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Ghostnets fish on: marine rubbish threatens northern Australian turtles

Each year around 640,000 tonnes of fishing gear is lost or thrown overboard by the fisheries around the world. These “ghostnets” drift through the oceans and can continue fishing for many years. They kill…

Rubbish in the ocean - marine debris - is a terrible threat to wildlife. Discarded fishing nets are among the worst. AAP Image/Department of the Environment and Heritage/Melbourne Zoo

Each year around 640,000 tonnes of fishing gear is lost or thrown overboard by the fisheries around the world. These “ghostnets” drift through the oceans and can continue fishing for many years. They kill huge numbers of marine mammals, sea turtles and sea birds, and cause significant loss of biodiversity.

One study showed fur seal populations declined around 5% each year. Ghostfishing of commercially valuable fish species also reduces food resources.

Ghostnets are a global problem: they’re found even on remote atolls thousands of kilometres from commercial ports. But they are a particular problem in Australia’s Gulf of Carpentaria. Here ghostnets wash ashore at densities reaching up to three tonnes/km, among the highest in the world.

We don’t know where more than half the nets come from, but of the nets we can identify, most come from fisheries in neighbouring Asian countries. About 4% come from Australia. Because of the large amount of illegal fishing that has occurred in the region it’s not clear whether they were lost or left behind intentionally.

Ghostfishing in the Gulf is known to kill sharks, crocodiles, and dugongs, as well as other fish and invertebrates. But it is turtles that are most at threat.

Australia is home to six of the world’s seven threatened species of marine turtle. During a recent cleanup of ghostnets on beaches in the Gulf, 80% of animals recorded in nets were marine turtles, including Olive Ridley, Hawksbill, Green and Flatback turtles. Getting tangled in ghostnets is one of the most common causes of death for marine turtles in Australia.

Understanding the impact of marine debris

It’s expensive to get out on planes and ships, so most of the data we have about marine debris comes not from the sea itself. Instead we use beach clean ups of rubbish washed ashore to estimate what might be drifting out there in the ocean.

In our research, we worked with GhostNets Australia and used data collected by Indigenous rangers on the number of ghostnets found during beach cleanups in the Gulf of Carpentaria. We combined that with a model of ocean currents. This let us simulate the likely paths that ghostnets take to get to their landing spots on beaches in the Gulf.

If you’re a marine turtle, your most likely cause of death is getting tangled in a discarded fishing net. AAP Image/Department of Heritage and Government

Beyond finding out where ghostnets occur in the Gulf, we wanted to actually estimate their impact on threatened marine turtles. So we combined our model with data about where turtles exist in the Gulf, using turtle by-catch records from the prawn trawl fishery that operates in the region. We crosschecked our predictions about where turtles would wash ashore tangled in ghostnets with real life data on turtles caught in ghostnets found by the rangers.

This showed us where the hotspots are. Ghostfishing for turtles is concentrated in an area along the eastern margin of the Gulf and in a wide section in the southwest extending up the west coast.

Taking action on ghostnets

Most ghostnets enter the Gulf from the northwest and move clockwise along its shores. This means we can help protect biodiversity in the region by intercepting nets as they enter the Gulf, before they reach the high-density turtle areas along the south and east coastlines.

Nets arriving here could be monitored by aerial or satellite surveys or coastal surveillance programs. Run from a nearby port, this surveillance could focus on a relatively small area north of the Gulf. Intercepting nets along the northeast of the Gulf should reduce much of their impact: they will no longer sweep through the Gulf and meet turtles along the south and east margins.

Encouraging fishers to recover lost or damaged nets could also reduce the prevalence of ghostnets in our seas. One idea is to offer incentives for fishing boats to return fishing gear. Another is to set up waste disposal sites at ports.

Finding a path through the debris

Our work points the way forward for understanding the global threat from marine debris and making predictions that can guide regulation, enforcement, and conservation action.

This approach can easily be expanded to the level of whole oceans for a huge range of different animals, from sea birds to seals. Models that predict global densities of marine debris already exist, thanks to other researchers.

Combining models such as these with species distribution data, even at coarse scales, would show us the global hotspots where marine debris meets commercially valuable or threatened marine species.

This could pinpoint where prevention and clean-ups could really make a difference to biodiversity and help us mitigate the impact of marine debris on the world’s marine wildlife.

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7 Comments sorted by

  1. Alex Serpo

    Garbologist

    It's a self solving problem. Wildlife eats plastics, wildlife dies. More plastics, wildlife becomes extinct.

    When there is no more wildlife, plastics aren't a problem any more. Bacteria and jelly fish don't care about plastics.

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    1. Mike Swinbourne

      logged in via Facebook

      In reply to Alex Serpo

      I think you missed a couple of steps there Alex.

      "....It's a self solving problem. Wildlife eats plastics, wildlife dies. More plastics, wildlife becomes extinct...."

      This should be followed by: "....wildlife becomes extinct, humans become extinct, no more plastic produced...."

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  2. Bernie Masters

    environmental consultant at FIA Technology Pty Ltd, B K Masters and Associates

    Sorry, Britta and Chris, but this article shows how the CSIRO has lost the plot in recent years. Ghost nets have been known to be a problem for over 20 years. The time for research on the damage they do ended 10 or more years ago and Australia should have put in place a program of ghost net removal from northern waters a long time ago. Yet here we are still researching where they are and what they do when such knowledge has been in the possession of researchers for years.
    Is it any wonder that the public become cynical or even hostile towards researchers when problems are unnecessarily researched at the expense of on-ground action to solve the problems?

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    1. Jamie Machin

      Experientialist

      In reply to Bernie Masters

      Bernie, there is an organisation called "Ghostnets Australia", mentioned in the article, which has done exactly that since 2004. They have removed over 10,000 nets from the shorelines of the gulf in that period working in partnership with indigenous ranger groups, Northern Gulf Resource Management group and - yes - CSIRO.

      Chill dude.

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    2. Bernie Masters

      environmental consultant at FIA Technology Pty Ltd, B K Masters and Associates

      In reply to Jamie Machin

      Thanks for the comment, Jamie, but can you help me understand what's going on here. You say that the group Ghostnets Australia has removed over 10,000 nets from beaches and other places around the Gulf but the research goal of this latest CSIRO study is to "simulate the likely paths that ghostnets take to get to their landing spots on beaches in the Gulf." As well, "Beyond finding out where ghostnets occur in the Gulf, we wanted to actually estimate their impact on threatened marine turtles."
      Sorry…

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    3. Britta Denise Hardesty

      Research Scientist, Ecosystem Sciences at CSIRO

      In reply to Bernie Masters

      Bernie, James, thanks for your interest in the work. This work explicitly involved GhostNets Australia and we worked with GNA data collected by rangers working on at multiple sites across the Gulf of Carpentaria for the analyses. We used actual net and beach data along with ranger-collected information on where turtles had washed up on beaches. Just to clarify, the work is not only based on oceanographic models, but models are incorporated to identify where nets washing up on beaches likely came from and where they were moving. Feel free to read the paper published in Conservation Letters for more detailed information (link: http://onlinelibrary.wiley.com/doi/10.1111/conl.12001/full).

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  3. Wade Macdonald

    Technician

    Quote...'One study showed fur seal populations declined around 5% each year.'

    The NZ fur seal population off Australia's southern coastline is increasing at a rapid rate. So much so they are believed to be causing declining rates to the giant cuttlefish, Australian sealion and fury sea penquins in SA. I would suspect that this article is referring to other fur seal populations e.g. Northern Hemisphere?

    The Australian sealion however, I could understand the original statement being plausible.

    Ghost nets are a big problem up North and many commercial/recreational fishers/divers in OZ remove them when and where possible.

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