Shark Attacks

Paul S. Auerbach, M.D.

Although dreaded, sharks are among the most graceful and magnificent denizens of the deep, and the subject of many ongoing behavioral investigations. Sharks may be found in oceans, tropical rivers and lakes. They range in size from 10 to15 centimeters up to the whale shark, which is more than 15 m long and weighs 18,000 kilos. The whale shark is, fortunately, a plankton feeder.

Approximately 32 out of the at least 368 species of sharks have been implicated in the 50 to100 shark attacks (with 6 to 10 deaths) reported annually worldwide, and another 35 to 40 species are considered potentially dangerous. The danger of sharks to humans is their combination of size, aggression and dentition. Even small sharks can be powerful and destructive. The white shark ("great white" or "white pointer") is likely responsible for more attacks on humans than is any other species, especially in the waters of southern Australia, the east coast of South Africa, the middle Atlantic coast of North America, and the American Pacific coast north of Point Conception, California. The latter is a breeding area for elephant seals, which yield 90-kilo pups, perfect food for the immense predators. Tiger sharks and bull sharks are also considered to be quite dangerous with regard to attacks upon humans. The great hammerhead has a reputation as a man-eater in equatorial waters.

Although sharks are not highly intelligent, they are endowed with remarkable sensory systems. Their color vision is poor, but well compensated for by their acute perception of motion. Keen olfactory and gustatory chemoreceptors permit taste and the recognition of blood, urine or peritoneal fluid in the water - in some cases, to a sensitivity of one part of blood in 100 million parts of water. Sharks are keenly aware of chemicals similar to those produced by normal prey, such as amino acids, amines and small fatty acids. Up to two-thirds of the shark brain may be devoted to smell. Sharks also possess skin chemoreceptors that detect chemical irritants, and are able to detect minute electrical impulses. These systems allow sharks to locate struggling fish, swimmers or divers. The common smooth dogfish shark can detect an electrical voltage gradient of 5/1000 of a microvolt. Sharks also have extremely sensitive hearing, which may detect prey underwater from a distance of up to 900 meters.

Shark Feeding and Attack. Sharks feed in two basic patterns: normal or subdued, with slow, purposeful group movements; and frenzied or mob, as the result of an inciting event such as the sudden presentation of commotion or food/blood in the water. Frenzied behavior is enhanced by the proximity of other sharks in large numbers. In a frenzy, sharks become fearless and savage, snapping at anything and everything, including each other. After a shark decides to attack, it "postures", swimming erratically with elevated snout, arched back, pectoral fin depression, stiff lateral bending of the body and rapid tail motion. In bursts of motion, a shark can use its powerful caudal fin musculature and attain speed in the water of up to 64 kilometers m per hour. As a shark prepares to strike, it typically opens and closes its jaws (up to three times each second), depresses the pectoral fins in a braking action and elevates its head. During a bite, the shark shakes its head and forebody in an effort to tear flesh from the victim. The shark may "bite and spit" in order to mortally wound the victim prior to consuming it. Sharks swallow food whole without chewing it.

It is difficult to postulate hunger as the sole attack motive, since more than 70% of victims are bitten only once or twice. "Hit and run" attacks are most common. Usually, the lower teeth are used first in feeding; solitary upper-tooth slashes might indicate an attack unrelated to feeding. Up to 60% of wounds involve only the upper teeth. Sharks are selective feeders with clear dietary preferences. They commonly attack the young, old, injured and/or sick prey individuals. Sea turtles, squid, penguins, seals and stingrays are consumed in preference to man. Sharks often eat other sharks.

Attacks on Humans. It is very difficult to generalize about shark attacks on humans. Current explanations favor aggression directed at the frightened victim. Aggression may be aggravated by purely anomalous behavior, violation of courtship patterns or territorial invasion. More docile behavior tends to be the rule with most reef sharks. The white shark (which may reach a size of 7.6 meters and a weight of 2,500 kilos) is a man-attacker but probably not a man-eater; it usually releases its victim after a single bite, after it recognizes that a mouthful of neoprene, fiberglass or lead weights is not normal dietary fare. This is small consolation to the unfortunate victim, who may have an entire hemithorax or limb removed. In circumstances where human victims have been largely consumed, one theory is that this was because they were solitary in the water. A breath-hold diver's behavior and the similarity in appearance of the silhouette of contemporary surfboards to that of a surface seal may be responsible for attacks upon humans. Most attacks upon humans occur at the water's surface. However, at least one fatal attack in 1989 with two victims was upon sea kayakers off the coast of southern California.

Shark attacks have occurred from the upper Adriatic Sea to southern New Zealand, with most between latitudes 46° N and 47° S. The odds of being attacked by a shark along the North American coastline is approximately one in five million. The danger is greater during the summer months (more persons in the water), in recreational areas, during late afternoon and nighttime, and in murky warm (greater than 20° C) water. White sharks frequently venture into colder water, and attacks have occurred in water as cold as 10° C. Contrary to the trends suggested by world-wide shark attack analysis, attacks in northern California occur more frequently in clearer water at temperatures less than 16° C.

Although most attacks occur within 30 meters of shore, the danger is probably greater further out, in deep channels or drop-offs. Because of their ability to detect contrasts, sharks have a predilection to attack bright, contrasting or reflective objects. Movement is an added attraction to sharks, which have been known to bite surfboards, boats and buoys.

Most victims are attacked by single sharks, violently and without warning. In the majority of attacks, the victim does not see the shark prior to the attack. The first contact may be a bumping, which is an attempt by the shark to wound the victim prior to the definitive strike. Severe skin abrasions from the shark skin ("shagreen") placoid scales (denticles) can be engendered in this manner.

Clinical Aspects. The jaws of the major carnivorous sharks are crescent-shaped and contain up to six rows of razor-sharp rip-saw triangular teeth, which are replaced every few weeks by advancing the inner rows. While normal tooth replacement takes 7 to 10 days, in some species a lost tooth can be replaced within 24 hours. The upper jaws generally have larger "cutting" teeth, while the sharp lower teeth are designed to fasten onto and hold prey as part of prey capture. The teeth are cartilaginous, strengthened by the deposition of calcium phosphate crystals (apatite) in a protein matrix, all covered by an enameloid substance. They are considered to be as hard as granite and as strong as steel. In a great white shark, the largest serrated triangular teeth can grow to a length of 6 cm with 26 upper and 24 lower teeth exposed in the front row. The upper jaw is advanced forward and protruded to allow its participation in the biting action. Severe shark bites result acutely in massive tissue loss, hemorrhage, shock and death. Even a smaller animal can bite with bone-crushing force. The potential for rapid destruction is unparalleled in the animal kingdom.

The human leg(s) is most frequently bitten, followed by the hands and arms, as the victim attempts to fend off the shark. Proximal femoral artery disruption carries a poor prognosis because of the torrential nature of hemorrhage. Fractures are not uncommon, and broken ribs are often accompanied by intrathoracic, intraperitoneal and retroperitoneal injuries. Because the victim is generally far from medical assistance, blood loss may be profound. The wounds have historically been fatal in 15 to 25% of attacks, with major causes of death listed as hemorrhage and drowning.

Treatment. In most cases, the immediate threat to life is hypovolemic shock. It is occasionally necessary to compress wounds or manually to occlude severed blood vessels while the victim is in the water. As soon as the victim is out of the water, all means available must be used to ligate large, disrupted blood vessels or to apply compression dressings. Pressure point compression or tourniquets have been needed on occasion, balancing critical blood loss with the complications associated with their misuse. If intravascular volume must be replaced in large quantities, at least two large-bore intravenous lines should be inserted into the uninvolved extremities in order to deliver crystalloid (lactated Ringer's solution, normal saline or hypertonic saline), colloid or blood products. Central venous cannulation should be reserved for the emergency department.

The victim should be kept well-oxygenated and warm while being transported to a facility equipped to handle major trauma. Blood losses should be replaced with whole blood or packed red blood cells and fresh frozen plasma. The precise ratio of crystalloid to blood products and proper mean arterial blood pressure end point of primary resuscitation in the presence of a major vascular injury are the subjects of ongoing investigations. The victim should be thoroughly examined for evidence of cervical, intrathoracic and intra-abdominal injuries. Because Clostridium can be cultured from ocean water, tetanus toxoid 0.5 mL IM and tetanus immune globulin must be given. The administration of prophylactic antibiotics is more controversial, and is guided by the risk for a Vibrio infection. The victim of a shark bite may be treated with a parenteral third generation cephalosporin, trimethoprim-sulfamethoxazole, chloramphenicol, an aminoglycoside, ciprofloxacin, or some reasonable combination of these agents. The rationale for prophylactic antibiotics is that shark wounds are prone to heavy contamination with sea water, sand, plant debris, shark teeth and shark mouth flora. After a clinical infection is recognized, wounds should be properly cultured for aerobes. Wound cultures prior to the establishment of an infection are less useful. Marine organisms can be virulent yet difficult to identify.

Proper operative intervention is mandatory. It is inappropriate to attempt emergency department exploration of what often prove to be extensive and complicated wounds. In the operating room, devitalized tissue should be widely debrided and the wound irrigated copiously to remove all foreign material. Unless absolutely necessary, the wound should be closed loosely with drains or packed open to await delayed primary closure. Although there is debate about whether to use internal or external fixation of grossly open and contaminated fractures, it seems logical to recommend surgical stabilization to facilitate vascular and soft tissue repair. The abrasions associated with a shark bumping should be managed like a second-degree burn, with daily debridement and application of antiseptic ointment. A reasonable "shark pack" should be available in emergency facilities and rescue vehicles near shark-infested waters. This must be portable and should include items necessary to control hemorrhage and initiate IV therapy.

Prevention. Shark behavior can be unpredictable. There is no such thing as a "friendly" shark. Shark-infested water should be avoided, particularly at dusk and at night. This fundamental rule is disregarded amazingly often. Surfers are generally at greater risk than divers. Do not disguise yourself as a pinniped (seal). Do not swim with animals (e.g., dogs or horses) in shark waters.

Swimmers should remain in groups. Isolation creates a primary target and eliminates companion surveillance. When diving, vigilance must be constant.

Turbid water, drop-offs, deep channels, breeding inlets and sanitation waste outlets are areas frequented by larger sharks. However, it appears that sharks attack at least half the time in clear water. Humans are most often attacked in shallow water and further from shore.

Blood and other body fluids (including peritoneal fluid) attract sharks. No person should be in shark waters with an open wound. Women have occasionally been advised to avoid diving during menstruation, but this is controversial and without proof of efficacy.

Game fish are attracted to brightly colored and flashy metallic lures. Brightly colored swimwear or diving equipment and shiny snorkeling gear may attract sharks. Orange appears to be attractive to sharks. Black is a less attractive color.

Captured fish must be removed from the water or stored at a considerable distance from divers. There is no greater chemical attractant for a shark than fish blood.

The presence of porpoises in the water does not preclude the presence of sharks. Be alert for the presence of a shark whenever schools of fish behave in an erratic manner.

Never tease or corner a shark. This is particularly true with captive animals. If a shark begins to act in an erratic manner, do not photograph it at close range using a strobe flash apparatus.

If a shark appears in shallow water, swimmers should leave the water with slow, purposeful movements, facing the shark if possible and avoiding erratic behavior that could be interpreted as distress.

If a shark approaches in deep water, the diver should remain submerged, rather than wildly surface to escape. He should move to defensive terrain with posterior protection in order to fend off, as best as possible, a frontal attack. It is inadvisable to trap a shark, so that it must attack to obtain freedom.

Fighting sharks is very difficult; they are best repulsed with blunt blows to the snout, eyes or gills. If possible, the bare hand should not be used, in order to avoid severe abrasions or lacerations. A stream of air bubbles from a SCUBA regulator directed into the face of a shark may be sufficient deterrent. Although spears, knives, shotgun shell- or 30-.06 loaded powerheads, strychnine-filled spears and CO2 darts can kill small sharks, they can worsen the situation if they are misapplied or their application promotes frenzy in a school of sharks.

Do not splash on the surface or create a commotion in a manner that might cause a shark to interpret your behavior as that of a struggling fish. Surface chop and perhaps the sounds created by helicopter rotors attract sharks.

Shark defense techniques and repellents are in constant evolution. Recreational beaches in Australia and South Africa are protected with extensive gill net systems to trap overly curious animals. These work to a certain degree, but are not foolproof. Electric shark barriers utilizing 0.8 millisecond pulses 15 times per second to create a field of 4 volts per meter seem to generate a fright response in sharks longer than 1.2 meters and are being investigated. Their benefits include repulsion rather than shark capture or destruction. Abalone divers in South Australia work from one-man, self-propelled shark cages. Experimental devices for individuals include chain mesh diving suits, inflatable dull-colored plastic protective bags (yellow is easy for aircraft to spot, but most attractive to sharks), acoustical and hand-held electrical field transmitters, and surfactants and other chemical repellents. Glandular extracts from the Moses sole flatfish Pardachirus marmoratus found in the Red Sea and western Indian Ocean contain shark-repellent lipophilic substances. However, it has been estimated that about 24 kg of any effective chemical repellent would have to be contained in the volume of water through which a slowly-approaching shark might swim as it attacked a human in the ocean. At the present time, the concept of a chemical carried in a lifejacket being useful in shark attack prevention is unrealistic.

Dr. Paul Auerbach is a partner at Delphi Ventures in Menlo Park, California and Clinical Professor of Surgery in the Division of Emergency Medicine at Stanford University. He is a editor of the textbook Wilderness Medicine and author of Medicine for the Outdoors. He is a member of the board of directors of the Wilderness Medical Society and serves as an advisor or consultant to numerous outdoor organizations, including the Divers Alert Network and the National Ski Patrol System.