Known Dangerous Sharks.
There are over 510 species of shark worldwide and 182 of these sharks have been found in Australian waters (as of 2010), but only a handful are known to be dangerous to humans.
The types of sharks implicated in the majority of unprovoked attacks on humans in Australian waters are the White Shark (Carcharodon carcharias), Tiger Shark (Galeocerdo cuvier) and the Bull Sharks (Carcharhinus leucas) one of several Whaler shark species that occur in Australia. These three species have been identified in 94% of all fatal unprovoked shark attacks in Australia.
The White, Tiger the Bull sharks are capable of inflicting much more damage to humans than they do. However, the majority of people that are bitten are usually bitten once and released. This would indicate that the shark is not biting to procure food in the majority of cases (as commonly thought) but rather they may be investigating an attractive object. It is possible they may have initially mistaken a person for their normal prey but called off the attack when they realise that the taste from the initial bite, or possibly because the objects behaviour is different, from their normal prey.
Analysis of all unprovoked case histories from the ASAF indicate that 28% of attacks are fatal. This figure is consistent with global figures of 30% fatality rate (ISAF). Around 30% of all Australian shark attack cases do not result in an injury to the human (eg a bite to a surfboard or the shark is pushed away before it can bite, etc). It must be remember that any large animal should be considered potentially dangerous to humans (on land or in the sea) and sharks 2 metres in length or larger should also be considered potentially dangerous.
Most shark encounters typically occur in near-shore coastal waters where most people go. Swimmers commonly encounter sharks inshore of a sandbar or near drop offs to deeper water. Surfers typically encounter sharks on the seaward side of the waves near deeper water where the larger free swimming sharks can approach without being observed. A vast majority of victims do not see the shark until it initiates the encounter.
There are several theories as to why sharks bite humans including hunger, inquisitiveness (i.e. testing an object with their mouth to see if it is edible), mistaken identity or invasion of the shark's personal space by the human. Many years ago there was a 'rogue' shark theory where it was thought that only injured sharks unable to feed normally starts to hunt humans. However, there is no scientific evidence to support this theory. It is not know if there are common motivating factor involved in all shark encounters because each shark/human interaction is unique and behavioural and environmental circumstances are different in every case. The only consitent circumstance is that people and sharks are in the water at the same time at the same location when the encounter occurs.
Modern sharks have been on this earth for over 200 million years, whereas humans have only been around for about 200,000 years (earliest recorded human remains found). However, it is only in the last century that people have entered the oceans on mass to swim - in Australia it was illegal to swim at the beach in daylight before 1904. Humans are not part of a sharks natural diet and sharks do not eat people regularly enough to sustain their energy requirements. There are some instances where a shark has consumed a human or was suspected of eating a human (no body recovered) but these are rare events. Records analysed since 1791 indicate only 37 case where the victims body was not recovered. There are also case where the limb of a victim was removed only to be spat out and the shark swims away. If a shark did eat humans as part of their natural diet they would have to eat many thousands of people each year and this clearly does not occur. Humans can only be considered an accidental prey item.
Estimating the number of Sharks in Our Oceans.
Shark populations are generally considered to be in steep decline in Australia and around the world due to overfishing. Scientists consider that populations of large predatory fish including sharks to have declined dramatically since the 1980s. One cause is the high price obtained for shark fins in Asia, where there is great status attached to eating shark fin soup. Australia along with many other countries have banned the shark fin trade and the selling of shark fins without an attached shark. Estimates vary from 26 to 78 million sharks killed per year for their fins. But some fisheries experts suggest that this figure could be over 100 million sharks killed due to the unregulated and illegal fishing activities around the world.
The simple answer is that there is still no well defined way of estimating the number of sharks in our oceans. In relation to the shark species known to bite humans there is no scientific evidence that indicates an increase in the numbers for those species. It is also known that sharks generally do not have the capacity to increase in numbers quickly. The shark’s life history (long lived, slow growing, late age at maturity and give birth to only a few offspring) means that shark populations cannot increase rapidly and so sudden explosions in the population of sharks is not possible. Sharks have a low resilience to fishing pressure and their capacity for recruitment makes it extremely hard for sharks to recover from overfishing (targeted and unmanaged shark fisheries, finning, long line fishing, etc) and may take decades to do so. This is evident with the protection of the harmless Grey Nurse Shark in 1984 along the east coast of Australia which still shows no sign of recovery over the last 30 years.
Some people have stated that they have seen more shark than ever - but observing a large number of sharks in an area does not mean there is a shark population increase but rather it is more likely due to a congregation of sharks feeding in an area which may give the impression there might be more sharks.