Addressing the assumptions that underpin the shark cull

Many of the arguments given in support of culling sharks can appear logical on face value, until you examine the assumptions that underpin them. Below are some facts to consider on seven of these key assumptions.

 Assumption 1 – Shark cull policies have been proven to protect swimmers in NSW and Queensland

 The major piece of evidence cited to support the theory that nets and drumlines have protected swimmers in NSW and Queensland is that there is a drop in shark attacks at the time that the net and drumline policies began.  However, scientists have long questioned this theory, such as in this recent piece in the Conversation that shows that the evidence does little to back the claims of culling protecting people, and that attack rates were low and had been dropping off in Queensland long before drumlines were installed.

The dates of net installation also coincide with another key date that could explain the drop off in shark attacks, the end of whaling in Sydney and off Brisbane at Morton Island.

Whaling was well known to bring in large sharks from the deep ocean, and the presence of whale carcasses in the water is known to be a key risk factor in both attracting and altering the normal behaviour of sharks.  The flensing of hundreds of whales near to popular swimming beaches would have been a major influence on the risk of shark attack.  On the other hand, the chances of the drop off in attacks being caused overnight by the phased implementation of shark culling by nets and drum lines over a number of years seems much lower.

 The ABC’s Fact Checker website cites this as evidence, saying “’In Queensland, there has been only one fatal attack on a controlled beach since 1962, compared to 27 fatal attacks between 1919 and 1961.Statistics from the NSW Department of Primary Industries indicate that before nets were introduced in NSW in 1936 there was an average of one fatal shark attack every year. There has been only one fatal attack on a protected beach since then and that was in 1951.”

 However, a quick check of the dates around the end of whaling on the east coast shows that from 1952 until 1962 a whaling station operated at Tangalooma, Queensland, on Moreton Island, which harvested and processed 6277 Humpback Whales during that period – over 600 whales per year. It was forced to close after it had drastically reduced the number of whales in the eastern Australian Humpback population.  This coincides exactly with the 1961 date given for the reduction on shark attacks.

 Additionally, whaling as an industry in Sydney died out in the 1840s.  From the 1820s whaling boats were based out of Sydney Harbour from an onshore station in Mosman Bay.  The station was in use from 1831 until Mosman sold it in 1839, boiling down and storing whale oil on site. This drop off and eventual end of whaling also coincides with the dates given for the drop in shark attacks in NSW.

 Whilst this not conclusive evidence, the coincidence cannot be ignored and the end of whaling represents at least an equally strong, and probably stronger, theory for the reason for the drop off in shark attacks when compared to the phased implementation of a decades long culling program. 

 Also, it is important to consider when looking at evidence presented on so called shark attack trends that it is not unusual for a series of shark attacks to occur in a period, and then drop off for decades for no known reason. This is because shark attacks are so rare they can only be considered random events. According to shark attack researcher Chris Neff at the University of Sydney “the odds of having one shark bite incident followed by none for three months are the same as having a group of them occur. Yet when a cluster does happen, the inclination is to look for some new factor that explains the apparent trend.” He warns against seeing a cluster of bites as trends.  Such clusters have happened all over the world, for example in Florida in 2001, yet Florida did not introduce culling and the apparent cluster of bites ended.  

 Additionally, in New South Wales between 1937 and 2008, 63% of shark bites occurred at beaches with shark nets installed, clearly indicating that these lethal shark measures do not stop bites to humans.  This is not surprising given the majority of sharks killed in Queensland and NSW are taken leaving the beach. The fact that there have been so few fatalities may just highlight the fact that shark attacks are rare. With such small numbers of fatalities statistics need to be used very carefully, or they can be highly misleading.

 During the Second World War, beach nets were removed in NSW from ocean beaches so fisheries ships could be used by the Americans. For three years, between 1943 and 1946, there were no fatal shark bites at these un-netted beaches.

 The reasons above are probably why there is no independent scientific endorsement of shark culling, drumline and netting programs. Recently, 100 shark scientists wrote to the WA Government opposing the cull.

 Other claims have been made that the Queensland and NSW culling programs are not controversial.  However, there have been long running efforts to have the nets removed, and these occasionally reach the local and national media.  For example in early 2013 when a 4.8m pregnant female tiger shark was killed off the sunshine coast the debate about shark netting was reignited.  The shark had been around in coastal waters for a few days and prior to being killed. ABC. Sunshine Coast Daily.

 Undeniably, the Queensland and New South Wales programs have killed lots of sharks and other marine life.  Records show that more than 53,000 sharks, turtles, dugong, dolphins, whales and other marine life have died in the nets and on drumlines. It is estimated that the nets catch five times more non-target species than large sharks. It is highly questionable whether this is necessary, or even effective in reducing shark attacks. 


 Assumption 2 – Culling sharks and reducing the shark population will reduce the (already very low) risk of shark attack

Scientists regularly oppose this theory publicly.  One example commonly cited is from Hawaii where between 1959 and 1976 a cull of 4,668 sharks, including 554 tiger sharks, was carried out to reduce the number of shark bite incidents, yet there was no significant decrease in the rate of shark bites after the cull. 

 The theory that the size of the population of sharks in an area is a predictor of shark attack risk is also not supported by the scientific evidence.  A recently released study from NSW identified a new nursery area where up to 250 white sharks share the surf zone with swimmers for much of the year. The presence of these sharks had gone largely unnoticed and despite the high numbers there has never been a bite. The researcher behind this, one of Australia’s most respected white shark experts said:

“One thing that is becoming clearer is that people and sharks (including white sharks) share the same space more frequently than we previously realised. This is not because numbers of sharks have suddenly increased; it is because we are getting better information on shark movements.

“The more we look the more we see. For example we know of some areas in eastern Australia where white sharks and people commonly swim at the same beach at the same time – yet there has never been an attack at these beaches – in fact in most cases you would not know they were there.”

So whilst there certainly has to be a shark in the area for there for a shark attack to occur, the size of the local population of sharks is not a predictor of risk, humans regularly share the water with sharks and yet there are no attacks. Especially with white sharks, with such a migratory animal, we could kill hundreds of probably harmless sharks in a local area, but a migrating white shark might be the one that makes an attack.  We just don’t know.  This also highlights another danger in this policy, that it can create a false sense  of security whilst not reducing the risk, albeit a very low one to begin with.

 The good news in this information is that if we are successful in recovering shark populations that will not necessarily mean an increase in risk to people, especially if coupled with the smart development of non-lethal shark risk mitigation measures.


Assumption 3 – Western Australia’s unfortunate three years are caused by a large increase in the white shark population

 Another theory behind the cull policy is that shark numbers have increased.  Whilst there is some limited and as yet inconclusive evidence from the Fisheries Department that the numbers of larger bronze and dusky whalers (not considered dangerous in most circumstances) may have increased as a result of changes to fishing regulations to protect breeding stock for commercial fisheries that target these species, there is no available evidence of an increase in population of white sharks. 

 In fact, the evidence we do have from helicopter data and fishery by catch , shows no change in numbers. A study by UWA scientists showed that despite being dedicated to looking for sharks, the helicopters only see about one shark every 2.5 hours, a number that has not changed over time, and the vast majority are not white sharks.

 Contrary to media and public perception, the best available estimate of the size of the population (from genetic studies released in 2012) suggest that the Western Australian and South Australian population contains about 700 breeding size white sharks.  This population is a separate population from the East Coast population that has about 800 breeding size sharks. This further suggest that the size of the white shark population is not the reason behind the recent attacks. 


Assumption 4 – Sharks are the major danger at Western Australian beaches

 Whilst the attacks of the last few years have been tragic for the families and communities they occurred in, the high profile given to these circumstances belies the real scale of the risk.  In reality, drowning claims 20 times the number of lives that shark attacks do at our beaches.  The results of these deaths can be just as tragic for families who lose a loved one.  Consider that there are millions of individual incidents of swims, surfs, dives and other water based activities each year in Western Australia.  In many, or perhaps even most, of these times, there would have been sharks in the water at the time and yet there is no interaction let alone a bite.

 The risk of death from a shark bite when swimming, surfing or diving, is estimated to be close to that of being struck by lightning.  We readily and without question accept much higher risks to our lives when we get in a car or head out for a night on the town. Anyone who has drunk alcohol and driven, even below the limit, driven above the speed limit, or used a phone whilst driving has taken a much higher risk to their life.

 Also, a Department of Fisheries report shows that the majority of shark fatalities in Western Australia occur offshore and in areas away from metropolitan or regional centres.  There has only been one recorded fatality in less than 5m water depth and within 30m of shore.  Combine this with the aerial surveillance and shark alarms at most beaches, and the risk to swimmers at patrolled Perth beaches during the day is very low.

 Obviously, the risk is slightly greater for surfers and divers outside of these areas.  But it still remains low.  Modern technologies such as electrical deterrent devices are promising to reduce this risk further.

 The immense focus on the low risk of shark death compared to the many other risks we readily accept can be explained partially by the massive focus given to shark attacks by the media.  A 2012 study found in Australia 58 per cent of media articles about sharks related to shark attacks on people and only 11.3 per cent related to the “positive effects of sharks

Assumption 5 – We can surgically reduce local shark populations and have little impact on the entire population

 This may, or may not,  be true for some more resident species that are harmless depending if they migrate to breed, but not for highly migratory white sharks or most other larger sharks. WA’s white sharks live in a population that spans from the Victorian border to Ningaloo, and sometimes animals from this population move much further afield, up the east coast and overseas, and animals from overseas visit our coasts.  Localised population control is not a viable option for white sharks or any other migratory animal.


Asusmption 6 – The cull policy will not have a significant impact on the environment because it is a light touch approach

 There are four good reasons why environmental concerns about the cull policy are valid.  

 Big sharks breed slowly. As apex predators, large shark species are late to mature and breed slowly.  For example, a female white shark needs to be 12-17 years old and over 4.5m long to breed. They will only breed every two to three years. Given the genetic study estimates of the WA and SA population at only 700 breeding age sharks, this means there may only be a few hundred in the population.  Like any population of apex predators, there is a relatively low abundance of large breeding animals and low resilience to mortality.

 The shark cull explicitly targets white sharks, and also large breeding age bull and tiger sharks.  For the reasons above, white sharks are protected by both State and Federal laws.  Despite this, they are still occasionally killed in gillnet, long line and aquaculture operations. The other target species are both whaler sharks.  Due to historical overfishing, all whaler sharks over 1.8m are protected by state Fisheries regulations for the explicit reason that these sharks are critical to the future sustainability of the shark population.

 In addition to the drumlines set 24 hours a day off the coast, the policy also requires the contractor, or Fisheries Department in the metropolitan kill zone, to mobilise for any sighting of a shark over 3m.  The policy explicitly targets the breeding individuals of these three target species that have been proven to be vulnerable to overfishing of large adults. 

 The targeted hunting part of the policy is also of great concern from a scientific perspective, and the perspective of the valuable information this science delivers for both conservation and human safety, because of the risk it poses to tagged sharks.  Any tagged shark that migrates through the kill zones are under even greater risk than other sharks because they are likely to set off a research beacon, and be actively hunted. These sharks may have been tagged by other Governments in Australia or overseas, and part of valuable research programs.

 The final reason that environmental concerns are valid is that the by catch of this policy is likely to include other species of conservation significance.  Grey nurse sharks and marine turtles are both victims of drum lines off the East Coast.  This is also a concern for local tourism and diving operators who obtain value from the presence of these animals.


Asumption 7 – We have to do something, therefore we should cull sharks

 As outlined above, it is a big assumption to jump to the conclusion that WA now has a ‘shark problem’ rather than an unfortunate couple of years.  It is also incorrect to assume that culling sharks will increase ocean user safety.  However, even if both of these were true, killing sharks would not be the only option available. 

 The current aerial surveillance and beach alarm methods in place in WA have been very effective in keeping people and sharks apart in most circumstances, and these are in place and effective in many other parts of the world. 

 These don’t work for remote users, but this is where increased development and availability of non-lethal options such as electronic deterrents and research to better understand the behaviour and migration patterns of sharks are effective at reducing risk.    

 Another option currently being promoted by scientists is to catch the sharks (only during the day time when other ocean users might be in the water), tag them for research, and then tow them to deeper water.  This technique has been extensively studied in Brazil where it has been proven that these sharks will continue on their migration and not return to the beaches where they were caught.

 Also, whilst continuing valuable investment in the options above, we need to educate people about the results of research into shark behaviour, and also that the risk, whilst scary, is still very low compared to many other risks we readily accept in our daily lives.


Further reading?

Also of interest may be the live coverage of the recent shark forum in Margaret River by PerthNow.



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