Great whites are the most lethal to humans. Since 1876 there
have been 254 confirmed nonprovoked attacks on humans by great
whites, 67 of which were fatal, according to statistics compiled
by the International Shark Attack File at the Florida Museum
of Natural History. Over the same period, tiger sharks have
attacked 83 times with 29 fatalities, and bull sharks have
attacked 69 times with 17 fatalities. Great white attacks
on humans generally involve just one bite. Researchers are
not sure, but most think the shark's sensory organs quickly
differentiate between humans and the blubber-rich seals it
prefers, so it effectively bites and spits out humans.
Researchers who have been observing great whites off the
Farallon Islands west of San Francisco think they know why
sharks mistake humans for seals. Peter Pyle, a biologist for
the Point Reyes Bird Observatory, notes that the majority
of great whites that attack humans are in the 8-to-12-ft.
range - inexperienced juveniles making the diet transition
from fish to bigger, more nourishing seals. "They are learning
a new hunting technique and may mistake surfers for seals,"
says Pyle. Once the sharks get bigger and more experienced,
they appear better able to differentiate between seals and
humans.
Unlike tigers and bulls, great whites hunt mostly during
the day, and their preferred method of attack is to shoot
up vertically from 30 ft. down, knocking their prey right
out of the water with the impact. Researchers in South Africa
have produced spectacular footage of great whites leaping
15 ft. into the air with a seal in their teeth.
TIGER SHARK
Jesse Spencer, now 18, from the Big Island of Hawaii, was
surfing near Kona in October 1999 when a 10-ft. tiger shark
came halfway out of the water and pushed him off his board.
The shark's nose struck Spencer's head, then its jaws locked
onto his arm. "I could almost see the whole shark. My elbow
was down his throat." The shark ripped muscles, tendons and
blood vessels, then chomped down on the surfboard before finally
disappearing. Spencer made it to shore, and today his arm
is recovering, although he still cannot grip with his hand.
His mistake? Surfing at sundown.
The tiger shark generally hunts at night. It is an indiscriminate
eater, "willing to try anything for food," says Rocky Strong,
a shark biologist associated with the Jean-Michel Cousteau
Institute. Not just fish, turtles and sea mammals but also
dogs, boots, beer bottles and unopened cans of beans. Its
teeth are serrated, with a notch to catch and cut through
ligament or shell tissue.
Tiger attacks on humans have been on the increase in Hawaii,
and one reason, says John Naughton of Hawaii's Habitat Conservation
Program, may be the increase in seagoing green turtles since
they were protected in the 1970s. "Turtles come close to the
shore, and the tigers follow them to prey on them. That puts
them in the same area as swimmers and surfers." Tigers are
slower swimmers than great whites and not as good at surprise.
Human victims often see the shark before it closes in to attack.
But tigers are persistent. "If you are bitten by a tiger,
you have a good chance of being chewed up. They come back,"
says John McCosker, a scientist at the California Academy
of Sciences.
After a spate of attacks in Hawaii in the early '90s, islanders
headed out to kill the rogue tigers. But scientists have since
learned that tigers are not territorial, and so chances of
catching the culprit at an attack site are minimal. Dr. Kim
Holland of the Hawaii Institute of Marine Biology at the University
of Hawaii has been monitoring tiger movements with the CHAT
(Communicating History Acoustic Transponder) tag. Implanted
in belly walls to log the shark's position and depth, the
CHAT tags upload their information to underwater receivers,
usually placed in shallow bays, which are retrieved every
three weeks. "We know they don't stake out declared territories.
They are inter-island travelers," says Holland.
BULL SHARK
When Dawn Schauman was attacked by an 8-to-10-ft. bull shark
in October 1993, she said, "it felt like a truck had slammed
into me, then I felt a compacting squeeze and an acute burning
in my left hand and my left leg." The shark spun her around,
leaving her disoriented as she hemorrhaged blood into the
water. The shark left, and willpower alone got Schauman -
61Z2 months pregnant - back to shore. Her baby was later
born prematurely but safely. For months Schauman woke at 3
a.m. replaying the attack in her head.
The bull shark, Carcharhinus leucas, usually grows no longer
than 10 ft. and weighs up to 500 lbs., but what it lacks in
size it makes up for in aggressiveness. Experts regard it
as the most pugnacious of sharks. It has, according to Robert
Hueter, director of the Center of Shark Research at the Mote
Marine Laboratory in Sarasota, Fla., the highest level of
testosterone in any animal, including lions and elephants.
Its lower spiked teeth are designed to hold prey while the
upper triangular serrated teeth gouge out flesh. "The bull
is an ambush type of predator, it makes this big mortal wound,"
says Hueter. It is fearless, taking on prey as large as it
is.
A unique feature of bull sharks is their ability to live
in both salt- and fresh water; they have attacked people in
Lake Nicaragua in Central America and have been seen above
St. Louis, Mo., in the Mississippi River. Those born in the
Mississippi delta usually spend about six months in the brackish
water before migrating along the coast to Florida to winter
in the Keys.
The bull is the only shark that prowls regularly in water
shallow enough for humans to walk in - and it may be territorial.
Australian shark biologist Ian Gordon has been getting into
the water off Florida beaches and deliberately agitating bull
sharks to observe their reaction. He says his research so
far suggests that underwater geography and a sense of territory
can provoke an attack. "Even if you don't know it's there,
the shark will feel like it is being cornered."
Human shark victims almost always seem to be inadvertent
intruders rather than targeted prey. Scientists who work with
sharks know how dangerous they can be, and many are critical
of the guided shark-feeding tours that are proliferating in
Florida and the Bahamas. Sharks there have begun to associate
the sound of an outboard motor with food, and there have been
attacks by sharks apparently impatient to be fed, according
to George Burgess, head of the International Shark Attack
File. Shark feeding is illegal in two Florida cities, and
a campaign to ban it statewide is under way. "When you are
training animals, you are changing their basic behavior and
their respect for human beings," says Burgess.
That would be a strange development: the ocean's fearsome
hunters lured unnaturally into the company of humans - then
learning to bite the hands that feed them. Nature has its
bounds.
- Reported by Alice Jackson Baughn/Ocean Springs, Paul Cuadros/Gainesville,
Lisa Clausen/Melbourne, Jeanne DeQuine/Bahamas and Jeannie
McCabe/Honolulu
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