This shark's deadly at both ends

Scientists film threshers off the Philippines using their tails like a bull whip to kill several prey in a single hit

Wild pelagic threshers are not found in New Zealand but the more common variety are. Photo / Rafn Ingi Finnsson
Wild pelagic threshers are not found in New Zealand but the more common variety are. Photo / Rafn Ingi Finnsson

While sharks are usually feared more for their fangs, researchers have captured video footage of thresher sharks which use their tails as a weapon to stun or kill prey.

A group of scientists from the University of Liverpool watched and filmed wild pelagic thresher sharks hunting large shoals of sardines in waters off the Philippines during the summer of 2010.

Their footage, shown on the National Geographic website, shows the thresher sharks are deadly at both ends - the top halves of their scythe-like tail fins are used like a bull whip.

"It was absolutely extraordinary," Simon Oliver, the founder of the Thresher Shark Research and Conservation Project, told ngm.com.

"We always expected this but there's never been any solid documented evidence."

The video shows the thresher accelerating towards a ball of fish and braking sharply by twisting its large pectoral fins.

It then lowers its snout, pitches its whole body forward, and flexes the base of its tail.

This slings the tail tip over its head like a trebuchet - a medieval mechanical thrower - with an average speed of 48km/h, although the researchers recorded a top speed of 128km/h. "When the tail hits sardines, the results aren't pretty. We saw everything from swim bladder ruptures to broken spines to parts afloat. The sharks then swim round and swallow the pieces at their leisure."

The threshers are successful on only a third of their strikes but during these victories they always kill several sardines at once.

"It's extraordinarily rare in the animal kingdom to see animals hunt with their tails," said Oliver.

"Killer whales and other dolphins sometimes do so, but the strategy is unique among sharks."

Department of Conservation marine scientist and shark expert Clinton Duffy said that while the wild pelagic thresher was not found in New Zealand waters, the common (Alopias vulpinus) and bigeye (Alopias superciliosus) threshers were seen.

Most people who spent time on the water were likely to have seen threshers surface feeding and using their tails to stun fish, he said.

Juvenile common threshers are common to abundant in the inner Hauraki Gulf during spring and summer, particularly off places like Tiritiri, where large schools of pilchards and anchovy gather. Mr Duffy said the juveniles were often hooked by accident - either on bait or on cast lures - by fishers in the Hauraki Gulf.

Adult commons and bigeye threshers were usually encountered in deep water off Great Barrier Island and the Mokohinau Islands.

He said almost nothing was known of the biology of either species in New Zealand waters.

"They only have two pups around 1m long at a time so are potentially quite vulnerable to overfishing but there are no directed fisheries for them here in New Zealand.

"Internationally their fins appear pretty commonly in the shark fin trade."

- NZ Herald

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