Last week the Australian Border Force and Australian Fisheries Management Authority spent 24 hours dragging two tonnes of ‘ghost net’ out of our north-eastern waters.
The rogue fishing equipment, littered with debris, was found drifting in the Arafura Sea, about 500 km (240 nautical miles) northeast of Darwin.
Ghost nets are fishing nets from commercial fishing vessels set adrift either deliberately - by cutting them loose - or accidentally, perhaps when snagged in shallow waters. This year alone, six ghost nets have been retrieved from the Torres Strait and Arafura Sea, weighing an estimated 25.6 tons combined, while hundreds more have been removed each year from our northern coastline.
These derelict nets are “ghosts” because they sweep through the ocean unattended, collecting marine life and other debris indiscriminately as they drift along ocean currents.
Oceanographer Erik van Sebille from the Grantham Institute, Imperial College London, studies the pathways of global ocean currents. “Marine debris can travel over thousands of kilometres, easily. Especially ghost nets, because they are so sturdy and buoyant, can travel very far. [Made of synthetic materials], they don't really decompose but just linger around, becoming these giant floating death traps,” he remarks.
Ocean currents transport nutrients and heat across vast distances – but they also collect our garbage. Debris accumulates out of sight, out of mind in slow-moving whirlpools called ocean gyres, and along exposed coastlines. Remote stretches of the Gulf of Carpentaria and Cape York can see ghost nets piling up at a rate of three tons per kilometre.
“The reason we see so much marine debris north of Australia is because it is close to some of the global hotspots of leakage; the regions where most of the ghost nets get released in the ocean,” van Sebille says.
The seas north of Australia are intensive international fishing grounds. Studies by GhostNets Australia in partnership with the CSIRO have shown that most ghost nets are not of Australian origin; by the make of nets, Indonesian and Taiwanese fishing operations contribute to this transborder problem, with nets coming from as far north as South Korea and Japan.
Impact on marine life
The Gulf of Carpentaria is a haven for marine and coastal species given its remoteness and protected waters – but for the same reasons it is a cul-de-sac for marine debris. Shallow waters and a clockwise gyre keep debris trapped in the Gulf.
The Gulf is home to five of the worlds’ seven marine turtle species and it also sustains a rich seagrass habitat for dugong; these animals are cultural keystone species of coastal Indigenous communities in the region.
“It’s also an important feeding habitat for several seabirds,” says Denise Hardesty, Senior Research Scientist at the CSIRO.
Marine debris has a shocking impact on wildlife. Hardesty, who has worked in the Gulf of Carpentaria alongside GhostNets Australia rangers, can attest, “Given their declining populations, ghost nets have a significant impact on turtles in the region.”
Drifting ghost nets attract large predators to entangled fish. Animals can ingest garbage or quickly become ensnared in the synthetic lines themselves, suffocating or starving to death. Sharks, sea snakes, crocodiles, dugong and turtles have been found in nets in the Gulf; often multiple animals are caught in individual nets. Nets also smother seagrass beds and mangrove habitats.
The clean up
Much information on marine debris – a global problem burdened locally – comes from coastal clean ups. Tangaroa Blue, which manages the Australian Marine Debris Database, has been working with Indigenous land and sea rangers in the Gulf of Carpentaria, as well as Cape York, to find, record and remove marine debris.
In the last year, they have reported a dramatic decrease in the number of nets making landfall at Mapoon Beach in the Gulf, which is likely because the Indonesian Minister of Fisheries imposed a ban on illegal fishing vessels in the nearby Arafura Sea.
To this end, Heidi Taylor, Director of Tangaroa Blue, says, “We need to find ways of preventing marine debris from occurring by stopping it at the source.”