What drives shark attacks?

[SkepticLance]

To Paralith

 

I don't want to pretend to be an expert on lions and their behaviour. I have only seen them in zoos and on TV. Sharks, on the other hand, I can speak about with the authority of first hand experience.

 

However, a couple of thoughts.

 

You said that :"If humans really were a prime menu option for lions, then I think my truck and many others like it or even more open than it would be highly unsafe"

 

When the first Spanish appeared in America, riding horses, the local natives thought that the horse/man combination was a single new creature. If a creature as smart as a human can be so easily fooled, why should a lion see a human on a truck as a small animal?

 

 

Your second point. I don't know any more about that incident than I told you. I could only guess why she didn't run back to the bus.

 

 

Your third point is, of course, valid. However, I am talking of killing AND eating. Or killing for food. There is no doubt that lions do this. Not being an expert on African wild-life, I have to rely on what I have seen. However, what I have seen makes it look as though each land predator is able and willing to kill and eat an almost infinite range of prey animals. Reptiles, birds, mammals etc. Sharks are more restricted.

 

Your fourth point, about acclimation changing behaviour definite does also apply to sharks. There are lots of places in the world where tourists chase the same sharks day after day all year long. Places where sharks are attracted each day by underwater tour guides handing out food. eg. Bahamas. Other places where sharks come regularly to cleaning stations, and where tourists are there each day by the dozen. eg. Red Sea.

 

In spite of the acclimation, it does not seem to make the sharks more prone to attacking people.

[Sayonara]

Argument from low likelihood. I'm not saying that it's incredulous that someone be eaten by a shark and no one know about it. I'm saying that the circumstances in which such a happenstance would occur are relatively rare.

Yes, they are likely to be rare. But my point is that with the low figure SkepticLance has for shark-related deaths per year, you only need 36 people out of all those who go missing at sea or from beaches per year to be killed due to shark activity, and the figure is then an under-estimate by an order of magnitude.

 

Sorry if you were taken aback by the "argument from incredulity" thing, it was late here and I was just being me

 

How else would someone eaten by a shark not be registered?

All the reasons given... except the aliens one I would imagine. Note that you don't have to be eaten, just killed. There is also the fairly common eventuality that bodies recovered at sea or on coastlines are often too badly damaged for coroners to properly identify a cause of death.

 

I am suggesting that shark-related deaths are under-reported for the simple reason that there are so very many more deaths of unknown cause and persons recorded missing at sea than there are identifiable deaths/disappearances.

 

Ok, put it another way. How many people of groups in any number find themselves in the ocean where sharks are without being near a boat or populated beach? Maybe its more that I think.

That's exactly my point; I was just pointing out to SL the danger of accepting that "only four per year" figure as being concrete.

 

I doubt that it would be so many, though, as to make the statement that "shark attacks are rare" be untrue considering all of the people who we know come into contact with sharks and are heard from again. Perhaps I'm wrong there, too.

Let's make no mistake about this: the people who survive shark attacks are very lucky. You only need a relatively small wound from a modest sized animal to do serious damage when you are in the water; without help being immediately at hand getting to safety and/or medical attention is very problematic.

 

I have to say I agree with CDarwin. Most people do not enter the water alone, whether swimming or snorkelling, surfing or scuba diving. People like company, and do these things in company.

Yes, because (a) nobody ever finds themselves in water under unexpected circumstances, and (b) nobody was ever separated from their group and met a fate the nature of which could not be established.

 

If, occasionally, a solo person gets killed and eaten, then we would expect much larger numbers of people with company to get killed and eaten, and that does not happen.

What does not happen, larger numbers are killed than are killed? I do not follow your logic. Are you countering the "under-reporting" argument, or suggesting that people in groups are not usually killed by sharks at all?

 

Unless a solo person presents a much less threatening situation than a mob of thrashing humans, making it more likely that the shark may approach and investigate the possibility of an easy meal.

There is a very good possibility that this happens frequently. A lot of predatory behaviour comes directly from a form of cost-benefit comparison.

[SkepticLance]

To Sayonara

 

While it is probable that a few people get killed by sharks, and their bodies never found, I think that the number would be small. I appreciate that your number 36 was just a f'rinstance, but that would be a major exaggeration. As I said earlier, people are gregarious, and most people who enter the water do so in company. It makes it more fun!

 

I am not sure that lethal shark attacks are under-reported. Sharks are a very convenient scapegoat to blame other deaths on. I am aware of two case. One where a murder took place, and one where a person ran over the victim in a power boat by accident. In both cases the culprit tried to blame the death on a shark attack. In fact, I believe that police are actually made MORE suspicious when a death is called a shark attack. I sometimes wonder how many so called shark attacks are actually murders that the perpetrator got away with!

 

In fact, if you look at official shark attack statistics, the great majority are non lethal. The place in the world where more shark attacks happen than anywhere else is apparently Florida. Partly this is due to the larger numbers of people in the water, but also due to the fact that Florida is a shark breeding area, and very large numbers of immature sharks (mainly lemon sharks, I think - but I am going by memory, so I could be wrong) are present. Those miniature sharks encounter humans in shallow water, and will bite if accidentally provoked. However, it is a bit like being bitten by a chihuahua.

 

I suspect that there are a lot of shark bites that go unreported, since any really minor bite will not result in hospitalisation.

 

The big thing that stands out to me, is the number of attacks - the vast majority - that are not followed up. In spite of the masses of blood in the water, and the reputation that sharks have for going into a feeding frenzy where blood is present, it just does not seem to happen. Perhaps human blood is not exciting.

[Sayonara]

While it is probable that a few people get killed by sharks, and their bodies never found, I think that the number would be small. I appreciate that your number 36 was just a f'rinstance, but that would be a major exaggeration.

You calling it a major exaggeration is no more likely to be accurate than the figure of 36.

 

In fact, if you look at official shark attack statistics, the great majority are non lethal.

Yes, it is much easier to communicate when one is not dead.

[JohnB]

While it is probable that a few people get killed by sharks, and their bodies never found, I think that the number would be small.

That would only be a guess. How many small craft go missing each year? How many fishermen are lost overboard?

 

We had a case recently. Three fishermen and their boat went down. One swan to shore (about 15 km, a mighty effort) and raised the alarm, the second was picked up by chopper, the third? "Missing presumed drowned."

 

People go missing every year. They may drown, or they may be taken by noahs, there's no way to tell. The only thing that we can be absolutely certain of is that the official figure is too low. It's only by how many that there is uncertainty.

The big thing that stands out to me, is the number of attacks - the vast majority - that are not followed up. In spite of the masses of blood in the water, and the reputation that sharks have for going into a feeding frenzy where blood is present, it just does not seem to happen. Perhaps human blood is not exciting.

AFAIK a feeding frenzy is triggered by vibration, not blood. A human tends to go into shock and become still after losing a limb, so that could be why the initial attack is not followed up.

 

Another possibility is that since most of the initial attacks occur near the beach the human may get to land while the shark is still deciding whether to go back for another bite. The reports of single attacks mostly come from people who got out of the water very quickly and survived.

[SkepticLance]

The idea of lots of people killed by sharks and the deed undetected does not make much sense to me. That requires that two things happen :

 

1. The attack is not witnessed

2. The body is never found.

 

For both to happen would seem to be a rarity.

 

A person who fell off a boat way out at sea might suffer such a fate. However, there are not many people in that position, and for those few it seems rather more likely that they simply drown. This statement is based on the fact that there are a number of cases like that every year where two or more people are drifting together at sea and one or more drowns. The survivors report drownings - not shark attacks. I have read the news items.

 

Lethal shark attacks do happen, but they average about 4 per year world wide. There will be more, since some peoples do not report deaths to authorities. But to suggest more than, say, 10 per year would be a stretch.

[Sayonara]

Lethal shark attacks do happen, but they average about 4 per year world wide. There will be more, since some peoples do not report deaths to authorities. But to suggest more than, say, 10 per year would be a stretch.

Would it? Globally, how many people go missing at sea or on coastlines in a year?

 

For someone with a stated interest in the topic, you are not making a very visible effort to check up with reputable sources and determine if this possible under-reporting can be written off safely. If it were me, I would not simply rely on conjecture and anecdote.

[SkepticLance]

Sayonara

 

There is a serious lack of evidence in the suggestion that lots of people are killed by sharks. The evidence that the number is few comes from official statistics, and the fact that most people who go into the water do it in company. This makes an unwitnessed attack likely to be rare. Since most people are in the water in company, then most shark attacks should be witnessed. Simple logic.

[Paralith]

I don't see why you find it so unlikely that a significant amount of lethal shark attacks could go unreported. We're talking about the ocean, here. Even in coastal areas, we're talking about vast areas with a very thin concentration of human life. As others have said, many people simply go missing without any actual knowledge about how they died, or what they may have experienced in the hours/moments before their death (a single weakened human floating in the ocean about to die from exposure is probably easy pickings compared to a pair of healthy swimmers). We're not saying for sure that there ARE lots of attacks that go unreported, but you can't deny the possibility. It's difficult to answer the question of why there are less than expected shark attacks if you're not even sure of the actual frequency of shark attacks in the first place.

 

Even so, I'm not even sure it's right to base your expected number of shark attacks on number of attacks by land predators. Once again, pure density is probably the largest culprit for any discrepancy. We humans are crawling all over the dry land, and are relatively nonexistant in the oceans when it comes to cubic space. We are simply not a typical factor in a shark's life, so their reactions to humans are probably highly variable depending on the specific circumstances of the encounter.

[Sayonara]

Sayonara

 

There is a serious lack of evidence in the suggestion that lots of people are killed by sharks. The evidence that the number is few comes from official statistics, and the fact that most people who go into the water do it in company. This makes an unwitnessed attack likely to be rare. Since most people are in the water in company, then most shark attacks should be witnessed. Simple logic.

 

"Rare" is a relative term. How many millions of people enter the Earth's seas every year? Additionally, you are ignoring the possibility of all members of a party being killed before making a report. Note that they do not all need to be killed by sharks - there is a reason why it is called "the cruel sea".

 

It seems to me that you are writing off an important uncertainty factor to make the facts fit, without being sure about your assumptions. Personally I don't care if the figures you use are right or wrong, but I would have thought you'd want to be very confident of them.

[SkepticLance]

A lot of the way I think about shark attacks comes from my own personal experience. I have, over the past 40 odd years, been in the water with sharks on many occasions. Only twice have I been threatened. Both times it was in very deep tropical water, in places where very few humans ever go (once at North Minerva Reef and once at the Rowley Shoals); both times it was the Oceanic White Tip which rarely encounters people (its home is deep tropical waters) and both times the sharks turned and fled from me anyway.

 

I have also snorkelled off popular swimming and surfing beaches and encountered sharks. People have no conception as to how often sharks come within range of their senses to human swimmers. From experience, I can tell you it is far more common than most realise. In spite of this common encounter (only perceived by the shark - the swimmer remains oblivious) attacks remain very rare.

 

From personal experience, I would judge that people coming within striking range of sharks is far more common than people coming in striking range of lions, tigers, hyenas etc. Yet attacks are rare.

[Sayonara]

Refer to my previous post.

[SkepticLance]

To Sayonara

 

I presume you want me to reply to your idea that sharks may occasionally kill a bunch of people and remove all trace of the deed.

 

This is, of course, possible in the deep ocean. As I said before, the one shark that I have personally seen act aggressively to me is the Oceanic White Tip, and it lives in deep water in the tropics. It is possible that groups of people adrift in deep tropical water may be killed and all traces removed by this shark. However, there are a number of recorded situations where groups have been rescued from deep water, and there has been no attacks. So lets call that a maybe.

 

The great majority of recorded shark attacks are coastal. Certain species are known to make these attacks - bull sharks, great whites, and tiger sharks especially. It is much more difficult for such an attack to be carried out with no witnesses or trace of a body. And those attacks are pretty much always fleeting. The shark strikes only once. We get repeated news reports of the victims 'fighting off' the shark, or rescuers risking their lives. I do not want to denigrate the courage of those who carry out such rescues, since they are no doubt terrified, but do the brave thing anyway. However, I doubt they are in much danger. The sharks strike and then fail to follow up.

 

My interest is why they strike. My own experience is that sharks often see humans but rarely attack. When they attack, they do not follow up. Why?

 

My theory is that marine predators have evolved to avoid eating unknown foods, because so many marine organisms are toxic. Thus sharks attack and eat only the organisms on their mental 'shopping list' in order to avoid being poisoned. Land predators do not suffer this risk, since few, if any, large prey animals are poisonous.

 

So why do sharks attack, even if rarely? My feeling is that is is curiosity - mouthing something to investigate it, or by accident when they strike at a white flashing object from reflex.

[Sayonara]

To Sayonara

 

I presume you want me to reply to your idea that sharks may occasionally kill a bunch of people and remove all trace of the deed.

To be fair, the multiple persons thing was not my idea (although it came from an off-the-cuff comment of mine), but I do agree that a party of more than one person falling foul of sharks is possible. I think we probably all do, if we completely lay aside the issue of frequency.

 

To be honest, I don't want to labour the issue too much. The possibility of under-reporting is less important in its own numerical right than it is for the purpose of reminding you to consider anything that might affect your hypotheses.

 

It is possible that groups of people adrift in deep tropical water may be killed and all traces removed by this shark. However, there are a number of recorded situations where groups have been rescued from deep water, and there has been no attacks. So lets call that a maybe.

I think that is sensible, but bear in mind that we can't just focus on groups who were successfully rescued and disregard those which were not. It stands to reason that people who are not attacked by sharks stand a better chance of being rescued than those who do. You can acknowledge certain possibilities without throwing your whole idea; it is all about managing your variables.

 

The great majority of recorded shark attacks are coastal. Certain species are known to make these attacks - bull sharks, great whites, and tiger sharks especially. It is much more difficult for such an attack to be carried out with no witnesses or trace of a body.

This is kind of the point. The implication of this paragraph is that it is much easier for attacks to be carried out with no witnesses or trace of a body away from the coast, where attacks go unrecorded. We know that attacks do occur away from the coast, since they are accounted for in the outstanding minority. The information we lack is the degree to which that minority represents the entire set of attacks made away from the coast.

 

My theory is that marine predators have evolved to avoid eating unknown foods, because so many marine organisms are toxic. Thus sharks attack and eat only the organisms on their mental 'shopping list' in order to avoid being poisoned. Land predators do not suffer this risk, since few, if any, large prey animals are poisonous.

 

So why do sharks attack, even if rarely? My feeling is that is is curiosity - mouthing something to investigate it, or by accident when they strike at a white flashing object from reflex.

Since you have your idea, a good way to progress this thread now would be to devise a reliable means of testing the hypothesis.

[SkepticLance]

To Sayonara

 

As you say, we need to devise a predictive test for the hypothesis.

 

How about this? The range of prey species that a predator feeds upon should be less in areas where poison species are common, such as coral reefs, and wider in areas, such as cold waters or the deep ocean, where poison species are less common.

 

Is there a marine biologist in the house? Someone who could comment on the prediction above?

[halogirl]

generally i have always felt that sharks generally attack people out of either curiosity or stupidity. i suspect that although humans can be eaten, we probably taste very sweet, due to the number of sugars and preservatives we eat. that probably doesn't taste very good combined whith salt water. sharks may be used to the salt,but that doesn't mean sweet and salty tastes good at all.

sharks very rarely attack us i beleive, also because we don't look very appealing. i mean, their eyes will see differently then ours, so mabe whith all our limbs we look like octopi. or mabe we just smell bad, but it seems to me that they just know we aren't going to make good eating.

[bombus]

Great Whites often just bite and wait for the prey to bleed to death. Humans (if lucky) can swim to shore and get medical help so survive, whereas this is not the case for seals!

 

However, Great Whites (and probably other sharks) do investigate objects with their lips and teeth, which would be fatal to humans. When they strike from below though, they are probably preying!

 

As has been said above, many Great White attacks are probably 'mistakes' by the shark. Humans are too boney and not blubbery enough to make a decent meal.

[halogirl]

hooray for not being nutritional to sharks!

 

well not tasty or filling at least, mabe we're what they binge on. :}

[SkepticLance]

In the last few days I saw a television program on fishing. This guy was swimming a baited line with a big hook and big bait - out past the breaker line on a surf beach, and catching big bronze whaler sharks - 2 to 3 metres long. Bronze whalers are listed as 'man-eaters'.

 

They stated that the sharks were numerous at that point and swam in and out of the surf regularly and routinely. Well, the beach in question is a very popular swimming beach. On a warm summer weekend day, you might see hundreds of people playing in the surf. There never has been an attack there, but must have been hundreds of occasions where the big sharks detected the humans and ignored them.

[halogirl]

exactly, sharks do not like us. we're probably like twinkies to them. disgusting and stuffed with preservatives. :}