Feared by man since times ancestral, the shark evokes
images of terror and horror to all who cross their path.
Yet much of our hysteria over this animal is misdirected,
and recent studies of sharks show them to be quite predictable
and not-so-single-minded in their behavior. By observing
sharks in their natural habitats and under natural conditions
(i.e. without chumming and turning the water blood red),
scientists have found several plausible reasons to explain
shark attacks on humans. Unfortunately, our wholesale
slaughter of these highly evolved animals threatens to
remove them permanently from our planet. Only through
careful scientific research and public education will
the monstrous myths of these creatures be put to rest.
Sharks and Shark Attacks
The first sharks to roam the ocean depths appeared more
than 350 million years ago. While these ancestral sharks
differ considerably from modern day sharks, the evolutionary
success of sharks is little disputed. From the basic body
plan of these ancestral sharks evolved the lean, mean,
eating machine that characterizes sharks today, perfectly
adapted with its torpedo-like body and hydrodynamic fins.
Interestingly, some of the earliest fossils of sharks
have been found in Ohio. Here, during the Upper Devonian,
a broad sea extended southwest from the St. Lawrence Seaway
to Arkansas. While the only hint of an ocean today are
the amber waves of grain blowing in the wind, within the
sediments along the banks of the Rocky River are the well-preserved
6-foot long bodies of Devonian sharks. Their sleek and
streamlined bodies and their mouths with carnivorous teeth
belie an efficient predator that fed on early fishes.
Some specimens still contain the remains of a whole fish
in their fossil bellies.
Today, sharks are among the most feared predators in
the ocean. While more than 350 species are known to exist,
only the great white, tiger, bull, and oceanic white tip
sharks readily attack humans. The smallest shark, the
6-inch cigar shark, lives at depths of 1500 feet in the
Atlantic, Indian, and western Pacific oceans, and feeds
at night on squid and luminescent fishes. The largest
shark (and fish, for that matter), the 60-foot whale shark,
eats plankton, hardly the mark of a ferocious predator.
Whale sharks are distributed worldwide in temperate and
Still, their reputation as killers is not totally undeserved.
In California, twelve people have been killed by sharks
since 1926. Less than 50 shark attacks have occurred in
the last 25 years. Worldwide, reported shark attacks number
in the low thousands, yet this number doesn't include
the toll in areas of the world where shark attacks are
rarely reported nor does it tell the story of thousands
of sailors who have survived shipwrecks only to die at
the jaws of a shark.
Consider these accounts from the annals of World War
November 28, 1942, a troopship carrying 900+ men was
torpedoed by a German submarine. Most of the men successfully
abandoned ship only to be ravaged by sharks while they
drifted helplessly in lifejackets. Only 192 men survived
and many of the bodies recovered had their legs chewed
November 11, 1943, a troopship carrying 1,429 men was
torpedoed by a Japanese submarine. Only 448 survived
while frenzied sharks climbed half out of the water
onto life rafts to snatch survivors.
July 30, 1945, the U.S. cruiser Indianapolis, the ship
that delivered the nuclear warhead to the Philippines
that eventually destroyed Hiroshima, was torpedoed by
a Japanese submarine 600 miles southwest of Guam. Although
most of the 1,199 men aboard the ship succeeded in getting
off the ship, only 316 survived. It took four days for
rescue ships to reach the men and many of those who
died were killed by sharks, "the blood spilling
into the sea attracting wave after wave of these voracious
killers." The recovered bodies were heavily mutilated
and even some of the survivors carried home scars inflicted
by sharks. The death toll of 883 men was the worst in
Despite their voracious reputation, many species of sharks
are in danger today. The recent popularity of sharks as
food and medicines has caused their populations to be
decimated in some parts of the world. Shark flesh has
long been favored as food, as have the fins (for soup).
A few tasty recipes from Texas A&M's Sea Grant Program
are reproduced at the end of this lecture. The shark liver
was once prized as a valuable source of vitamin A until
synthetic vitamins became widely available and cheaper.
Shark skin is widely used for leather goods and teeth
are used in jewelry. Shark eyes have even been used for
Among some fisherman, the practice of catching a shark,
removing its fin, and throwing it back in the water is
considered acceptable. One conscientious fisherman describes
pulling up a 400-pound tiger shark flopping on his line,
but completely finless. Overfishing and the shark's slow
rate of reproduction has brought some species to the brink
of extinction. Such practices are repulsive and major
international efforts are underway to prevent such occurrences.
Believe it or not, California law protects white sharks
because they are a key predator for controlling populations
of seals and pinnipeds. As a top level predator, they
ultimately control populations of several other trophic
levels. Without them, oceanic ecosystems would become
unbalanced, having potentially catastrophic consequences
for other organisms.
For that reason, and in the interest of advancing scientific
knowledge, a number of researchers from various institutions
are joining efforts to study shark biology and behavior
and their interactions with humans. The Academy of Sciences,
San Francisco State University, the Point Reyes Bird Observatory,
and the California Department of Fish and Game are cooperating
to learn more about sharks. Additional efforts are underway
by the Pelagic Shark Research Foundation in Santa Cruz.
Such studies are essential for understanding not only
the role of sharks in oceanic ecosystems, but also for
insuring a peaceful coexistence with humans.
The Blue Shark
Along the coast of southern California, and in most waters
of the oceans, swims a shark who would qualify for all
the benefits of a frequent-roamer mileage program, if
such a program existed. The blue shark, or Prionace
glauca, is a veteran swimmer of the world ocean, known
for its extensive migrations in temperate and tropical
waters. One blue shark, tagged off New York, was captured
16 months later off the coast of Brazil, a 3,740-mile
Cousteau calls the blue shark "the most majestic
of all sharks." The blue shark is named for the brilliant
blue color of its sides and back. Like most sharks, who
exhibit countershading, the blue shark's belly is bright
white. Presumably, the blue shark's blue topside provides
camouflage as it approaches its prey from below. Blending
in with the brilliant blue waters of Catalina, this shark
would be very difficult to see from below.
Perhaps the most distinctive feature about this sharks
are its eyes. Its coal-black pupils rimmed with white
have an impassive look to them. Blue shark eyes, like
all shark eyes, are highly developed. In fact, shark eyes
function much like cat eyes to given them excellent night
vision. Behind the retina of the eye is a reflective structure
called a tapetum lucidum. The granular, silvery crystals
of the tapetum lucidum act to capture scattered light
under low light intensities and improve the ability of
the shark to see objects in dim light.
In addition to their reputation as long-distance swimmers,
blue sharks are also fast swimmers. Their sleek, slender
body, long pointed snout, and long curved pectoral fins
provide powerful and swift locomotion when necessary.
Blue sharks will even jump out of the water when hooked.
This speed allows them to devour large numbers of squid
and small bony fishes of which they are fond. When feeding
on squid, blue sharks may race through the of squid with
their mouth wide open or they may swim slowly sweeping
their heads back and forth. They also can charge upwards
in a vertical position to engulf their prey. Blue sharks
are also well known for their love of whales. Whalers
have long noted the ferocity with which blue sharks attack
whale carcasses. In the midst of a full-on frenzy, blue
sharks are even oblivious to the injurious pokes of a
whale spade wielded by a wary seaman.
Growing to lengths up to 16 feet, the blue shark is the
most abundant shark along the east and west American coasts.
They may travel alone or in groups, unlike most other
species. This behavior of traveling in groups makes them
especially susceptible to feeding frenzies and is one
reason they are considered dangerous. Blue shark attacks
on humans have been reported but they are not as common
as the attacks of "man-eaters." Other than the
four listed above, seventeen species, including the blue
shark, are considered dangerous to man.
Blue sharks, like most elasmobranchs, give live birth
to their young. However, blue sharks are viviparous, meaning
their young develop by receiving nourishment from a placenta.
This is in contrast to ovoviviparous species, who give
live birth but whose young nourish on the yolk of their
egg. Gestation periods in blue sharks last from 9 to 12
months with as few as 4 and as many as 135 pups being
born per litter. During courting, the male blue sharks
appear to bite the females. Female blue sharks are easily
distinguished from males by the teeth scars on their backs.
The skin of female blue sharks is twice as thick as male
blue sharks and thicker than the male's teeth are long,
an adaptation to their mating rituals. Following copulation,
the female stores the sperm until the following spring,
whereupon ovulation and fertilization occurs.
Sharks typically rely on several senses to discern and
track prey in the water. Sharks can see, smell, hear,
feel, touch, taste, detect vibrations and movements, and
sense electric and magnetic fields. Their shark's keen
sense of sound allows them to detect potential prey for
a mile or more. Sharks use hearing, possibly through their
inner ear and also the lateral line system, to detect
low frequency vibrations (40 Hz and below) such as a struggling
or splashing fish or mammal.
At somewhat closer distances, on the scales of perhaps
a quarter of a mile or more, sharks rely on their sense
of smell to locate prey. Sharks have large olfactory organs
on their snout into which water flows. Sharks are very
selective in their sense of smell, able to differentiate
amino acids, amines, and small fatty acids, stimulating
smells, from sugars and simple carbohydrates, which don't
appear to attract sharks. Sharks track down the source
of a preferred smell by turning towards the direction
of the smell as the swim, tracing a sinuous path back
and forth like a hunting dog to find its prey.
At distances up to 300 feet, sharks rely on their lateral
line system to detect vibrations, changes in pressure,
and movements in the water. Because all living organisms
produce electrical fields, sharks can locate their prey
at close range with surprising accuracy; they can even
locate and catch flounder and stingrays buried in the
At distances of tens of feet, a shark's far-sighted vision
allow it to home in on its prey. Sharks are most sensitive
to light, movement, and contrast. It is also likely that
they can determine shape, to some extent, as this is the
commonly offered reason for surfers being attacked, i.e.
they look like seals.
At very close distances, on the scales of inches, sharks
use another type of electrical sense organs called the
ampullae. These organs give the shark another level of
electrical sensing. At point-blank range, sharks have
a sense of taste that is highly refined. Many sharks bump
their prey prior to biting it, apparently as a means to
identify the prey. Many sharks will bite their prey and
discontinue their attack if the victim is not the preferred
Blue sharks are well-noted for their keen sense of perception.
They appear to have a highly-developed lateral line system
and studies are underway to better determine the nature
of this sense. Only by examining the true nature of these
animals will we come to appreciate the extent to which
they have fine-tuned their sensory systems for life in
Undoubtedly one of the most distinctive sharks is the
hammerhead. With its spade-shaped head punctuated by singular
eyes and nostrils at each end, the hammerhead is a shark
uniquely its own.
Hammerheads are found around the world in shallow coastal
waters and occur locally in the Gulf of California. Several
species exist; the largest, the great hammerhead, grows
to lengths of more than 18 feet.
One of the most interesting behaviors about hammerheads
is their penchant to form groups. Schools of scalloped
hammerheads (Sphyrna lewini) with more than 100
individuals have been observed in the Gulf of California.
Why they form groups is not clear. Groups appear to form
during the day in association with seamounts. All the
individuals swim in the same direction, apparently following
the designated leaders. Some theorize that grouping is
related to breeding, but this appears to be only part
of the answer since no copulation has ever been observed.
Grouping for defense has been ruled out because hammerheads
have no natural enemies. Research on these sharks, conducted
in the Gulf of California over a period of several years,
are only now beginning to yield some clues as to why hammerheads
Here's the story so far. Unlike typical schools of fishes
where individuals of nearly equal size swim in close formation
at the same speed, schools of hammerheads contain individuals
of many sizes who swim in haphazard and uncoordinated
patterns. Large female hammerheads dominate the center
of the school while smaller ones circle along the edges.
This central position appears to be a power position;
younger females constantly "battle" for the
center by striking their rivals with the undersides of
their jaws. Dominant females also bully their rivals by
performing what is known as a corkscrew display. In this
behavior, the female performs a twisting loop, rotating
her body as she accelerates into a tight somersault. At
the height of her loop, a white flash of light reflects
off her body, which appears to intimidate the other females
and cause them to retreat to the sidelines, shaking their
This center position is important to the females because
this is where the most desirable male hammerheads can
be found. Sexually mature males will dash into the cluster
of females and twist his body, revealing to the "queen"
his handsome pair of claspers, the male reproductive organs
If the central female takes a liking to the male, the
pair will leave and swim to the bottom of the seamount
where they will mate. Thus, it is clear that one function
of schools is to identify the most fit mates, a process
that would be difficult if hammerheads were solitary.
Scientists studying hammerheads in the Gulf of California
have also observed that hammerheads leave their schools
and seamounts at night when they go to feed. Every evening,
hammerheads complete a ten to fifteen miles journey into
deeper waters, always returning at dawn. By attaching
transmitters to the animal's body, scientists discovered
that hammerheads travel to abundant feeding grounds, sometimes
near another seamount. What is so extraordinary about
their travels is their ability to find their way back
and forth between seamounts. Hammerheads travel in a yo-yo
pattern at mid-depths, following each other like cars
on a highway. When they reached the outermost point in
their journeys, the stayed in one place and made jerky,
random movements, as if they were feeding. In one case,
scientists were able to confirm that one individual had
traveled to a distant seamount where abundant squid were
From these observations has come the extraordinary hypothesis
that hammerhead sharks use a magnetic sense to navigate
within the oceans. Geophysical data collected at the sites
using magnetometers reveals distinctive patterns and magnetic
anomalies that could serve as navigational points, just
like landmarks serve as convenient reference points for
hikers or sailors. Magnetic navigation has been proposed
for other species of fish, including salmon, but no direct
observations of magnetic sense organs in hammerheads has
yet been found.
One way that sharks -- and hammerheads in particular
-- might sense magnetic fields is through use of their
ampullae, the electrical sensing organs in the snout of
sharks. In hammerheads, the distance between ampullae
is exaggerated due to the shape of its head, and this
feature could allow hammerheads to detect magnetic field
lines. Even the yo-yo behavior of their swimming would
be consistent with magnetic navigation as hammerheads
would be better able to distinguish local magnetic features.
Whether or not hammerheads can actually detect magnetic
fields is the subject of a study to be conducted at Bodega
Bay Research Station in northern California. By burying
electric cables in a maze-like pen, scientists will be
better able to determine whether hammerheads are actually
capable of following a magnetic field. In any event, the
hammerhead's relationship with seamounts will continue
to intrigue us for some time, but the implications for
other species of migrating fish could be profound.
The Great White Shark
The Great White shark needs no introduction. Its reputation
as a killer, an "eating" machine, the lord of
the sea, etc. has been heralded for centuries. Its scientific
name, in fact, Carcharodon carcharias, means "ragged
tooth," an all too descript pseudonym for a shark
that kills ruthlessly. Great whites have been reported
in practically all oceans, but they seem to prefer cool,
temperate and coastal waters. Regardless, they have also
been reported at depths greater than 3000 feet and seen
in the surfline and in shallow bays. Apparently, the white
shark goes where it wants to because it can!
As with all sharks, females tend to be larger than males.
The average length of a females is reported at 15 feet,
but specimens longer than 25 feet have been caught. The
largest white shark ever taken was harpooned in the Azores,
a 29.25-foot-long giant with a pectoral span of nearly
14 feet and teeth as long as 3 inches. Weights of these
large sharks range from more than 2,000 pounds up to 7,000
The great white shark gets its name from its pure white
belly, which is often the first (or last) image presented
to a fisherman or victim. However, the back and sides
of the great white are a dark grayish black and some have
suggested that "black" shark would be a more
The shark's reputation as a killer of humans is not without
substantiation. As far back as the 16th century, naturalists
reported finding whole men in armor in the stomachs of
great whites. While these reports may be approached with
skepticism, there is no doubt that great whites can swallow
huge prey. In July 1976, a Los Angeles fisherman caught
a 16-foot white shark that "contained the bodies
of two whole sea lions, one weighing 175 pounds and the
other 125 pounds." In 1954, the body of a 13-year-old
boy was apparently found in the stomach of a great white
caught off Nagasaki.
The preferred prey of adult great white sharks off the
coast of California are seals and sea lions, although
any marine mammal will probably do. The blubber of whales
and porpoises is especially satisfying to a great white
shark. Juvenile sharks will feed on fishes, such as menhaden
or tuna, and even other sharks, such as houndsharks, requiem
sharks, hammerheads, and the spiny dogfish. Recent studies
on the dietary cuisine of great whites indicate that they
prefer meals with a high fat content (fat-free is not
in the great white's vocabulary). They typically will
reject low-fat prey, such as birds or sea otters, an observation
that suggest one reason why great whites call off their
attacks on humans (see below).
This pattern of food preference correlates well with
the reproductive habits of great whites along the coast
of California. Great white sharks are viviparous, like
blue sharks, and give live birth to their young, which
may weigh from 36 to 60 pounds! In California, most young
sharks are born in southern California between San Diego
and Catalina Island. As the sharks mature, they move further
up the coast towards the Farallons, as their youthful
diet of fish gives way to their adult preference for seals
and sea lions.
While identification and reporting of great white attacks
have become more accurate in recent years, there seems
little doubt that the occurrence of great whites is on
the rise. Along the coast of California, sightings and
attacks have grown in number, from one or two per year
in the 1950s to nearly five per year in the 70s and 80s.
In the period from 1973-1983, surfers were attacked thirteen
times in waters near San Francisco.
So notorious have attacks become off San Francisco that
a zone called the "red triangle" has been designated.
The red triangle extends from Tomales Point in the north,
south to Monterey Bay, and west to the Farallon Islands
off the coast. One reason for the high concentration of
great whites attributed to this area appears to be the
abundant populations of seals and sea lions. Since implementation
of the Marine Mammal Act, populations of marine mammals
of all species have grown in number; so too, have populations
of great white sharks.
The feeding behavior of great whites has been a topic
of intense research in the Farallon Islands and elsewhere.
Because great whites are elusive and highly mobile, natural
observations of its behavior are difficult to obtain.
Still, a picture of their attack patterns and feeding
preferences is emerging that suggests predictable patterns
in time and space.
One of the more famous great white attacks occurred in
Australia in 1963. An Australian skin diver, Rodney Fox,
was participating in a spearfishing tournament when he
felt jaws close on his chest and back, hurtling him through
the water with the impact of the strike. Fox drove his
fist at the shark's eyes but his arm slipped into the
shark's mouth tearing his hand and arm to the bone. The
shark regrouped to attack again only this time it went
for the fish bag strapped to Fox's waist. As the shark
pulled him downwards, Fox desperately struggled to free
the bag. Just at the end of his air, the bag snapped and
Fox raced to the surface, where he was picked up by a
nearby boat, who noticed an unusual amount of blood in
Fox's condition was horrifying: "his rib cage, lungs
and the upper part of his stomach were exposed, the flesh
had been stripped from his arm, his lung was punctured
and his ribs were crushed." Miraculously, he lived,
albeit with the scars of 462 stitches in his body.
A few years earlier, a friend of Fox's, Brain Rodgers,
had been attacked by a great white. Only after firing
his spear into the shark's head did it call off its attack.
Rodgers managed to struggle to shore, where he was rushed
to a hospital. After 3 hours in the operating room, he
managed to survive.
All of these attacks reveal a single pattern and a decidedly
unusual response. These divers and many others attacked
since that time were subjected to the "bite-and-spit"
kind of attack. Observations on white shark attacks on
seals and sea lion populations reveal a couple different
kinds of attacks, depending on the type of prey.
In attacks on seals, the shark apparently grabs the prey
and holds it until it bleeds to death, a killing mode
called exsanguination, or blood deprivation. Once the
seal quits bleeding, the shark begins to eat.
Great white shark attacks on sea lions begin somewhat
differently. The strike begins with an explosive splash‹the
shark appears to ram its victim‹whereupon the sea lion
often struggles free. However, the shark is relentless
and soon grabs its victim again until it stops bleeding.
Once it has stopped bleeding, the shark finishes off its
Sharks are opportunistic feeders and their feeding patterns
on seals and sea lions are quite interesting when contrasted
with the attacks on divers or other "non-food"
items, such as pelicans and sea otters. A commercial abalone
diver recalls being attacked while swimming at a depth
of 15-20 feet. The shark grabbed him by the leg and carried
him downwards. All the while the diver was bleeding profusely.
Only by pounding on the shark's head with a metal rod
was he able to convince the shark to set him free. This
attack is very similar to the kind of attacks great whites
mount on seals.
It appears that white sharks release people because they
find them unpalatable, not suited to their palate. The
Farallon shark scientists observed a great white attack
on a brown pelican, in which the animals was attacked
and disabled, but never pursued, even though the bird
was incapable of going anywhere. The pelican died two
minutes later. Similar attacks have been postulated for
sea otters, whose dead bodies was ashore intact, but with
great white teeth in their wounds. A sea otter has never
been found in the stomach of a great white.
As mentioned above, great white sharks appear to prefer
the fat and blubber of marine mammals. Scientists have
hypothesizes that this diet of fat enables the great whites
to maintain high rates of growth, about 5% per year, which
is twice the growth rate of other sharks. A diet of blubber
and high fat is consistent with high growth rates and
makes sense for a predator who prefers cooler waters.
One other interesting observation of great white sharks
bears mention. Farallon Island scientists often observed
"confrontations" between two great white sharks
to decide who would eat a freshly killed prey. In what
these scientists call a "ritualized combat,"
two great whites would approach each other head on, then
sharply slap their tails in the water, splashing water
towards their opponent. Water splashing was quite vigorous
on occasion and some sharks would lift their bodies two-thirds
of the way out of the water to make a larger splash. The
sharks would circle and repeat their tail slapping until
one shark called it quits. The victor fed on the remains
of the prey.
This "before-dinner dance" has been interpreted
as a kind of communication between great whites. A stronger
shark sees a rival shark as a threat to its feeding, and
warns it away. In this way, great white sharks may avoid
killing each other.
Finally, there appears to be some evidence that great
white populations are in danger in California. Intensive
sport fishing appears to have reduced the numbers of great
whites and their population is though to be small. Because
they reproduce approximately once every two years, and
because their litter sizes are small (7-9 pups per litter),
it is difficult for great whites to make a comeback. For
that reason, the state of California has passed legislation
protecting the great white shark.
While the threats of these great animals on humans still
remains, the knowledge to be gained through scientific
research on these animals far outweighs their danger.
Through research, we may find a way to peacefully coexist
with these magnificent creatures. Certainly their success
in surviving catastrophic changes in our planet over the
past 300 million years deserves some attention. They may
have much to teach us that we have yet to learn.