Challenging Common Misconceptions of Sharks

With an understanding of basic facts on shark behaviour and their sensory systems, some of the more common misconceptions are easily corrected...

• Sharks will attack and try to eat anything in their path, including humans.
• Diving or swimming with sharks is suicidal.
• Where there are dolphins, there are no sharks.
• A shark circling a human being is getting ready to attack.
• Bending down the pectoral fins is a sign of aggression in sharks.
• Sharks are attracted to human blood.

Sharks will attack and try to eat anything in their path, including humans.

Most of the about 500 shark species in the world's oceans are fairly specialised in their diet, proven by specific adaptations of their teeth and other morphological features. Only very few species are large enough and have such general and opportunistic feeding habits, that they would even theoretically consider something the size and shape of a human as potential prey.

The large oceanic sharks, e.g. oceanic whitetip sharks (Carcharhinus longimanus), mako sharks (Isurus sp.), silky sharks (C. falciformis), blue sharks (Prionace glauca) are most likely to take a closer interest in humans and their activities. After all, they do have very few natural enemies, and the open sea is a difficult environment; these sharks cannot afford to be picky or let a potential food source pass by, not knowing when the next one comes along.

Even if any of these sharks display any kind of interest in a human, e.g. a scuba diver, they generally approach carefully. Keep in mind that they are approaching something rather large, loud and unknown. As a predator, they need to stay healthy and avoid injuries, which explains their cautious behaviour. A lot of their subsequent behaviours will depend on the human's response to the approach (read more...)

Diving or swimming with sharks is suicidal.

All available statistics on shark attacks (or shark incidents) clearly contradict that statement. Actually scuba diving is probably the safest way to interact with sharks in their natural environment, while there is some reasonable concerns about swimming with them. The position on the surface is the most vulnerable one. For a marine predator, seeing a large 'something' on the surface probably leaves three main possibilities: an inanimate floating object (very often accompanied by a variety of smaller marine organisms underneath), the carcass of a marine animal, or an animal in trouble that is unable to leave the surface due to injury or sickness. All these situations invite a large predator to approach safely from underneath, and check this object for its food potential.

The minimum safety requirement for somebody swimming in an area where sharks are likely to occur is wearing a mask. This will at least allow you to be aware of an approaching shark and react to it. The reaction can be as simple as following its movement with your head and/or calmly changing your body position in relation to it. Again, sharks are very successful and efficient predators, they do register if they have been detected by their environment or not. Once they have been seen, they loose the element of surprise and stay cautious; they might even turn away and leave.

Where there are dolphins, there are no sharks.

For the open sea, the opposite is closer to the truth. Dolphins travelling or hunting over deep water in the tropics or subtropics are often accompanied by sharks, e.g. oceanic whitetip sharks (Carcharhinus longimanus) or silky sharks (C. falciformis).

A shark circling a human being is getting ready to attack.

Contrary to common beliefs, a shark that calmly circles a human in the water is by no means preparing for an attack. A more realistic explanation is that the shark is simply making use of its full range of senses, in this case especially the lateral line system, to explore and identify the unknown object it encounters. After one or two circles they normally turn away and disappear again, unless their attention is held by one of their other senses.

Bending down the pectoral fins is a sign of aggression in sharks.

Bent-down pectoral fins is one of the behaviours that have been identified from threatening displays shown by a variety of sharks in the presence of humans. Like many other agonistic displays in the animal kingdom, they consist of a sequence of stereotyped behaviours that evolved to avoid physical confrontations, which could result in injury for both opponents.

Because of the ritualistic and sequential nature of these displays, one cannot simply take one of the behaviours out of this complex context and assign the same meaning to it.

To qualify as part of an aggressive display, the bent down pectoral fins would have to be accompanied at least by stiff, exaggerated swimming movements, an arched back and a lifted head. By itself, bending down pectoral fins in an otherwise slow swimming, relaxed shark simply functions to steer this animal away from the object it approaches. The closer the approach, the more marked the angle of the pectorals (see images below). In some cases, the use of only one pectoral fin indicates in which direction the shark will turn away, as shown in image 4; the right pectoral is almost vertical, the animal turns away to its right side.

Sharks are attracted to human blood.

While their acute sense of smell clearly enables them to detect and follow chemical trails, sharks never had the time to adapt their chemo-receptors to the composition of human blood. To identify blood or other body fluids of their prey organisms (e.g. tuna or other bony fish), sharks mainly analyse the composition of amino acids, which is clearly different in humans.

The presence of human blood in the water can be detected by sharks, it might even catch their attention as an interesting scent. But it will not ring the dinner bell, because they don't know what it is!