live for twenty years or more.


No distinct breeding season is observed by thresher sharks.


THRESHER DIET

Thresher sharks are active predators; the tail is actually used as a weapon to stun prey. When hunting schooling fish, thresher sharks are known to "slap" the water, herding and stunning prey. The primary food items of the thresher sharks are pelagic schooling fish (such as bluefish, juvenile tuna, and mackerel), however squid and cuttlefish as well as crustaceans and the odd unlucky seabird are also eaten.