by Matthew Norton
Few ocean predators have quite the reputation of the White shark, Carcharodon carcharias. They are accomplished hunters of a variety of prey, including tuna, sea turtles and other sharks, but we have been especially interested in how they pursue various seals, such as cape fur and elephant seals.
As with most active predators, they first need to find their prey and White sharks have numerous sensory tools at their disposal. In particular they have an especially acute sense of smell, able to detect 1 drop of blood in 1 million drops of water, and are able to detect the weak electrical fields produced by critical biological processes, such as beating hearts and moving muscles. However, in the vast oceans even the most effective sensory tools have their limits.
Fortunately for White sharks, seals live in colonies for part of the year, allowing them to maximise their chances in coming into contact with their prey by simply patrolling the waters around these colonies. These areas are so ideal that the larger White sharks will even drive away smaller sharks that try to access these hunting grounds.
Having found their prey, the next step is for them to approach the seal so that they can attack. This will not be easy as seals are agile in the water, and if given enough warning will outmaneuver their attacker. White sharks therefore, adopt a hunting strategy that minimises the chance of being detected by the seal, stalking them from below and mostly attacking at sunrise and sunset, when their approach is well hidden by the poorly lit water below.
Attack speed is also important for taking the seal by surprise, and as evidence by their ability to launch themselves out of the water they clearly have phenomenal swimming power. The torpedo-like shape of their body gives them the aerodynamic edge, but the real power comes from their internal physiology. The circulation system of White sharks, and some of their close relatives, is structured in such a way that it conserves their internal body heat, effectively making them warm blooded. This increased body temperature raises their metabolic rate, which gives them the energy bursts needed to attack seals at speed.
Clearly white sharks are an intimidating predators, but this does not necessarily make the seals they target helpless. Where possible, seals can swim along the seabed or among structurally complex habitats, such as kelp forests, where their agility gives them the advantage over the shark’s straight line speed. In open water they often rely on safety in numbers, the larger the group is, the lower the probability is for each seal that they will be the one targeted. On some occasions a group of seals will also make aggressive displays towards a shark to drive it away.
In the end white sharks and seals are stuck in an evolutionary arms race, as the hunting behaviour of white shark improves through natural selection, so does the ability of seals to avoid being eaten by the sharks, which is why the white shark is such an impressive predator today.
From a human perspective
White sharks, as well as many other sharks, are large, powerful animals with intimidating jaws and teeth, so the fear that some people feel towards them is understandable, but not really justified given the actual threat they pose to humans. Sharks cause far fewer fatalities than rare natural disasters, such as tornadoes and lightning strikes, or seemingly low risk activities, such as using a toaster. We are not even remotely desirable as a food source for sharks compared to their natural prey, due to our low fat content.
Yet, attacks do occur and many theories have been put forward to explain why. One of the oldest put forward is the ‘rogue shark’ theory, which suggested that attacks along any stretch of shoreline are perpetrated by a single shark that has moved away from its natural prey. The theory has been considered outdated for decades, but it was very influential for its time, having been loosely used in the film ‘Jaws’. More recently numerous other theories have been proposed, such as mistaken identity between human and a shark’s natural prey, sharks being inquisitive about an unusual animal in the water, and that a human victim unknowingly invaded a shark’s personal space, or territory. What most of these theories have in common is that sharks do not intentionally target humans and this is supported by both the low likelihood of attack and that in most attacks the shark only bites once and then swims away.
Yet the sharks’ undeserved reputation as mindless killers has lingered, which sadly is a key reason why shark finning is such a widespread practice that is driving sharks to extinction. For anyone who isn’t aware shark finning is the practice of catching sharks at sea, cutting off their fins while they are still alive, and then chucking them overboard to suffer slow and painful deaths. The market for these fins, for shark fin soup, is so great that an estimated 63-273 million sharks are killed each year. With such a large scale slaughter (few other words feel more appropriate) it is not surprising that 181 species, around ¼ of all sharks and rays, are at risk of extinction, being classified as vulnerable, endangered or critically endangered.
Fortunately, attitudes towards sharks are changing for the better, so there may be hope for them. Today there are numerous shark and ocean conservation charities and NGOs (non-government organisations) which are working to spread the word that sharks are worth protecting so that their population may recover. There is one such organisation that I want to acknowledge, Fin Fighters. I have seen their efforts first hand, having attended talks and their first Sharkfest, and although I was unable to attend their second Sharkfest I wish them luck in the future.
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