杏吧原创

IF YOU are going to jump into the sea with sharks, it鈥檚 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 鈥� a shark expert at the University of British Columbia in Vancouver, Canada, until his death at home in February 鈥� 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 鈥� and one that every diver should know about 鈥� 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 鈥淪hark attack: the warning signs鈥�).

鈥淚n [great] white sharks the hunch is almost a flicker 鈥� just 3 or 4 seconds in duration 鈥� compared to the blatant 30 to 40-second signal of the grey reef shark,鈥� says Martin鈥檚 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 鈥減rovoked鈥�, for example by feeding or touching sharks.

However, ISAF鈥檚 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 ecotourism industry.

鈥淪harks have gone from being a menace to a resource. We don鈥檛 want to see a situation where a father and kids in Tahiti try taking a photo of a shark doing something 鈥榠nteresting鈥� and end up getting hurt,鈥� says Peter Klimley, a shark behaviourist at the University of California, Davis. 鈥淚f it arches its back, bares its teeth and pulls its fins down, it鈥檚 not a good situation.鈥�

Most of the threat or 鈥渁gonistic鈥� 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 鈥� although the difficulty of observing the animals in the wild means that it is hard to draw conclusions.

鈥淔ew have seen these displays, but with more people entering the water in locations where they are more likely to see sharks, there鈥檚 a need to know when it鈥檚 time to depart,鈥� says J眉rg 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.