White Shark at Ano Nuevo, January 2000; Reproduced without permission but with all acknowledgement and support to the Pelagic Shark Research Foundation at www.pelagic.org.
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. In today's lecture, we spotlight three local species, the Blue, the Hammerhead, and the Great White.
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 tropical waters.
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 II:
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 off.
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 American history.
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 cornea transplant.
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 journey.
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 sand.
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 food.
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 the sea.
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 form groups.
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 heads.
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 located.
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 pounds.
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 apt name.
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 the water.
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 prey.
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.
The Pelagic Shark
Research Foundation, Santa Cruz, California
The Shark Research
Institute, Princeton, New Jersey
The South African
White Shark Research Institute, Cape Town,