Shark season

Summer in Australia comes with cicadas, sunburn and, in the media at least, sharks. So far, I have learned that aerial shark patrols are inefficient (or perhaps not) and that the Western Australian government plans to keep swimmers safe by shooting big sharks.

Sharks are compelling objects of fear, right up there with spiders and snakes in the package of special terrors for visitors to Australia. As good hosts, we are quick to reassure: sharks may be the stuff of nightmares and 70s horror movies, but attacks are rare.

But, exactly how rare is death by shark? Over a Boxing Day lunch, I heard an excellent ‘statistic’, designed to reassure a visiting American. Apparently, more people are killed each year in the US by falling vending machines than are killed by sharks around the world. I was skeptical, but had no data to hand. Later, with the help of Google, I discovered that this statistic is 10 years old and the source? Los Angeles life guards. The tale has, however, become taller over time. Originally, vending machine deaths in the US were compared to shark attack fatalities in the US, not the entire world.

While data on vending machine related deaths are hard to come by, subsequent attempts to validate the story concluded that it was plausible, on the basis that there were two vending machine deaths in 2005 in the US but no fatal shark attacks.

Fun though the vending machine line may be, it is not relevant to Australia and, if you are on the beach contemplating a quick dip, then the risk of a shark attack is certainly higher in the sea than death by vending machine. Local data is in order.

According to the Taronga Zoo Australian Shark Attack File (ASAF):

 In the last 50 years, there have been 50 recorded unprovoked fatalities due to shark attack, which averages one per year.

Fatalities have been higher than average over the last couple of years. The ASAF recorded two deaths in 2012 and, although validated figures for 2013 are yet to be published, six deaths have been reported over the last two years, suggesting that fatalities rose further to four this year.

To compare shark fatalities to other causes of mortality, a common scale is useful. My unit of choice is the micromort. A one-in-a-million chance of death corresponds to a micromort of 1.0, a one-in-ten-million chance of death to a micromort of 0.1. Taking the recent average death rate of three per year (more conservative than the longer run average of one), and a population of 23 million in Australia leads to a figure of 0.13 micromorts for the annual risk of death for a randomly chosen Australian.

The most recent data on causes of death published by the Australian Bureau of Statistics (ABS) are for 2009. That year, three people were killed by crocodiles. Sharks are not specifically identified, but any fatal shark attacks would be included among the three deaths due to ‘contact with marine animals’. The chart below illustrates the risk of death associated with a number of ‘external causes’. None of these come close to heart disease, cancer or car accidents. Death by shark ranks well below drowning, even drowning in the bath, as well as below a variety of different types of falls, whether from stairs, cliffs or ladders.

Shark barplot

Annual risk of death in Australia (2009 data)*

Of course, you and I are not randomly chosen Australians and our choices change the risks we face. I am far less likely to suffer death by vending machine if I steer clear of the infernal things and I am far less likely to be devoured by a shark if I stay out of the water.

So, care should be taken when interpreting the data in the chart. Drug addicts (or perhaps very serious Hendrix imitators) are far more likely to asphyxiate on their own vomit than summer beach-goers. The fairest point of comparison is drowning in natural waters. At almost 3.5 micromorts, drownings in the sea (or lakes and rivers) is more than 25 times more common than fatal shark attacks. And the risk of both can be reduced by swimming between the flags.

What does that leave us with for conversations with foreign visitors? If you are headed to the beach, the risk of shark attack would be higher than death by vending machine, but it is still very low. The drive there (at 34.3 micromorts) is almost certainly more dangerous.

I will be taking comfort from my own analysis as I am heading to Jervis Bay tomorrow and sharks were sighted there this weekend:

Bendigo Bank Aerial Patrol spotted up to 14 sharks between 50 and 100 metres from shore at various beaches in Jervis Bay. [The] crew estimated the sharks at between 2.5 and 3.5 metres in length at Nelsons, Blenheim, Greenfields, Chinaman’s Beach and Hyams Beaches.

The beaches are un-patrolled, so wish me luck…but I don’t think I’ll need it.

* The figure for ‘Shark attack’ is based on the estimate of three deaths per year rather than the ABS data.

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21 thoughts on “Shark season

  1. Zebra

    Hi Mule – I have a theory that the reason why indigenous Australians weren’t regularly swimming at Bondi, or Jervis Bay, when Europeans arrived is because sharks were much more common and lethal. Whale hunting in the 19th century decimated both the whales, and the sharks who preyed on their calves. Sea swimming only became popular in Australia in the early 20th century, surfing about half a century later, at times when whale numbers, and sharks, were at historically low numbers.

    I grew up sailing sabots in Queensland in total fear of sharks. While sailing I once saw a shark fin in the water near the swimming beach at Yeppoon and proceeded to yell out to the swimmers to get out of the water. It turned out to be a rubber mat with one corner sticking out of the water. True story. Many years later when learning about the concept of “false positives” I was able to draw on this experience.

    Other tips: swim with someone else (reduces risk of shark attack by 50%) or a dog (80%). That is why, should I ever visit Jervis Bay, I will bring a dog with me.

  2. Stubborn Mule Post author

    It’s an intriguing theory. It will be interesting to see whether the recent uptick in fatalities is statistical noise or a trend.

    Your tip is reminiscent of the adage that you don’t have to outrun the bear, just outrun the other guy. As for pets though, your tip may not be so good. According to the Zoo files:

    Do not swim with pets and domestic animals (sharks can be attracted to the disturbance animals make in the water).

  3. Thomas Speidel

    What if I don’t buy from vending machines, but do swim daily in the ocean in WA: is the risk the same? I think risk, besides being subjective, needs a Bayesian approach. The frequentist approach is useful for policies and overall assessment, but useless for individual assessment (what if I don’t swim?).
    People are really killed by vending machines?! Sounds like Steven Spielberg picked the wrong plot for jaws ;-)

  4. Ken

    I expect the main reason that Aborigines weren’t swimming at Sydney 200 years ago, is that there was no benefit to them. Spearfishing in open water would be quite difficult without modern equipment. Collect shellfish from a depth would be difficult in the often murky waters. It is much easier to fish from a canoe or herd fish into fish traps and collect shellfish from shallow water.

  5. Stubborn Mule Post author

    @thomas a simple comparison of vending machine deaths to shark fatalities is not really a fair comparison, which is why I prefer to compare to natural drowning deaths. I’m interested in a potential Bayesian approach though: how would you go about forming prions here?

  6. Zebra

    I heard the “more people die from vending machines that are killed by sharks” again, on ABC radio today in relation to the WA shark attack brouhaha. It seems it has become a meme.

  7. Stubborn Mule Post author

    Sounds like it has a life of its own alright, despite being a rather dodgy analogy.

    So far I have still survived my beach holiday, although the aerial spotter plane did chase people out of the water a few days ago because of sharks…I was back in soon afterwards.

  8. dan

    Zebra’s theory may be right, but I suspect the reason for the lack of sharks in the Harbour of our youth was the same for the lack of pretty much any wildlife. It was terribly polluted from years of toxic dumping industry. (Think Colgate Palmolive in Balmain, Union Carbide in Rhodes, BHP Wire in Chiswick and on and on and on).

    It is remarkable today to see the seals and pelicans and penguins return to the Harbour, now that we have outsourced our pollution to Guangzhou. And with them, the sharks are coming back.

    Them’s my 2 c.

    By the way, Stubbies, for those with a stat bent (and anyone who’s into Box jokes applies) this is always worth a listen:

  9. Zebra

    Not sure its entirely relevant Sean, but probably the best quote I’ve read recently, by Upton Sinclair “It is difficult to get a man to understand something, when his salary depends on his not understanding it”.

  10. Simon

    Nothing to do with shark attacks or vending machines but an interesting take on interpreting the news.

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