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This shark is telling you something
This shark is telling you something RACHEL NOWAK
IF YOU are going to jump into the sea with sharks, it’s probably best to have some understanding of how they behave. A study of possible threat displays in 23 shark species could make it easier to read the signs. Aidan Martin – a shark expert at the University of British Columbia in Vancouver, Canada, until his death at home in February – described 29 different components to threat displays by sharks in a study published posthumously this week (Marine and Freshwater Behaviour and
Physiology, vol 40, p 3). For about two-thirds of the species studied, this is the first detailed record of any such behaviour. By far the most common display – and one that every diver should know about – is pointing the pectoral fins downward. It was first described in grey reef sharks in 1973. Martin found it had been seen in all of the 23 species he studied, including great white, tiger and bull sharks, which pose the biggest danger to humans. The most obvious display, though, is the hunch (see “Shark attack: the warning signs”). “In [great] white sharks the
DOUG PERRINE/NATUREPL.COM
This week–
–A threat display, possibly?–
IF ONLY WE HAD A MAGIC BIN THAT WE COULD THROW STUFF IN AND MAKE IT DISAPPEAR FOREVER. WHAT WE CAN DO IS FIND CREATIVE WAYS TO RECYCLE. GREENHOUSES USE OUR WASTE CO2 TO GROW FLOWERS. AND OUR WASTE SULPHUR TO MAKE SUPER-STRONG CONCRETE. REAL ENERGY SOLUTIONS FOR THE REAL WORLD. WWW.SHELL.CO.UK/REALENERGY
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hunch is almost a flicker – just 3 or 4 seconds in duration – compared to the blatant 30 to 40-second signal of the grey reef shark,” says Martin’s widow, fish biologist Anne Martin. Sharks rarely attack people. There were only 78 such attacks worldwide in 2006, according to the International Shark Attack File, a compilation of all known incidents based at the Florida Museum of Natural History in Gainesville. Of those 78, 16 were said to be “provoked”, for example by feeding or touching sharks. However, ISAF’s director, George Burgess, says that the number of attacks is increasing as a result of larger human populations, growing popularity of water sports and better reporting of attacks. Shark experts are therefore emphasising ways of preventing attacks, in part to protect a multimillion-dollar
070428_N_p12_p13_Sharks.indd 13
SHARK ATTACK: THE WARNING SIGNS ●
The hunch: nose up, pectoral fins down, back hunched. Signals high degree of stress. Common to many species, including great white. ● Pectoral fins down. Almost universal. ● Body shiver: shark appears to stall in water, shuddering. Only in silvertip. ● Jaw gaping: like a yawn, displaying teeth. Many species, including tiger, great white and bull sharks. ● Flank displaying: turning sideways to target, slowing swimming. Many
species, including great white and tiger. ● Tail popping: a shotgun-like report from exaggerated tail beats. Possible agonistic display in sandtiger shark. ● Laterally exaggerated swimming: eellike swimming, folding almost in half. A few species, including Galapagos shark. ● Give way: shark swims straight at target but turns off at last moment. Typical of great white. ● Gill-pouch billowing. Galapagos shark, possibly great white.
ecotourism industry. “Sharks have gone from being a menace to a resource. We don’t want to see a situation where a father and kids in Tahiti try taking a photo of a shark doing something ‘interesting’ and end up getting hurt,” says Peter Klimley, a shark behaviourist at
the University of California, Davis. “If it arches its back, bares its teeth and pulls its fins down, it’s not a good situation.” Most of the threat or “agonistic” displays reported by Martin were recorded after he or another diver had rapidly approached a shark without
leaving the shark an escape route, or had pursued it. Usually the observer left without waiting to see if the shark would have carried out its threat. Fewer displays were recorded when sharks were feeding, suggesting that they are to do with self-protection rather than defending resources – although the difficulty of observing the animals in the wild means that it is hard to draw conclusions. “Few have seen these displays, but with more people entering the water in locations where they are more likely to see sharks, there’s a need to know when it’s time to depart,” says Jürg Brunnschweiler, a shark ecologist at the University of Zurich, Switzerland. Brunnschweiler is preparing a Professional Association of Diving Instructors course that will cover the threat displays reported by Martin. ●
24/4/07 2:28:43 pm
IF YOU are going to jump into the sea with sharks, it’s probably best to have some understanding of how they behave. A study of possible threat displays in 23 shark species could make it easier to read the signs. Aidan Martin – a shark expert at the University of British Columbia in Vancouver, Canada, until his death at home in February – described 29 different components to threat displays by sharks in a study published posthumously this week (Marine and Freshwater Behaviour and
Physiology, vol 40, p 3). For about two-thirds of the species studied, this is the first detailed record of any such behaviour. By far the most common display – and one that every diver should know about – is pointing the pectoral fins downward. It was first described in grey reef sharks in 1973. Martin found it had been seen in all of the 23 species he studied, including great white, tiger and bull sharks, which pose the biggest danger to humans. The most obvious display, though, is the hunch (see “Shark attack: the warning signs”). “In [great] white sharks the
DOUG PERRINE/NATUREPL.COM
This week–
–A threat display, possibly?–
IF ONLY WE HAD A MAGIC BIN THAT WE COULD THROW STUFF IN AND MAKE IT DISAPPEAR FOREVER. WHAT WE CAN DO IS FIND CREATIVE WAYS TO RECYCLE. GREENHOUSES USE OUR WASTE CO2 TO GROW FLOWERS. AND OUR WASTE SULPHUR TO MAKE SUPER-STRONG CONCRETE. REAL ENERGY SOLUTIONS FOR THE REAL WORLD. WWW.SHELL.CO.UK/REALENERGY
070428_N_p12_p13_Sharks.indd 12
24/4/07 2:28:18 pm
hunch is almost a flicker – just 3 or 4 seconds in duration – compared to the blatant 30 to 40-second signal of the grey reef shark,” says Martin’s widow, fish biologist Anne Martin. Sharks rarely attack people. There were only 78 such attacks worldwide in 2006, according to the International Shark Attack File, a compilation of all known incidents based at the Florida Museum of Natural History in Gainesville. Of those 78, 16 were said to be “provoked”, for example by feeding or touching sharks. However, ISAF’s director, George Burgess, says that the number of attacks is increasing as a result of larger human populations, growing popularity of water sports and better reporting of attacks. Shark experts are therefore emphasising ways of preventing attacks, in part to protect a multimillion-dollar
070428_N_p12_p13_Sharks.indd 13
SHARK ATTACK: THE WARNING SIGNS ●
The hunch: nose up, pectoral fins down, back hunched. Signals high degree of stress. Common to many species, including great white. ● Pectoral fins down. Almost universal. ● Body shiver: shark appears to stall in water, shuddering. Only in silvertip. ● Jaw gaping: like a yawn, displaying teeth. Many species, including tiger, great white and bull sharks. ● Flank displaying: turning sideways to target, slowing swimming. Many
species, including great white and tiger. ● Tail popping: a shotgun-like report from exaggerated tail beats. Possible agonistic display in sandtiger shark. ● Laterally exaggerated swimming: eellike swimming, folding almost in half. A few species, including Galapagos shark. ● Give way: shark swims straight at target but turns off at last moment. Typical of great white. ● Gill-pouch billowing. Galapagos shark, possibly great white.
ecotourism industry. “Sharks have gone from being a menace to a resource. We don’t want to see a situation where a father and kids in Tahiti try taking a photo of a shark doing something ‘interesting’ and end up getting hurt,” says Peter Klimley, a shark behaviourist at
the University of California, Davis. “If it arches its back, bares its teeth and pulls its fins down, it’s not a good situation.” Most of the threat or “agonistic” displays reported by Martin were recorded after he or another diver had rapidly approached a shark without
leaving the shark an escape route, or had pursued it. Usually the observer left without waiting to see if the shark would have carried out its threat. Fewer displays were recorded when sharks were feeding, suggesting that they are to do with self-protection rather than defending resources – although the difficulty of observing the animals in the wild means that it is hard to draw conclusions. “Few have seen these displays, but with more people entering the water in locations where they are more likely to see sharks, there’s a need to know when it’s time to depart,” says Jürg Brunnschweiler, a shark ecologist at the University of Zurich, Switzerland. Brunnschweiler is preparing a Professional Association of Diving Instructors course that will cover the threat displays reported by Martin. ●
24/4/07 2:28:43 pm