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Killing sharks is killing coral reefs too

The growing demand for shark fin as an ingredient in Chinese cuisine has caused an explosion in the number of shark fisheries in recent decades. But sharks are important members of ocean food chains, and…

Without Grey Reef Sharks, reefs struggle to recover from disturbances. Peter Verhoog/Dutch Shark Society

The growing demand for shark fin as an ingredient in Chinese cuisine has caused an explosion in the number of shark fisheries in recent decades. But sharks are important members of ocean food chains, and removing them can have unintended consequences. Our new study, conducted off the coast of northwest Australia, shows that killing sharks isn’t just bad for sharks; it can also harm coral reefs.

What do sharks have to do with coral?

Sharks are apex predators - they live at the top of the food chain. They grow slowly, mature late and have relatively low rates of reproduction. This means that their populations have little resilience to harvest and as a result, over-fishing of sharks has now become a worldwide problem.

Nowhere is immune to this phenomenon, as rising prices for shark fin drive fishermen to search every corner of the oceans to harvest sharks. Coral reefs, once renowned for their abundance of sharks, have been targeted by both legal and illegal fishermen so that today even areas as large and as well-managed as the Great Barrier Reef show alarming signs of diminishing shark populations.

While we recognise this loss is occurring, we still have very little idea of what effect the removal of sharks has on coral reef ecosystems.

This is because in most places, fishing is just one of many processes such as coral bleaching, cyclone damage, attacks by crown of thorns starfish, pollution and eutrophication that can occur simultaneously, all of which alter the structure of the reef in fundamental ways. Disentangling the effects of the loss of sharks from these other influences can be a daunting task.

But a unique combination of circumstances now allows us to test the impact of sharks on coral reefs, on offshore atolls in the remote north-west of Western Australia. Our results are published in open journal PLOS ONE today.

Study sites on remote atolls off the NW coast of Australia. Reefs open to Indonesian fishing are shown in the hatched area, reefs closed to all fishing – the Rowley Shoals – are to the south. Mark Meekan

For hundreds of years, fishermen from Indonesia travelled south into what are now Australian waters to fish for sharks. This long-standing tradition was recognised when our Exclusive Economic Zone was established; some reefs were set aside so that these fishermen could continue their harvest using traditional methods.

Although traditional, the methods used by Indonesian fishermen are still highly effective at removing reef sharks. Nearby are pristine reefs that are completely protected from fishing. Our decade-long monitoring programs at both these fished and unfished reefs allowed us to compare what happens to reefs with, and without sharks.

The difference between the two is striking. On reefs without sharks, smaller predators (known as “mesopredators”) such as snappers and emperors were many times more abundant.

This phenomenon is called “mesopredator release” in ecology. It’s common wherever top-level predators are removed from food chains both in the ocean and on land. For fishers, having more fish like snappers and emperors might seem like a good thing, but unfortunately the effects of the loss of sharks did not stop at that level in the food chain.

In contrast to the smaller predatory fish, herbivorous or algae-eating fishes (parrotfishes, rabbitfishes and the like) were less abundant on fished reefs. Herbivorous fish are vitally important to coral reefs because they eat algae that otherwise overwhelms young corals, particularly as they recover from disturbances such as cyclones and bleaching.

Given the predicted future of coral reefs under climate change, with more bleaching and cyclones at greater intensity, anything that potentially weakens the ability of reefs to recover is worrying.

We have limited ability to alter the trajectory of our warming climate. At least some of the challenges facing coral reefs are now locked in. But this is not necessarily the case with the loss of reef sharks.

Tracking studies show that in many cases individual reef sharks are closely attached to certain coral reefs, so even relatively small marine protected areas could be an effective way to protect these top-level predators.

Ultimately, this could mean that coral reefs are better able to recover from the serious disturbances they will face in the future.

Smaller predatory fish increase when sharks disappear. Peter Verhoog/Dutch Shark Society

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22 Comments sorted by

  1. karen griffiths
    karen griffiths is a Friend of The Conversation.

    retired teacher

    Shark fins an 'important' part of Chinese cuisine! Elephant tusks vital to Chinese culture, for some reason. Bear bile also a vital ingredient to the continuation of Chinese culture. Three of the cruelest and most unnecessary practises that cause unbelievable suffering to animals, and threaten their very existence on this planet. How can a country that wants to be seen as a world leader [be allowed to] continue to behave in such a reprehensible manner?

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    1. Paul Prociv

      ex medical academic; botanical engineer

      In reply to karen griffiths

      An extremely pertinent comment. You overlooked tiger penises, and rhino horns. China wants to be viewed as a modern, enlightened member of the global community, yet I bet even their political leaders are into the mediaeval thinking and behaviour underlying these barbaric and totally unjustifiable destructive activities. Years ago, before understanding its devastating consequence, I actually did try shark-fin soup; it tasted hardly different from gelatine soup with noodles. How about some effective public education, especially among the wealthy classes of China?

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  2. Decortes Fleur

    Writer Researcher Producer at creative industry

    How will the 'decommissioning' of marine parks - by new OZ Govt - affect sharks?
    SOme fishermen say that without Australian fishing vessels in our waters poachers 'Shark FIN PIRATES' will enter and 'destroy' stocks and gelignite or pair trawl for maximum results.

    Look forward to what Greg Hunt will do with his $200 million AUD Great - Barrier Reef fund.

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  3. Mike Swinbourne

    logged in via Facebook

    Thank you for this article Mark.

    The concept of ecosystem degradation following the removal of an apex predator is an extremely important one, and has been demonstrated in the terrestrial environment as well. The classic example of the removal of wolves from Yellowstone NP is well described in the literature. I know there is some other work on the effects of dingo removal from grazing land, but in this case the mesopredator release hypothesis (cats and foxes most notably) is still controversial…

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    1. Nick Kermode

      logged in via email @hotmail.com

      In reply to Mike Swinbourne

      Hi Mike, your Yellowstone comment made me immediately think of this talk I watched just the other day

      http://www.ted.com/talks/george_monbiot_for_more_wonder_rewild_the_world.html

      Worked really well in that case but I would caution that like you say, more research is crucial.
      I just returned from diving Wolf and Darwin Islands and it is just incredible to see ecosystems that are relatively untouched by man. There is just thousands of sharks and abundance of everything, it blows your mind! Some fishing is carried out here but also regulated very well as to methods, times and allowances. Such a stark contrast to diving most spots around Aus where if you see a handful of pelagics you've had a lucky dive

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    2. Mike Swinbourne

      logged in via Facebook

      In reply to Nick Kermode

      Hi Nick. Thanks for that link. I will have a look at it when I get a chance, although I must say I have read a lot of what Monbiot has written on the environment, and while I agree with some of what he says, I do have a number of problems with his views.

      And on the subject of diving and sharks etc, I am not sure where you live in Australia, but you may wish to visit my favourite dive site at Fish Rock Cave in NSW. It may not be up to the places you mentioned, but it is spectacular nonetheless and there are lots of sharks to be seen up close.

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    3. Nick Kermode

      logged in via email @hotmail.com

      In reply to Mike Swinbourne

      Hi Mike, yes agree on Monbiot, but no one has all the answers ;) best way I would describe him is thought provoking.
      Yes fish rock is fantastic, although not protected. My favourite site in Aus as well! Last night was at the launch of the 3rd instalment of a 6 part series on Australia's sharks, made independently by friends who have a project called Sarah Shark. Was excellent and all about the grey nurse and comparing protected and not protected known aggregation sites. It is sad to see so many…

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  4. Nicolas Bertin

    Physicist

    To be precise, serving shark fin soup is now becoming illegal in many Chinese provinces, as well as turtle soup etc... I think the Chinese culture is the number one cause of poaching in the world right now, but let's be fair, they're not doing nothing against it. The Chinese government isn't stupid, they realize all that image is bad for business and Chinese people, being more open to the world, start to realize how outdated and dangerous some of their customs are. Let's not forget that in Thailand and Vietnam they have such customs as well and over there absolutely nothing is done.

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  5. Bruce WILDCARD Davey

    4th generation Professional Fisherman at WILDCARD WILDCAUGHT Pty Ltd

    Conceding there has been quote "an explosion" of shark fisheries in recent decades; only internationally.
    This is incorrect in Australia's "over managed + over regulated shark fisheries domestically.
    In fact quite the opposite, we have been literally shut down. Thanks to alarmist style untrue article's like Mark's.
    It is also generally incorrect quote "the growing demand for shark fin as an ingredient in Chinese cuisine".
    In fact the past 2 years WORLD demand, + prices for shark fin has plummeted…

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    1. Mike Swinbourne

      logged in via Facebook

      In reply to Bruce WILDCARD Davey

      Well Bruce, your rant was long in rhetoric, but woefully short on facts. It would help your credibility if you didn't spread misinformation poorly disguised as fact.

      First up - sharks do get cancer.
      http://jnci.oxfordjournals.org/content/97/21/1562.full

      And even if they didn't, so what? Shark cartilage is not useful as a cancer treatment, so to claim that it is a valid reason to fish for sharks is on the par with claiming that we should kill rhinos as treatment for erectile disfunction…

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    2. Joe Smythe

      Fisherman

      In reply to Mike Swinbourne

      As a professional fisherman I certainly think Bruce could polish up on his arguing skills quite a lot and try to present our side of the story in a more calm manner. However, while I think this is a very good article, I do wish the author made clear that in northern Western Australia there is no domestic fishery targeting these large demersal shark species. North of Sharks Bay has been closed to gillnet fishing for sharks since the late 80's as a protection measure for the breeding stock of bronze…

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    3. Nick Kermode

      logged in via email @hotmail.com

      In reply to Joe Smythe

      Hi Joe, I think that the article does make clear the protected status of the area and the fact that the sharks are not targeted commercially. In fact I believe that that is the "unique combination of circumstances" where "some reefs were set aside so that these fishermen could continue their harvest using traditional methods" whilst "nearby are pristine reefs that are completely protected from fishing" that is the whole rationale behind the study.

      I also think that referring to Australia as world…

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    4. Mike Swinbourne

      logged in via Facebook

      In reply to Joe Smythe

      Thanks for your response Joe.

      I would like to say that I concur with most of the sentiments of your post, especially the final paragraph. I can understand that some fishermen may resent MPAs being 'forced' upon them, and you are correct - they are not, nor should they be, the only tool for fisheries management. But then, I don't ever think that was the intention, and other forms of management, such as quotas, will remain extant.

      However, I remain an advocate of MPAs, and it is important that…

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    5. Joe Smythe

      Fisherman

      In reply to Nick Kermode

      Thanks for your reply Nick and sorry for the lateness of my reply ( I thought I had comment notifications switched on). The tenor of your reply doesn't inspire much confidence in me. In particular your comment "to do a little better than awful or non-existent" is the type of loaded sentiment that sets people like Bruce and myself off. While you may be dissapointed that Australian fish stocks are not pristine or fully recoverd the fact remains that compared to the situation globally and considering…

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    6. Nick Kermode

      logged in via email @hotmail.com

      In reply to Joe Smythe

      Thanks Joe, sorry my comment perhaps did go a bit far and exposed some of my bias (that I try to manage) on the issue, but Mr Hunts facts do prove the situation is not as good as the report suggests. I thought that Mr Hunts analysis was more accurate and also exposed some of the clever language the report used that softened and shrouded its conclusions, for whatever reason. In this situation being pessimistic is safer than being optimistic, if he was anything.
      I believe the state of the fisheries…

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    7. Joe Smythe

      Fisherman

      In reply to Nick Kermode

      Cheers Nick, I agree with pretty much all your points there especially the disconnect between environmentalists and the fishing industry. The thing which irritates most in the industry is that it is so obvious that many of the environmental activists have no clue about the history of Australian fisheries management and what has been painstakingly achieved over a long period and indeed I think many don't want to know. Your comment about fishermen with bad attitudes is sadly so true and I battle with those as well, indeed I started a Facebook page to try and stimulate rational discussion in the industry and have been pleasantly surprised how many forward thinking and pragmatic fishermen are out there. Thanks again for your consider'd and intelligent reply

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  6. Comment removed by moderator.

  7. Montagu Douglas Halls

    Secretary - Shark and Coral Conservation Trust

    Thank you Mark Meehan for hlighting a problem we have been addressing for more than 7 years - please see www.sharkandcoralconservation.com . Can you please get in touch via contact@sharkandcoralconservation.com- - - there exists a much greater threat to sharks and reefs. an idea may be gleaned from the following link:: http://www.youtube.com/watch?v=9EaLRcVdTbM
    Best wishes,
    Monty Halls (Senior)
    Sec SCCT
    Charity Commission Reg: 11216

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