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Explainer: sharks — why size and species matter

Dozens of sharks have reportedly been caught since Western Australia’s “catch-and-kill” drum line program began two weeks ago. Firm numbers are not available given the WA government’s unwillingness to…

When it comes to sharks, it’s important we know size and species. Scubaben/Flickr

Dozens of sharks have reportedly been caught since Western Australia’s “catch-and-kill” drum line program began two weeks ago. Firm numbers are not available given the WA government’s unwillingness to release figures and scientists are relying instead on observations from the public and tweets from Surf Life Saving WA. Reports from media, conservation groups and surf life saving suggest up to ten sharks have been killed, either by shooting or drowning.

All captured animals so far are reportedly tiger sharks, with white and bull sharks also targeted by the program. While the program was designed to reduce the risk of fatal shark attacks, it also poses a threat to shark populations.

Endangered sharks, across the world

Sharks are in trouble across the world. A recent global analysis on the status of all sharks and rays found that their overall extinction risk is high, with only one-third of species considered safe. This is in large part due to over-fishing.

This should worry us because sharks are the ocean’s equivalent of lions and tigers. Known as “apex predators”, these animals play a key ecological role in maintaining healthy oceans.

Size really does matter

To reduce the risk of fatal shark attacks, the Western Australian government will kill all sharks over three metres caught on baited drum lines.

But three metres is also an important size for sharks. Animals greater than three metres are likely reproductively mature animals. If these animals are killed, it could damage the reproductive capability of the population. In fact, the program is a direct contrast to 2010/2011 WA fisheries policy that protects breeding stock.

Target species

Sharks are a diverse group of wildlife and understanding their biology and ecology is important to their conservation.

Alexis Bachofen

White sharks (also known as great whites or white pointers) have been implicated in a number of attacks in Australia’s southwest. Found in marine environments ranging from the tropics to cool temperate zones, they can be semi-permanent residents in coastal waters. Tagging data show that they also undergo very long migrations across the open ocean.

White sharks are listed as Vulnerable under the IUCN Red List and protected in Australia.

The reason is their reproductive behaviour. White sharks are slow-growing and can live for over 70 years, reaching sexual maturity at 17 years. When they reach maturity they are around 4.8 metres in length.

Consequently, white sharks reproduce very slowly, with females giving birth to between two and 10 very large pups — between 1.1 and 1.7 metres — every 2-3 years.

This means that white shark numbers also increase only slowly, and are very vulnerable to over-exploitation. Where data exist, most estimates suggest white sharks are decreasing. And while accurate estimates are difficult, genetic work suggest that there may be as few as 700 breeding individuals in southwestern Australia.

Operation Blue Pride

Despite never being implicated in a fatality off the coasts of Perth or the southwest, tiger sharks are also targeted by the WA drum line program, and appear to be comprising the bulk of the catch.

Like many other large sharks, tiger sharks undertake long coastal and oceanic migrations.

Growing to a maximum of about 4.5 metres, they can live to 50 years and are reproductively mature when 2.5-3.5 metres in length at 4-11 years of age. Tiger sharks produce relatively large litters, typically between 30 and 35 pups every 2 years.

Although these relatively rapid rates of reproduction mean that tiger sharks are more resilient to exploitation than white sharks, they are listed as “Near Threatened” on the IUCN Redlist. This is largely due to commercial fishing and by-catch driven by finning for shark-fin soup.

Albert Kok

Bull sharks are the smallest of the three sharks, growing to 3.6 metres long. They are found globally in tropical and warm temperate waters but sometimes move into cool temperate environments on a seasonal basis.

Bull sharks can live for 30 years, maturing at 15 years and 1.9 metres long. They have litters of about six pups, probably once every two years. Genetics suggest that female bull sharks return to same estuary to breed.

Adults are typically found on the shelf in shallow coastal and estuarine waters less than 30 metres in depth. Unusually for a shark, they also travel into fresh water and can be found long distances up rivers and in freshwater lakes and canals connected to the sea.

Because estuaries often border sites of urban development and are a focus for industry, the species is threatened by habitat loss as well as being targeted by both recreational and commercial fishers. They are listed as “Near Threatened” on the IUCN Redlist.

By-catch and tourism

It’s not only whites, tigers, and bulls threatened by WA’s shark program. Drum lines may also catch other threatened shark species in the region, including “endangered” hammerhead sharks, “vulnerable” sandbar sharks, “vulnerable” dusky whalers and “near threatened” bronze whalers.

The danger to these animals and undersize sharks lies in setting drum lines for extended periods of time. If the lines are not checked regularly, non-target wildlife may be dead on the line or seriously compromised, rather than being released as intended.

It’s also worth noting that these animals form the basis of lucrative tourism industries. The global shark ecotourism sector is worth over US$314 million annually. White shark-diving in South Australia generates US$6.5 million annually, tiger shark-diving industry generates annual revenues of approximately US$1.8 million in South Africa, and in Fiji, a well-established shark-diving industry generates approximately US$42 million annually, largely based on bull sharks.

We need to implement non-lethal approaches that improve ocean safety such as tagging and removing sharks further out to sea or better beach behaviour.

Otherwise we’re just placing additional pressure on already threatened wildlife.

Join the conversation

38 Comments sorted by

  1. Wade Macdonald

    Technician

    Quote "While the program was designed to reduce the risk of fatal shark attacks, it also poses a threat to shark populations."

    Localised drum lines off Perth will not threaten these 3 species in any holistic sense. Have you assessed these sharks geographical ranges?

    Quote "A recent global analysis on the status of all sharks and rays found that their overall extinction risk is high, with only one-third of species considered safe."

    Well given this other article stated the following how can…

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    1. Nick Kermode

      logged in via email @hotmail.com

      In reply to Wade Macdonald

      Hi Wade,
      I really don't think it is that long a bow to draw for scientists say "their overall extinction risk is high". For example, in the time it takes for one female white shark to produce her first small litter of pups we will have killed a couple of BILLION sharks. The other species are not so extreme an example but what we do know is they take a long time to mature and produce very few young biennially that have a high natural mortality rate as they have to fend for themselves immediately from birth in an ecosystem with nowhere to hide. I think the risk is high that we are removing more than are being born in at least the slowest reproducing shark species and agree with the above quoted assessment.

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    2. Nick Kermode

      logged in via email @hotmail.com

      In reply to Wade Macdonald

      Thanks Wade, although I did write "other species" were vulnerable to natural mortality and had "nowhere to hide", not whites. It is thought that White Sharks have a low natural mortality rate, probably because they are born around 1.5m.
      Yes I have been aware for years of the Port Stephens sharks. I have been to film there. Trouble is the scientists who have been actually the doing study there (and elsewhere in Aus) say "there appears to be an overall, long term decline in the abundance of white…

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    3. Jessica Meeuwig

      Professor & Director, Centre for Marine Futures at University of Western Australia

      In reply to Wade Macdonald

      Wade raises a number of issues.

      He first suggests that global distributions insulate species from threats. This view ignores the large body of published research that shows spatial structure in the populations of many marine species. With respect to white sharks, this is reflected in Blower’s estimates of the number of breeding white sharks in southwestern Australia as potentially being as low as 700 individuals. Note that referencing juvenile aggregations is not relevant to the number of breeding…

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    4. James Hammond

      Ecologist

      In reply to Wade Macdonald

      Wade, your denigration of the IUCN and the robust and transparent criteria used to assess threat status does nothing but make YOUR post look like 'emotive dribble'.

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    5. Wade Macdonald

      Technician

      In reply to James Hammond

      James, vulnerable is 6 catagories down the list but still appeals to the bleeding hearts in the mmeeedja!

      Only a bias fool would condone those terms as sensible.

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    6. Nick Kermode

      logged in via email @hotmail.com

      In reply to Jessica Meeuwig

      Hi Jessica, thanks for a great article, spot on, and taking the time to reply to comments.
      I totally agree with you that we know enough about the biology of these species to say that the current human pressures puts them at risk and anything that adds to that could be the straw that breaks the camels back.
      I spend a big chunk my life in the ocean looking for sharks but would not use this to form my judgement. Having done some volunteering with shark researchers I recognise their knowledge is…

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    7. Wade Macdonald

      Technician

      In reply to Jessica Meeuwig

      To be clear also, I don't support the drum lines either and think it is a stupid way of diverting responsibility away from those who enter the water at their own risk. However, it will not threaten the existence of these species in any holistic sense and this is where we differ Jessica.

      I also dispute the terms used to catagorise species in the IUCN red list as it assumes that everything is either in trouble or not studied...why? I could create a catagory termed 'near plague' which is just as…

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    8. Jessica Meeuwig

      Professor & Director, Centre for Marine Futures at University of Western Australia

      In reply to Wade Macdonald

      For those that are interested in how IUCN redlisting actually works, the criteria are (1) Extinct; (2) Extinct in the Wild; (3) Critically Endangered; (4) Endangered; (5) Vulnerable; (6) Near Threatened; and (7) Least Concern. “Vulnerable” is the third worst outcome after going Extinct.

      There are specific criteria to determine status (see http://jr.iucnredlist.org/documents/redlist_cats_crit_en.pdf).

      The white shark’s classification as Vulnerable was based on criteria A2cd and A3cd which I…

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    9. Wade Macdonald

      Technician

      In reply to Jessica Meeuwig

      Jessica, outside of the localised drum lines off Perth targeting 3 plus meter sharks, the GW shark is protected in Australia and so are some of the species like NZ fur seals they prey upon. How many fishermen are killing Great Whites in our EEZ?

      While I respect that recent observations 'suspect' a decline globally, I dispute the claim that fishing here in OZ is a threat to the great white shark in any measurable way against sustainability of said species.

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    10. Wade Macdonald

      Technician

      In reply to Nick Kermode

      Nick, like yourself I may be an interested amateur but I have met enough scientists to understand rec fishers observations are often more accurate and critical to balanced conservation outcomes.

      I am fully appreciative of the work scientists do but that appreciation stops when I cannot question statements that contradict other scientific efforts or statements and terms that invoke unobstanciated fear and alarm to the public.

      Lets face it, the conservation movement rarely put anything out in the media these days that is completely factual or not based around bans on specific activities.

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    11. Hal Harvey

      Interested observer

      In reply to Jessica Meeuwig

      I'm sure we're still keen to see an answer to Wade's last question, re beach fishing.

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  2. Michael Shand

    Software Tester

    kind of interesting, it's great that you are highlighting some of the problems with the current action but can't we just leave the sharks alone, get out of their habitat

    Are we going to start tagging crocs as well? it's a stupid idea, you are never going to win this battle, just waste huge sums of tax payer money

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  3. Russell Hamilton

    Librarian

    Could the author please comment on the oft-repeated claim that because the population of whales migrating along the south-west coast has increased so much, the number of sharks following them has accordingly increased, and that is why we are seeing so many attacks?

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    1. Wade Macdonald

      Technician

      In reply to Russell Hamilton

      It ain't just whales, it the exploding nz fur seal population as well as increased southern bluefin tuna stocks.

      The tuna pros caught their quota easy and are back on land with plenty left over for the future.

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    2. Jessica Meeuwig

      Professor & Director, Centre for Marine Futures at University of Western Australia

      In reply to Russell Hamilton

      Hi Russell – That’s an important question and I think the answer is that we don’t know. There was a recent article by Professor Sprivulis from UWA (http://www.washarkattacks.net/sprivulis-shark-paper.pdf) that suggests shark incidents covary with humpback whale presence (not numbers) in the winter, which ironically is not when we are deploying these lethal drum lines.

      However, there could simply be other drivers that have sharks and whales doing the same thing at the same time of year (temperature…

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  4. Chris Gillham

    Journalist

    Russell ... it's not an "oft-repeated" claim that the shark attack increase is due to rapidly recovering whale numbers.

    I'm the owner of the website on which the Peter Sprivulis study is hosted, and for about 15 years I've been trying to fracture the media's wall of silence to let the public know what is causing the surge in attacks by great whites in WA.

    The Sprivulis paper was given some publicity by The West Australian newspaper a couple of days ago but, as usual, they utterly missed the…

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    1. Wade Macdonald

      Technician

      In reply to Chris Gillham

      Well said chris,

      Every fishermen knows that when the tiger sharks are about the whites are usually somewhere else. Notably chasing the seal poulations and snapper aggregations along the southern coastline of OZ.

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  5. Hal Harvey

    Interested observer

    Jessica, "Despite never being implicated in a fatality off the coasts of Perth or the southwest" - referring to tiger sharks - is as fundamentally wrong as it gets. The first death at Cottesloe Beach was in shallow water and was a tiger shark, as evidenced by the fact that the fish was subsequently caught and the human body parts retrieved.

    Many other deaths, e.g. George Wainwright, Brian Guest are 'thought to be great whites', which of course soon becomes a guess recited as fact, hence a fact... in reality they and many other 'great white victims' are at least as likely, possibly more likely to have died as a result of a tiger shark attack.

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    1. Nick Kermode

      logged in via email @hotmail.com

      In reply to Hal Harvey

      Bad examples there Hal. Both those incidences were confirmed as white sharks. Saying "many other deaths" is an exaggeration. There have been so few fatalities in the 211 years of records and so few of those are unconfirmed.
      Also any guesses that experts make are not 50/50 chances. When they say "thought" to be a white shark they consider a range of factors first of which is water temp. All of the serious Perth and SW attacks happen in 18-19 degree water and that means it's more like a 90% chance it's a white. Likewise but reversed if it is water over about 22 deg.

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    2. Hal Harvey

      Interested observer

      In reply to Nick Kermode

      No, both good examples Nick. Neither were or could ever have been confirmed to be great whites, but like most people, you heard it repeated often enough that you thought it must be true. Same with many more. The 211 years is another furphy - the statistics that influence what's happening now come from the last few years compared to the few years before them.

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    3. Nick Kermode

      logged in via email @hotmail.com

      In reply to Hal Harvey

      A great white surfaced right next to George Wainwrights body, witnesses on the boat said it was a great white and analysis of his wetsuit "pointed to a great white shark". With Brian Guest horrifically people saw the shark with him in its mouth. About as confirmed as you can get.

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    4. Hal Harvey

      Interested observer

      In reply to Nick Kermode

      I can only advise against using the Daily Mail for your research. Brian Guest, who was a close friend of mine incidentally, was taken by a shark. That's all we know. The incident wasn't witnessed, despite your dreamy literacy. George Wainwright, I can agree, was most likely taken by a GW; though as Lloyd Rayney will be the first to tell you, leaving a card with your name on it in the vicinity of a victim does not constitute a conviction.

      If you want to persist, tell us about Nick Edwards, Kyle…

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    5. Nick Kermode

      logged in via email @hotmail.com

      In reply to Hal Harvey

      Thanks Hal, I didn't use the Daily mail but a statement to police I read a while back from a Luke someone I think, but with your access to better knowledge I'll defer to you on that one as unconfirmed. Lets call that one all then ;)
      However my point about water temperature still makes it far more likely it was indeed a white shark. There is lots of study that shows that tiger sharks move from the tropics to the temperate waters during the warmer months and return during winter. Water temperatures…

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    6. Hal Harvey

      Interested observer

      In reply to Nick Kermode

      Your defence of Jessica's unequivocal wrong statement is hard going. Samuel Ettelton died when he was bitten by a tiger shark at Cottesloe in November - a month in which drum lining would be taking place, if it continues - which is a Perth beach, and was fatal. And it was mid afternoon, for good measure. Hugh Edwards has the tiger shark's head at his house in nearby Swanbourne to this day.

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    7. Nick Kermode

      logged in via email @hotmail.com

      In reply to Wade Macdonald

      I didn't mention SA at all Wade so I am not sure what you are talking about.

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    8. Nick Kermode

      logged in via email @hotmail.com

      In reply to Hal Harvey

      Thanks Hal, I was not defending it I was speculating on a possible reason for it seeing as there have never been any attacks in the months they are trialling the drum line. Should the trial continue, well that's a different argument which I wasn't making either.

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  6. Mary Elgar

    Worker

    Hi Jessica
    My simple question is this: why didn't the Barnett Govt. return to the tried and tested aerial surveillance we've had in previous years instead of implementing, at great cost to the tax payer and mounting consumer angst. There is absolutely no need to have such a knee-jerk reaction to shark attacks as we've seen last month, and it does nothing except waste resources that could be put to less invasive protection measures. No-one is forced to surf, we do have safer beaches (Mettams Pool for example) and it would even be possible to build a sea pool as is found in Sydney.

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    1. Hal Harvey

      Interested observer

      In reply to Mary Elgar

      Mary, as I understand there was no question of 'return to the tried and tested'; that aerial surveillance continued on unchanged, and will continue on in to the future as normal. Tried and tested may be another conversation though, as it too is very expensive and hasn't been what you could call a complete success.

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    2. Mary Elgar

      Worker

      In reply to Hal Harvey

      Yes I agree, nothing is 100% foolproof except staying out of the water!

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    3. Wade Macdonald

      Technician

      In reply to Mary Elgar

      Mary, estimates from New South Wales suggest arial surveillance only pick up 12% of sharks.

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