Hero image

The Ultimate Marine Predator

Removing the fear and exploring the significance of one of the ocean’s most critical species

Shark! The mere word is enough to strike fear and loathing into most people’s hearts. Shout it on any popular Australian beach and you’re likely to trigger a floundering stampede of bathers and board riders from a refreshing surf on a hot summer day.

Our perception of sharks as malevolent maneaters is a lingering stereotype created by the 1975 blockbuster movie Jaws

, and a gross mischaracterisation of one of the ocean’s most majestic creatures. Far from being the monstrous villains portrayed by Hollywood, sharks are crucial to the health of all marine ecosystems, maintaining a balance in the natural order of a global food chain that ultimately supports humankind.

Sharks have been swimming the world’s oceans for millions of years and become perfectly adapted for their role as an apex marine predator with few rivals. However, sharks have good cause to fear humans — more than we have to fear from them. Within the last 50 years, global populations have been decimated by a host of man-made pressures that have pushed some species to the brink of extinction. Sharks now represent the largest group of threatened marine species on the planet.

It’s time to set the record straight and shed some light on the most feared and fascinating of all sea creatures.

SHARKS AND THEIR HABITATS

Sharks belong to an order of cartilaginous fish that includes rays, skates and sawfish. This group first appeared in the evolutionary calendar about 420 million years ago. Worldwide, there are more than 510 species of sharks — about one percent of all living fishes — with new species being discovered every year. About 182 of them inhabit Australian waters, 70 of which are found nowhere else. They are highly diverse, ranging from the world’s largest, the whale shark (18m), to one of the smallest, the pygmy shark (25cm), and the awesome great white. Sharks are found in all seas and are common to depths of 2000 metres. They occur in all habitats around the Australian coastline, typically on the continental slope or shelf, and are widespread in tropical to temperate zones. A few species have adapted to freshwater environments, such as the bull shark and northern river shark.

APEX PREDATORS

Most sharks are carnivorous, operating at the top of the food chain. They have few natural enemies, although the great white is sometimes hunted by killer whales — a pod of orcas killed a great white off South Australia in 2015. Sharks usually become active after dusk and hunt during the night. The majority feed on other fish, while large sharks prey on marine mammals such as seals, sea-lions and dolphins.

All sharks have multiple rows of teeth embedded in their gums, which they constantly shed and replace — some species may lose as many as 35,000 during their lifetime.

Tooth shape depends on the shark’s diet. Species adapted for bottom-dwelling feed on molluscs and crustaceans and have dense, flattened teeth used for crushing. Those that hunt fish have needle-like teeth for gripping, and those that prey on larger animals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting.

The basking shark, megamouth shark and whale shark are filter feeders, straining enormous quantities of plankton from the water through specially adapted sieves similar to the baleen plates in whales. The teeth of plankton-feeders are small and non-functional.

Sharks have also developed different methods of hunting. Many sharks, like the whitetip reef shark, are cooperative feeders and hunt in packs to herd and capture elusive prey. Thresher sharks use their long tails to stun shoaling fish and saw sharks either stir prey from the seabed or disable swimming prey by slashing with a long, saw-like snout edged with sharp teeth. Some seabed–dwelling species, like angel sharks and wobbegongs, are highly effective ambush predators, using camouflage to lie in wait before sucking prey into their mouths.

ANATOMY OF A HUNTER

Sharks have a unique anatomy, with many features designed to enhance their role as a hunter. Made of cartilage and connective tissue, their skeletons are flexible and about half the normal density of bone. Around the skeleton is wrapped a complex ‘corset’ of flexible fibres arranged in a helical network, which provides attachment for the swimming muscles.

Unlike bony fish, sharks don’t have gas-filled swim bladders for buoyancy. Instead, they rely on a large liver (up to 30 per cent of their body mass) filled with an oil that is less dense than seawater. Neither do sharks have scales as such, but rather a skin covered by tightly packed dermal denticles, like modified teeth covered with hard enamel. Like chainmail armour, they protect the skin from damage and parasites, and also provide hydrodynamic benefits of reducing turbulence and drag, allowing the shark to swim quickly and covertly.

Most sharks have eight fins that aid stability while cruising and manoeuvrability when hunting. The tail provides thrust with the upper part usually noticeably larger than the lower portion, but these relative proportions vary considerably between species. For example, the tails of tiger sharks have a large upper lobe, which allows for slow cruising and sudden acceleration, whereas the porbeagle shark, which hunts schooling fish such as mackerel and herring, has a large lower lobe to help it keep pace with its fast-swimming prey.

In general, sharks cruise at an average speed of 8km/h and may travel 70–80km in a day. When feeding or attacking they can reach speeds of 20km/h. The shortfin mako is the fastest shark and is capable of bursts up to 50km/h. A hunting technique employed by great white sharks against seals is to launch their attack from deep below the victim, reaching speeds of 40km/h that may carry them (seal in jaws) more than 3m into the air.

Sharks’ senses are perfectly matched to their physical hunting attributes. They have keen olfactory senses, located in a short duct between their nasal openings. Some species are able to detect as little as one part per million of blood in seawater and can even determine the direction of a source based on the timing of scent detection in each nostril.

Shark eyes are similar to those of other vertebrates, with similar lenses, corneas and retinas, but their eyesight is well adapted to the marine environment, featuring a tissue behind the retina that reflects light back to it, increasing visibility in dark or murky waters. They may also be able to hear prey from many kilometres away.

Sharks also have the greatest electrical sensitivity of any animal. This is provided by an array of special sensing organs (electroreceptors) in their snouts, capable of detecting electric and magnetic fields, as well as temperature gradients which helps in the detection of prey and possibly navigation. Scientists have determined that sharks possess an internal navigation system similar to GPS that allows them to use Earth’s magnetic forces to travel long distances with accuracy.

Bull sharks are one of the ocean’s apex predators


DANGER TO HUMANS

The vast majority of shark species present little or no threat to humans. In Australian waters, only three kinds of shark account for 96 per cent of all fatal, unprovoked attacks on humans — the great white, tiger and bull sharks. Oceanic whitetips have also been implicated in some fatalities. 

Of all species, the great white is responsible for the most recorded shark bite incidents and the most fatalities. This is hardly surprising, given their sheer size. Analysis of some white shark attacks suggests that the attack may have been a ‘test bite’ — they are known to do this with unfamiliar objects such as surfboards, buoys and flotsam, to identify what it is. But, as one expert has put it, a blacktip bite can give you four stitches, while a nibble from a white shark can remove your leg, leading to catastrophic blood loss and shock, either of which could well prove fatal.

Shark encounters typically occur in near-shore coastal waters. Surfers are likely to encounter sharks on the seaward side of the waves near deeper water where the animals can approach without being observed. They and other board sport athletes experienced 61 per cent of bites worldwide in 2020. Swimmers commonly encounter sharks inshore of a sandbar or near drop offs to deeper water.

According to the Australian Shark Attack File, kept by researchers at Sydney’s Taronga Conservation Society, there have been 877 shark attacks in Australia since records began in 1791, and 216 (25 per cent) of these have been fatal. (This figure is consistent with the global rate of 30 per cent.) New South Wales has had the most attacks (156) for 7 fatalities, the Western Australian coast is the deadliest, with 18 fatal shark attacks, followed by South Australia (11), Queensland (9) and Tasmania (2). Australia ranks second in the world behind the USA for the number of shark attacks but has had more fatalities — 8 in 2020 alone.

ENCOUNTERS IN PERSPECTIVE

While these statistics may present a grim record, consideration of other facts puts them into perspective.

First, in the lexicon of researchers and record-keepers, the term ‘shark attack’ is generally applied to almost all interactions between humans and sharks, even those that involve no physical contact and do not lead to injury, for example where the shark bit a surfboard or was pushed away before it could strike — around 27 per cent of all Australian shark encounters.

Second, despite the global increase in population and the growing number of people engaging in recreation in coastal waters, the average number of reported shark bites has been falling steadily in recent years. Some experts attribute this to a significant decline in shark populations in the world’s oceans.

Third, statistically, fatal shark attacks are rare occurrences. It has been estimated that the odds of being killed by a shark in Australia are one in 3.75 million, with at least 17 other things more likely to kill you. On raw figures, in 2020, eight people died in shark attacks, about the same number who died from a lightning strike; in the same period, more than 18,000 Australians died from ischaemic heart disease — you do the math.

SHARK ATTACK THEORIES

Unprovoked shark attacks are seemingly random events, with little in common apart from the fact that human and shark are in the water at the same location at the same time. The motivation for an unprovoked shark attack remains unclear in most cases and predicting them is virtually impossible. Nevertheless, many theories have been advanced to explain shark attacks around the world.

One obvious explanation is that the shark is hungry, but there is no evidence sharks actively or routinely hunt humans as a food source. Although there have been instances where humans have been consumed, partly, or in rare instances entirely, we are not a regular part of their natural diet. On the contrary, research suggests that sharks appear to not like the taste of humans, or at least find the taste unfamiliar.

For the most part, humans are too bony — they much prefer seals, which are fat and rich in protein. A shark’s digestion is too slow to cope with our high bone to muscle/fat ratio. Experiments show sharks can tell in one bite whether or not the object is worth eating, which would explain why many encounters are broken off after the first sample. Other analysis indicates that when humans do become the object of a shark attack, it is possible the shark has mistaken the human for species that are its normal prey.

An ‘attack’ may be motivated by nothing more sinister than curiosity. The usual offenders are easily capable of inflicting much more damage to humans than usually occurs, but the majority of victims are bitten once and released. This would suggest that the shark is not biting to procure food but rather to investigate an interesting object. Or it could be that they are simply confused by competing sensory stimuli that prompt it to investigate.

Of course, sharks may attack for good reason: to defend an invasion of its territory or personal space, to protest an interference with courtship or mating, or in self-defence to perceived aggression.

WHY WE NEED SHARKS

Despite their fearsome reputation and bad press, humans and the planet need healthy shark populations in the oceans for many good reasons.

Sharks are essential to maintaining the natural balance of marine ecosystems. As apex predators, sharks have few natural enemies and play a major role at the top of the food chain. They feed on animals in lower tiers, regulating populations of their prey and encouraging natural selection among those species by removing the sick and weak ones from their communities.

Without sharks, populations of prey species can grow too big, increasing pressure on competitors at that level and the animals they in turn feed on. By limiting the abundance of their prey, sharks help maintain overall species diversity in the entire ecosystem.

Recent studies have shown that coral reefs, in particular, benefit from the protection of sharks. The removal of sharks from these ecosystems led to an increase in groupers that feed on herbivorous fish. With fewer herbivores, algae quickly dominated the reef, smothering coral, reducing biodiversity and decreasing the reef’s resilience to storms and marine heat waves.

Sharks contribute to the ‘circle of life’ in other ways. Scraps from their meals provide food for smaller fish. After they die, shark carcasses sink (literally and metaphorically) to the bottom of the food chain to support scavengers and create nutrients essential to the growth of phytoplankton, the foundation of marine ecosystems and an important contributor of oxygen in the Earth’s atmosphere.

Sharks also play an important role in the carbon cycle. Seagrasses absorb vast quantities of carbon dioxide — 35 times faster than tropical rainforests — and store it in their leaves and roots, reducing the amount of greenhouse gases in the atmosphere. In the absence of sharks, dugongs and turtles tend to graze heavily in concentrated patches, quickly consuming large swathes of seagrass. By prowling the meadows, sharks intimidate these prey species, which move around more often and feed more widely without overgrazing any particular area. Less continuous and less concentrated grazing encourages more abundant seagrass and promotes the capture of carbon.

Sharks not only provide a valuable source of protein for many communities around the world, they also contribute substantially to the global economy. Sharks are harvested not only for their meat and fins but also for their skin, liver oil and jaws. The estimated value of the world trade in shark commodities approaches US$1 billion per year. From an economic perspective, therefore, it makes good sense to maintain healthy, sustainable shark fisheries.

Leopard sharks are a beautiful adornment to their environment

At the same time, many communities around the world are learning sharks are worth more alive than they are dead, as the stars in a burgeoning ecotourism industry. In 2016, shark diving brought over $65 million to the economy of the small island nation of the Maldives as the country’s most popular tourist activity. In Palau, part of the Federated States of Micronesia, shark ecotourism generated more than $314 million in 2013, a figure that is expected to more than double over 20 years.

Shark diving tourism is a growing industry in Australia, conservatively estimated to be worth more than $25.5 million annually to regional economies. The main venues for this unique underwater experience include snorkelling with whale sharks off Ningaloo Reef in Western Australia, cage diving with white sharks off Port Lincoln in South Australia, diving with grey nurse sharks off the coasts of New South Wales and South East Queensland, and swimming with reef sharks at Osprey Reef in Far North Queensland.