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 derelict fishing nets, or ‘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 examined the paths of 920 satellite-tracked drifting buoys (satellite drifters) that had been recorded in the region (19ºS-15ºN, 110ºE-156ºE) since 1990, and used a global ocean model, ‘Bluelink ReANalysis’3, to simulate the paths of virtual floating objects (virtual drifters) through Asian and Australian waters. The geographic scope of the study was large enough to allow the possibility of finding pathways to the Gulf of Carpentaria from nearly as far away as Taiwan. The project found no evidence from satellite drifters that nets stranding in the Gulf were likely to have originated in south-east Asian waters. 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 also found no evidence that regions in south-east Asia, (other than those nearby in the Arafura Sea) are a source of ghost nets in the Gulf. Instead, the modelling suggested that nets entering the Gulf could come from the Coral Sea and South 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/ 1 Pacific, via Torres Strait. While a few satellite drifters have passed 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 2 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 some ghost nets do not hang this deeply in the water an additional three percent of the wind speed was added to simulate the drift of shallow submerged nets, as well as deeply submerged ones. The simulations did not include other complex variables affecting where marine debris strands, such as snagging on rocks or reefs. Model limitations The Bluelink model had two major weaknesses potentially limiting its direct applicability to this study. Firstly, because of the cost, tides were not included in the simulations. This has some impact on model accuracy even for long periods of drift, because of the complex role that tides play in mixing water, horizontally and vertically. 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 (averaged over a tidal cycle) 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 winddriven shear that exists in the surface layer of the ocean. While three percent of the wind velocity was added to account for this near-surface effect of the wind, it is still a crude 4 http://www.aoml.noaa.gov/phod/dac/gdp_drifter.html 3 approximation. An alternative strategy of running a regional, high-resolution model nested within the global Bluelink model was 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. Note the movement of three drifters from the Coral Sea through the Torres Strait. 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. 4 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. Two drifters entered the Arafura Sea from the northern Indian Ocean. 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 18 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. 5 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. 6 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). 7 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 suggests that the South Pacific region is a major contributor to ghost nets in the Gulf of Carpentaria. 8 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. 9 If this work confirms that the existing 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 the north-west during the ‘dry season’ (MaySeptember) 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). This would entail the release of drifters in May in the northern Arafura Sea, which is where this presently cryptic flow is most likely to occur. 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 5 6 http://www.imos.org.au/ http://imos.org.au/acorn.html 10