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Content Updated: 18th October 2012

Shark Attacks on Humans

According to the International Shark Attack File (ISAF), housed at the University of Florida:

For the period 1580 to 2011, the ISAF has records of 2,463 shark attacks, of which 471 (19%) were fatal.

Sharks probably attack 50 – 100 people annually (not all attacks are reported).

Of those attacked, 5 – 15 people are killed.

Currently, 32 species of shark are known to attack humans, whilst a further 36 are considered dangerous.

Over the last three years, unprovoked shark attacks have fluctuated (64 in 2009; 81 in 2010; and 75 in 2011); the decadal average has increased.

Perhaps reflecting a slow news flow, shark attacks were reported by the media so frequently in 2001, that it was dubbed the “Summer of the Shark” by Time magazine.

Attacking Species

According to ISAF records (1580 – 2011) the top ten sharks most commonly implicated in unprovoked attacks are:

Great White SharkGreat white shark (Carcharodon carcharias - right) in 272 attacks

Tiger shark (Galeocerdo cuvier) in 97 attacks

Bull shark (Carcharhinus leucas) in 91 attacks

Sandtiger shark (Carcharias taurus) in 31 attacks

Blacktip shark (Carcharhinus limbatus) in 28 attacks

Bronze whaler shark (Carcharhinus brachyurus) in 19 attacks

Spinner shark (Carcharhinus brevipinna) in 16 attacks

Blue shark (Prionace glauca) in 15 attacks

Hammerhead (unidentified species) in 15 attacks

Blacktip Reef shark (Carcharhinus melanopterus) in 11 attacks

It is important to note that this list does not necessarily translate to 'top 10 most dangerous sharks' and, in many cases, reflects the species most likely to be found in the habitats in which humans also frequent. Moreover, shark identification is something of an acquired skill, particularly when it comes to separating some of the Reef sharks, which makes establishing the true culprit (which are seldom actually seen by the victim) a difficult process. Consequently, this list should be interpreted with caution.

White shark breachingTypes of Shark Attack

In his contribution to the 1990 American Littoral Society book, Discovering Sharks, curator of the ISAF George Burgess explained how shark behaviourists recognise three types of sharks attack: Hit and Run; Bump and Bite; and Sneak attacks.

Hit and Run: This is the most common form of attack recorded by the ISAF. Hit and Run attacks typically occur in the surf zone, with swimmers and surfers the most common target. The victim seldom sees the shark and the shark does not return after the initial contact. Injuries to hit and run victims are usually confined to relatively small lacerations, frequently on the legs, and are not usually fatal.

Bump and Bite: Characterized by the shark circling and often ‘bumping’ their victim prior to instigation of an attack. Unfortunately for the victim, this behaviour is considered by some to be associated with feeding, frequently leading to serious -- often fatal -- injuries. Multiple bites are not uncommon.

Sneak: Sneak attacks occur without warning and, like Bump and Bite attacks, may be associated with feeding. Multiple bites are not uncommon. Burgess suggests that many victims of aviation disasters probably suffer either Bump and Bite, or Sneak attacks.

Shark attacks on seals and seal lions (collectively termed pinnipeds) are generally different to shark-human incidents, often varying according to geography and prey type. Scientists currently recognise two broad categories of attack on pinnipeds. In a 1984 contribution to the Proceedings of the California Academy of Sciences, Tim Tricas and John McCosker reported on their bite-and-spit hypothesis while, in a 1996 paper, Peter Klimley and his colleagues described an exsanguination (bloodletting) tactic used by sharks on pinnipeds.

Bite and Spit: These attacks are characterized by a forceful initial strike, often lifting the prey and shark clean out of the water. The prey is then released and the shark moves away, leaving the animal to bleed to death. It is considered that the shark backs off to allow the prey to die without the risk that wounded animal could fight back and injure the shark. This method of attack, as inferred by Tricas and McCosker, was based on bite wounds observed on surviving pinnipeds and accounts of White sharks leaving the prey after an initial incapacitating attack.

Exsanguination (or Bloodletting): This attack procedure is characterized by a shark maintaining hold of a prey item following the initial bite (i.e. the shark swims with the prey in its jaws). It is considered that the shark will continue swimming until the prey stops bleeding, after which it will surface with the prey and begin feeding. In their fascinating 1996 paper, Klimley and his co-workers noted intervals of a minute or more between an initial strike and the appearance of a seal on the surface in two-thirds of cases. They suggest that during this interval, the shark was exsanguinating the animal, before returning to the surface to feed. Further evidence in support of this theory came from observations by the authors that seals on the surface were rarely bleeding. There are, however, alternative explanations for this interval – the time underwater could equally be indicative of the shark chasing the wounded seal or carrying the carcass away from the initial striking point (perhaps to increase the time it takes competition to arrive).

During their research, Klimley and his colleagues found no evidence to support Tricas and McCosker’s ‘Bite, Spit and Wait’ hypothesis.

Why Do Sharks Sometimes Attack Humans?

Shark jawsMistaken identity is frequently cited by the media to explain shark attacks on humans. It is now considered far more probable, however, that such human-shark interactions are the result of a shark’s curiosity. Considering the majority of attacks on humans are ‘Hit and Run’ (i.e. non-predatory), it seems unlikely that the shark mistook the surfer, swimmer or kayaker for a potential meal. Many ichthyologists (people who study fish) consider that, because sharks don’t have hands, they use their highly dextrous jaws and sensitive teeth and gums as a substitute. Unfortunately for us (and indeed the sharks), humans have a high density of blood vessels very close to the skin and an investigatory bite that would do little more than wound a pinniped, can have considerably more distressing results for Homo sapiens. Indeed, one square inch of human skin contains somewhere in the region of 4.5 m (15ft) of blood vessels and 78 nerves. Similarly, human skin is thinner than that of pinnipeds. Humans have an epidermis (the thin protective layer of the skin) approximately 0.03 to 0.13 mm thick, whilst pinnipeds have epidermal thickness between 5mm and 1 cm; below the epidermis, is the dermis (the region containing the blood vessels).

Further credence to the 'jaws as hands' theory came from a series of experiments in the Spencer Gulf off Australia, by Rocky Strong of the Cousteau Society. Strong found that White sharks investigated almost every object floating on the surface, irrespective of size and shape - although naïve sharks showed significant preference for objects shaped like seals or sea lions. Strong’s experiments and further work by Scott Anderson and his colleagues have demonstrated that Great white sharks are primarily visual predators (the decoys had no scent, no electrical field and did not splash). In addition, many shark biologists have remarked on how White sharks are incredibly curious animals, investigating any novel items in their domain.

Rejection of Victims

In 1987, Klimley proposed that a White shark may decide whether or not to keep a prey item based on its fat content - a hypothesis based on observations that sharks would seize and consume pinniped bait, but reject sheep carcasses. Later, during a 1996 investigation, Klimley et al. observed similar scenarios, with a shark seizing and releasing an immature, highly decomposed otariid (an ‘eared seal’). Citing this and other examples -- of sea otters (Enhydra lutris) and Jackass penguins (Sphensicus demersus) having been seized and then released -- the authors again presented this hypothesis. This hypothesis for human rejection certainly seems plausible when we consider that, even though between 80 and 90% of human fat is stored subcutaneously (under the skin), subcutaneous fat is meagre by comparison to most marine mammals. For example, if you're a fit healthy adult male, the SCF on your forearm is about 1.2mm thick. Compare this to a blubber thickness of up to 7 cm (almost 3 inches) in seals, up to 15 cm (6in) in a walrus (Odobenus rosmarus) and 30 cm (1ft) in Grey whales (Eschrichtius robustus).

This theory appears, however, to overlook the fact that fish (bony fishes and especially elasmobranchs in senior life stages) are the primary dietary component of Great whites at all stages of their lifecycle. Furthermore, White sharks are known to consume small prey species that seem to be of low energy cost-efficiency (e.g. abalone and crabs). In the context of deciding what to eat, the Optimal Foraging Theory deals with two components of an organism's predatory strategy: energy maximization and number maximization. Predators can rarely afford to be finicky about their choice of prey and, unfortunately for Klimley's theory, data presented by Prof. Leonard Compagno and Aidan Martin suggest that White sharks lean towards ‘number maximization’, rather than ‘energy maximization’. Moreover, as Aidan Martin noted in his SHARK-L posting of 12th August 2001, pinniped blubber actually represents a relatively poor source of energy for a Great white. As Aidan explained, seal blubber is stratified and internally supported by disulphide cross-linked collagen fibres arranged in a complex and remarkably tough matrix. Consequently, although, in terms of per unit mass, fat has twice the calorific content of protein, blubber is fairly resistant to digestion and (on a calories liberated per unit time basis) doesn’t represent a particularly rich food type.

Klimley’s fat-rejection hypothesis suggests that a low-quality food item could remain in the stomach of a White shark until the animal can digest it. Many species of sharks are, however, known to evert their stomachs in a bid to remove any unwanted or indigestible articles; emetic responses are well documented in smaller sharks. While, it could be argued that no documentation exists to suggest stomach eversion is present in Great whites I see little reason to assume they are unable to do this.

The sum of this is that, from a maximization perspective, fishes simply constitute the most consistently available prey type for a Great white. Furthermore, victims probably aren't "rejected" in the context commonly implied; if victims are merely the unfortunate object of selachian curiosity, then predation was never the intention and rejection of the prey is a null concept.

Crested Bullhead sharkConclusion

Sharks are intelligent (and a growing body of evidence suggests socially complex) animals, not the ruthless killing machines frequently portrayed in the popular media. As has been said before, if sharks were truly interested in eating humans, they could have a veritable smorgasbord every weekend along beaches in almost every country. Unfortunately for humans, a curious shark can be a deadly shark – blood vessels close to the skin and a fragile frame (by comparison to a pinniped) make us prone to damage from many investigatory ‘shark–human’ interactions.

One should also consider that we currently know of about 483 species of shark, the vast majority of which grow to less than 2m (6ft) and are consequently not considered to be a threat to humans - a good example are the bullhead sharks (such as the Crested bullhead, Heterodontus galeatus, pictured left), that attain maximum lengths of only about 1.2m (4ft). There is no getting around the fact that sharks -- and many other animals -- attack humans, but it is an understanding of the reasons behind these attacks and education that provide the greatest promise for being able to move forward. Ultimately, without education we cannot hope to arrive at a situation where people understand the importance of sharks in the oceans and understand how to lower the risks of having a negative encounter with them. There is seldom a better weapon than knowledge!

Note: I am aware that shark attacks cause great stress, suffering and sometimes loss to those involved. The fact that very few people are attacked each year by sharks provides little solace to someone who has been seriously injured by, or to someone who has lost a loved-one to, a shark. I have not attempted to undermine the seriousness of shark attacks on humans, I have merely tried to provide an understanding of the factors involved. It is up to the reader to draw his or her own conclusions based on the information available to them.

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