Divers wishing to get closer to (or further from) this most misunderstood of species should take heed of these pointers.
Sharks Respond Quickly to Sound
This is the shark's primary reason for moving into an area rather than sight or smell. Irregular and low frequency sounds suggesting damaged fish will catch a shark's attention. They can hear sounds with frequencies ranging from 10 Hertz to about 800 Hertz, but are most responsive to sounds lower than 375 Hertz. In comparison, most adult humans can hear sounds ranging from 25 Hertz to approximately 16,000 Hertz.
According to Thomas Allen in The Shark Almanac (The Lyons Press, 1999), the impact of an aircraft falling into the ocean or the cacophony of noises released by a sinking ship both appear to attract sharks. Low frequency sounds travel long distances which is why some instructors will crumple half air-filled plastic bottle underwater.
Do Sharks See in Colour?
Although most divers refer to yellow neon wetsuits as "yum-yum yellow", the truth is that we do not yet know whether sharks can see colour. According to the Cyber Diver News Network, University of Queensland researcher, Dr Nathan Hart, intends to find out. His research project, due for completion in 2010, stems from experiments made by the US Navy some three decades ago to find out why its downed pilots and sailors were getting eaten by sharks.
"The fascinating thing about all those tests that the navy was doing was that some sharks were attracted to yellow and other sharks were repelled by yellow," he says. "Whether that means they're attracted or repelled by bright things, depending on species, or whether there is a colour [connection], remains to be seen."
In the meantime, Andy Murch of Elasmodiver (a web-based field-guide pertaining to sharks and rays) suggests that "as a rule of thumb, tropical sharks are mainly fish eaters and as such are attracted to bright and shiny objects. If you're planning on regularly putting yourself in the presence of tropical fish eating sharks it may be a good idea to tone down your fashion statement and choose a more muted color or black."
The Use of the Lateral Line
Sharks have a water filled canal system beneath their skin called the lateral line. Pressure sensitive cells can detect differences in pressure between one point and another allowing the shark to sense water movement and acceleration. "They are looking for the tell tale signature of a wounded fish or other animal," says Murch. "Once they find one they carry out their civic duty and remove the wounded creature from the gene pool."
A Keen Sense of Smell
Studying the brain of a shark shows us the relative importance of smell. According to The Encyclopaedia of Sharks (Firefly Books, 1999), up to two-thirds of the total weight of the brain is made up of the olfactory lobes (which analyse smell). Experiments have shown that sharks respond most strongly to odours produced by injured or distressed prey. They will respond to one part of blood to one million parts of water – the equivalent to one teaspoon in an average swimming pool.
In his book, Allen describes a scuba diver whose nose began to bleed. "Some of the blood was draining into his mouth exiting in a stream of blood-tainted bubbles," he writes. "A small shark, apparently aiming for the source of the alluring blood, twice struck at the diver's head and face, then darted away. The diver was only slightly injured."
Shy Creatures
Despite their relative sensitiveness and predatory nature, sharks are primarily shy creatures. Unless ruthlessly encouraged, they tend to avoid divers and, more often than not, their ghost-like form will will only be glimpsed on the periphery.
Justin Schamotta - Justin was born in South Africa, raised in the Welsh Valleys and now resides in Brighton. He has been co-editor of Bulb magazine, deputy ...
