Pilot investigation of the origins and pathways of marine debris found
in the northern Australian marine environment: summary report.
Introduction
Fishing nets that have been lost accidentally, or deliberately abandoned at sea, travel the
oceans of the world with the currents and tides, fishing indiscriminately. A large number
of these ‘ghost nets’ wash up along the coastline of the Gulf of Carpentaria. On the
eastern side of the Gulf (Cape York) the nets arrive during the monsoonal season from
November to March, while on the western shores (Arnhem Land) they arrive with the
south-east trade winds, mainly between May and September. In 2005 an aerial survey1
recorded more than 100 beached nets in the region.
Most of the nets are of Asian manufacture, but their origin is unknown. A study in 20032
concluded that many of the nets originated from Indonesian, Korean, Japanese, Chinese
or Taiwanese fisheries, operating in waters north of Australia. The study author
hypothesized that the combined influence of the north-west monsoon and the Indonesian
Throughflow (a series of currents that transport water between the Pacific and Indian
oceans via the Indonesian Archipelago), transported the nets into the Gulf of Carpentaria
(Figure 1).
To test this hypothesis a pilot project was set up through the Centre for Australian
Weather and Climate Research (a CSIRO and Bureau of Meteorology collaboration),
funded by the Department of the Environment, Water, Heritage and the Arts. The project
used a global ocean model, ‘Bluelink ReANalysis’3, to simulate the paths of virtual
floating objects (virtual drifters) through Asian and Australian waters, and examined the
paths of 920 satellite-tracked drifting buoys (satellite drifters) that had been recorded in
the region (19ºS-15ºN, 110ºE-156ºE) since 1990. The geographic scope of the study
was large enough to allow the possibility of finding pathways to the Gulf of Carpentaria
from as far away as Taiwan.
The project found no evidence from virtual or satellite drifters that nets stranding in the
Gulf were likely to have originated in south-east Asian waters farther away than the
Arafura Sea. However, Asian origins still cannot be ruled out as many satellite drifters
are thought to be intercepted by small boats before they exit the Indonesian Archipelago.
The deployment of ‘hidden’ satellite drifters into the Arafura Sea is suggested to help
resolve this question.
Modelling of virtual drifters suggested that nets entering the Gulf could come from the
Coral Sea and South Pacific, via Torres Strait. While a few satellite drifters have passed
1
Roelofs A., Coles R. and Smit N. (2005). A survey of intertidal seagrass from Van Diemen Gulf to
Castlereagh Bay, Northern Territory, and from Gove to Horn Island, Queensland. Report to the National
Oceans Office – Australian Government Department of Environment and Heritage.
2
White D. (2003). Marine debris in Northern Territory waters 2002. WWF report, WWF, Sydney.
http://www.nt.gov.au/nreta/wildlife/marine/pdf/marine_debris2002.pdf
3
http://www.cmar.csiro.au/bluelink/
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through the Strait, many are snagged on the Great Barrier Reef, off the north Queensland
coast. This raises the question of whether ghost nets originating in the South Pacific or
Coral Sea also snag on the reef. An answer to this question is needed to confirm the
modelling results.
Figure 1: Study region map reproduced from White (2003) showing the winds and currents
hypothesized to transport marine debris from south-east Asia to the Gulf of Carpentaria.
Modelling satellite and virtual drifters
To test the hypothesis that the Indonesian Throughflow and north-west monsoon
transport ghost nets into the Gulf of Carpentaria, from south-east Asian waters, the paths
of 920 satellite drifters recorded in the study region were analysed, and the movements of
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virtual drifters were simulated using the Bluelink ReANalysis model, developed by the
CSIRO, Bureau of Meteorology and Royal Australian Navy.
Satellite drifters have been deployed in the world’s oceans by various meteorological and
oceanographic agencies to obtain measurements of sea surface temperature, sea surface
current speeds and atmospheric pressure4. The drifters comprise a 30-40 cm diameter
surface buoy, holding the sensor and communication electronics, and a 12 m-long wire
connected to a 6 m-long sea-anchor that ensures the drifter moves with the water, rather
than the wind. The drag profile of the drifter is similar to that of a derelict fishing net, so
its drift characteristics are likely to be essentially the same. Importantly, satellite drifters
also provide examples of paths taken by real objects in the ocean and confidence in the
model’s ability to simulate these paths.
For the computer simulation of virtual drifters, the study focussed only on the horizontal
movement of the drifters in the water. Marine debris moves in response to the combined
effect of the movement of the water in which it floats, and the force of the wind on the
exposed portion of the debris. Large items, such as ghost nets that hang several metres
down from buoys, will move at a speed that is the average, over a given vertical range, of
the speed of the water. The Bluelink model simulates the movement of items that have an
equal drag down to 10 m. As most ghost nets do not hang that deeply in the water an
additional three percent of the wind speed was added to correct for this (in some
simulations?). The simulations did not include other complex variables affecting the
movement of water and marine debris, such as snagging on rocks or reefs, convergence
(where two water flows meet), and subduction (where one flow of water slides under
another).
Model limitations
The Bluelink model had two major weaknesses in this study.
Firstly, because of the cost of simulations, tides were not included in the two-year virtual
drifter simulations. This has some impact on model accuracy in deep water because of the
role that tides play in mixing water and slowing its movement. However, tides were
included for a six-month run between April and September and their effect was dramatic
through the Torres Strait. When the tides weren’t included in model simulations, water
flowed through the Torres Strait at a speed of 0.9 m/s. When tides were included, this
flow slowed to 0.4 m/s. The next version of the Bluelink model, due in late 2008, will
include a correction to reduce this error, without incurring the cost of explicitly including
tides.
The second major weakness of the model was its 10 m vertical resolution (the horizontal
resolution is 10 km). This vertical resolution means that the speed of the surface water is
represented by the average speed over the top 10 m. This is too coarse to simulate the
‘shear’ that exists in the surface layers of the ocean, which speeds the movement of water
(what is ‘shear’?). Three percent of the wind velocity was added to correct for this, but it
4
http://www.aoml.noaa.gov/phod/dac/gdp_drifter.html
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is still an approximation. An alternative strategy is to run a regional, high-resolution
model nested within the global Bluelink model, but is beyond the scope of this study. The
vertical resolution is expected to improve in the next version of the Bluelink model.
Results from satellite drifters
Of the 920 drifters recorded in the study region, only five entered the Arafura Sea. Three
of these drifters, deployed in the Coral Sea, passed through Torres Strait during the southeast trade wind season and moved west into the Arafura Sea. Two immediately stranded
on the Papua New Guinea Coast. The third eventually stranded on the island of Palau
Jamdena, 72 days after exiting the Strait, crossing the Arafura Sea at an average speed of
0.2 m/s (Figure 2).
Figure 2: Tracks of satellite drifters during the south-east trade wind season (April-November), for
1990-2007. Start and end points of track segments are shown in green and red, respectively.
Two drifters entered the Arafura Sea from the northern Indian Ocean during the monsoon
season, but their tracks ended in the northern Arafura Sea (Figure 3). Five drifters were
tracked in the Gulf of Carpentaria, near where they were deployed. No drifters were
tracked entering the Arafura Sea through the Indonesian Archipelago.
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Figure 3: Tracks of satellite drifters during the monsoon season (December-March) when the
wind blows from the north-west. Start and end points of track segments are shown in green and
red, respectively.
These results do not support the hypothesis that nets originating from south-east Asian
waters travel through the Indonesian Archipelago, via the Indonesian Throughflow, into
the Gulf of Carpentaria. However, the absence of drifters completing a passage through
the Indonesian Archipelago does not prove that ghost nets do not travel this way. Many
drifters are thought to be salvaged by small fishing boats in the Archipelago (satellite data
shows a change in drifters’ speed when they are intercepted by fast-moving fishing
boats). It is also possible that satellite drifters, with their 20 m-long sea anchors, are
snagged in deep water, preventing them traversing the archipelagos and reef systems
bordering the Gulf of Carpentaria.
Results from virtual drifters
The virtual drifter simulations involved ‘seeding’ the model with drifters every second
day for a year at a time. Drifters were released at 12 ‘far field’ locations and 11 ‘near
field’ locations (Figure 4). The drifters were then tracked for a second year, during which
no more were released from the starting points. The model was run several times for all
years between 1992 and 2006.
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Figure 4: Map of the study region showing the ‘far field’ (solid circles) and ‘near field’ (open
circles) points where virtual drifters were released in the model.
Two net depths were modelled; ‘deep submerged’, representing nets occupying the upper
10 m of ocean, and ‘shallow submerged’ representing nets in the upper 1 m of ocean. The
shallow submerged simulations effectively amplified the effect of wind and accounted for
the possibility that many ghost nets have a centre of drag closer to the surface.
The modelling showed that even when virtual drifters were released in the Indonesian
Archipelago, very few entered the Gulf of Carpentaria. Most dispersed into either the
northern Pacific Ocean or the Indian Ocean. Those that did enter the Gulf originated
nearby, in the northern Arafura Sea (Figure 5), eastern Banda Sea, or near Darwin, during
the monsoon season, and were only a small fraction of the total number of drifters
released. No virtual drifters arrived in the Gulf of Carpentaria from release points farther
west or north.
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Figure 5: Tracks of ‘deep-submerged’ virtual drifters released in 2002 in the northeast Arafura
Sea (centre of the green square).
However, in all model runs, a large number of virtual drifters released in the Coral Sea,
and further east in the South Pacific, moved west with the south-east trade winds through
Torres Strait and entered the Gulf of Carpentaria, or passed westward, close by the
Arnhem Land coast (Figure 6). That marine debris can pass through the shallow maze of
reefs and islands that make up the Torres Strait is evidenced by the movement of three
real satellite drifters through the Strait (Figure 2).
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Figure 6: Tracks of ‘shallow-submerged’ virtual drifters released in 2003 in the Coral Sea (centre
of the green square).
These results suggest a new hypothesis for the origin of ghost nets in the Gulf of
Carpentaria – the Coral Sea and South Pacific (South Pacific region) via Torres Strait.
This hypothesis, however, raises a key question. Large numbers of satellite drifters are
known to strand on the Great Barrier Reef, off the northern Queensland coast. The paths
of these drifters reveal that the Pacific Ocean, south of the Equator, is the probable source
of items washing up on the Great Barrier Reef and/or passing through Torres Strait. If
these drifters are stranding on the reef, do many ghost nets strand on it also? Diving and
aerial reconnaissance surveys have found very few nets, but due to the low number of
observers and the difficulty of surveying the region, the possibility of larger net
strandings cannot yet be ruled out.
Further Work
The questions raised in this study need to be answered to confirm or refute the two
hypotheses tested and proposed.
To confirm the hypothesis that ghost nets enter the Gulf of Carpentaria via the South
Pacific region and Torres Strait, a study is recommended to investigate whether nets are
stranding on the Great Barrier Reef.
If no or few nets strand on the Great Barrier Reef, then it can be assumed that even fewer
will pass through Torres Strait, ruling out the South Pacific region as the primary source
of ghost nets in the Gulf. Conversely, if many nets do strand on the Great Barrier Reef,
then this study confirms that the South Pacific region is a major source of ghost nets in
the Gulf of Carpentaria.
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To answer the question of whether satellite drifters and therefore ghost nets can enter the
Gulf of Carpentaria from waters north or west of the Arafura Sea, it is recommended that
low visibility satellite drifters be deployed at intervals in the Arafura Sea (near 10ºS
132ºE; Figure 7), during the beginning of the north-west monsoon season. Any
movement towards the Gulf can then be compared to Bluelink model estimates for the
prevailing conditions. This will provide examples of tracks that marine debris could take
and more confidence that the model-derived estimates of surface water speeds (and
therefore the modelled movement of virtual drifters) are correct.
Figure 7: Tracks of deep-submerged virtual drifters released throughout 1999 at 10ºS 132ºE. The release of
low visibility satellite drifters from this location, during the monsoon season, is recommended to confirm
whether the drifters, and therefore ghost nets, can transit the Arafura Sea and enter the Gulf of Carpentaria.
If they do, the hypothesis that ghost nets originate from Asian waters and enter the Gulf under the combined
influence of the north-west monsoon and the Indonesia Throughflow, would be confirmed.
The drifters should be designed to be as invisible as possible, to counter any interception
by boats. They should also be equipped with a sea-anchor designed to mimic the vertical
distribution of hydrodynamic drag typical of the types of nets of greatest concern. As
these drifters cost about $5000 each it is not possible to release enough to derive robust
statistics on the pathways of nets. Releasing 10 or 20 would, however, reveal any gross
errors in the present model that may exist, and help validate improvements to future
models.
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If this work confirms that the virtual drifter simulation is essentially correct during the
monsoon season, the next step should be to investigate the possibility that items are
drawn into the Arafura Sea (from where?) during the ‘dry season’ (May-September) in an
upwind return flow that does not exist in the present model because too much water is
flowing through Torres Strait from the Coral Sea (due to the absence of tides).
Once the next version of the Bluelink model is complete, the virtual drifter simulation
should be re-run to see if a reduction in flow through the Torres Strait, as a result of
incorporating tidal corrections, alters the results.
Other observational techniques that should also be considered for application across
Australia’s northern seas are those that are soon to be used in other regions of Australia,
for the first time, as part of the Integrated Marine Observing System (IMOS)5. The
observational method of most relevance to the drift of marine debris is HF radar6. A HF
radar in Torres Strait would provide real-time maps of surface current speed, of value to
shipping, and important data for scientific use. HF radars are normally deployed in pairs,
but in Torres Strait a single unit in the centre of the Strait would be enough to measure
the dominant east-west flow. These radars cost about $500 000.
The present study focussed on the question of how marine debris moves around, but not
on the processes which result in it stopping or stranding. Exploring these processes is also
recommended if a complete picture of marine debris accumulation is sought.
Further information
Full details of this study are available in the full technical report: D.Griffin (2008). Pilot
investigation of the origins and pathways of marine debris found in the northern
Australian marine environment. Department of the Environment, Water, Heritage and the
Arts.
Satellite drifter track segments for the individual calendar months
http://www.marine.csiro.au/~griffin/debris/drifters/monthly/index.html
Animation of ‘deep submerged’ virtual drifters released in the ‘far field’
http://www.marine.csiro.au/~griffin/debris/e007/2004e007.AVI
Animation of ‘deep submerged’ virtual drifters released in the ‘near field’
http://www.marine.csiro.au/~griffin/debris/e006/2004e006.AVI
Animation of ‘shallow submerged’ virtual drifters released in the ‘near field’
http://www.marine.csiro.au/~griffin/debris/e013/2004e013.AVI
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6
http://www.imos.org.au/
http://imos.org.au/acorn.html
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I don’t plan to include this in final summary:
Testing the Bluelink model
Before it was commissioned for ocean forecasting in Australia, the Bluelink model was
run for many years in ‘hindcast’ (rather than forecast) or ‘reanalysis’ mode, both as a test
of the system and to provide a detailed dataset of value to marine science and
engineering. Ocean current and wind speed observations, among others, made between
1992 and 2006, were used as input into the model.
One reanalysis in the study region showed that the seasonal cycle of surface water flow is
closely tied to the seasonal cycle of winds. In February, in the middle of the wet season,
when the wind is from the north-west, the surface flow is eastward in the Arafura Sea and
a clockwise, western-intensified rotation exists in the Gulf of Carpentaria. By May, the
influence of the south-east trade wind that blows during the dry season is established and
the surface flow is to the west or west-south-west throughout the Arafura Sea and Gulf of
Carpentaria. The westward surface flow reaches a peak in August and is still strong in
November when there is also some anticlockwise surface flow in the Gulf of Carpentaria.
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