Thursday, 13 May 2010
Matt Walker
Editor, Earth News
The mystery of why thresher sharks have such huge tails has been solved.
For years, biologists have been unsure why thresher sharks uniquely sport tails that can grow as long as the shark's body.
Now video footage has confirmed their true function: thresher sharks use their huge tails to swat and stun much smaller prey fish.
The discovery explains why thresher sharks are often caught by their tails by baited long-line fishing gear.
Three species of thresher shark swim the oceans: the common thresher (Alopias vulpinus), bigeye thresher (A. superciliosus) and pelagic thresher (A. pelagicus).
Each species has a similar general appearance, including an elongated tail which can extend up to half the total shark's length.
Biologists have guessed that the sharks might use their tails to stun prey.
Thresher sharks are frequently caught by the tail by long-line fishing hooks baited with small fish, or by fishing boats dragging slow-moving lure fish.
There is also a report dating back to 1923 of a thresher shark being observed to use its tail while feeding.
But until now, there has been no documentary evidence proving what the sharks use their tails for.
That was until marine biologist Dr Chugey Sepulveda and colleagues at The Pfleger Institute of Environmental Research in Oceanside, California, US and the University of Massachusetts in Dartmouth, US decided to film thresher sharks feeding in the wild.
To flick or strike?
To do so, the researchers towed a submersible video camera behind a research boat cruising the waters of southern California.
In front of the camera, they towed two baited lines, hoping to lure in common thresher sharks, which commonly feed on dense schools of anchovy and sardine.
Over 27 separate days, the team recorded 650 minutes of footage.
During this time, they filmed 33 common thresher sharks swimming near or approaching the bait.
Remarkably, 14 of the 33 sharks attempted to strike the bait fish with their tails, hitting the fish with a success rate of 65%.
The sharks struck with their tails in two distinct ways: either they waggled their body and surged forward, creating a wave down their body that ended in a tail flick, or they positioned their body alongside the bait fish, before making a sideways strike with their tail.
These observations confirm that common thresher sharks use their long caudal fins to pursue and stun their prey, which are then easy to catch, the scientists report in the Journal of Fish Biology.
"The common thresher is for now, the only species that this feeding behaviour has been documented for. But we hypothesise that all three actively pursue prey with their elongate caudal fins," Dr Sepulveda told the BBC.
It also explains why so many thresher sharks are caught by their tail by long-line fisheries, which dangle fish on hooks attached to long drifting fishing lines.
Surviving the hook
The study by Dr Chugey Sepulveda and colleagues was supported by the National Science Foundation, the George T. Pfleger Foundation, the William H. and Mattie Wattis Harris Foundation and the National Oceanic and Atmospheric Administration Bycatch Reduction and Engineering Program.
They are now researching how many thresher sharks caught by recreational fisheries survive.
Fishermen often return sharks after the struggle of the catch, but because threshers tend to be caught by their tails, they have difficult breathing underwater when hooked.
Most sharks breathe by swimming forward, ensuring their gills are ventilated by the water moving over them.
Thresher sharks caught by their tail face the wrong way and are unable to do this, and Dr Sepulveda's initial results suggest more die after release than other shark species.
The researchers now hope to encourage fishermen to use more traditional methods that capture thresher sharks by the mouth, which would at least allow the sharks to continue breathing until they are released.
http://news.bbc.co.uk/earth/hi/earth_news/newsid_8678000/8678061.stm
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