Within the marine environment, marine debris is sourced from both the land (rubbish flushed out to sea; see further detail on coastal pollution in the Coasts report) and marine industries (loss of equipment, often from fishing operations). Although marine debris can be found in all areas of the marine environment in the Australian EEZ (see Box MAR4), northern Australia is especially vulnerable because of the proximity of intensive fishing operations (including international operations) to the north of Australia, regional difficulties in surveillance and enforcement, and ocean circulation and wind patterns that appear to promote accumulation of floating debris (Kiessling 2003).
Entanglement of marine animals in debris can cause restricted mobility, drowning, starvation, smothering, wounding and amputation of limbs—all of which can result in death. One of the major threats to marine wildlife through entanglement is ghost nets. These are fishing nets that drift through the ocean unattended for years or even decades, ‘ghost fishing’ and entangling and killing commercially valuable or threatened species (Laist 1987, 1997, Macfayden et al. 2009). To date, more than 13,000 nets have been removed from beaches and estuaries by organisations such as GhostNets Australia. Surveys of ghost nets in the northern Australian region found that, of the approximately 50 per cent that could be identified, only 4 per cent originated from Australian fisheries (Gunn et al. 2010).
Floating plastics are also a major threat because they are resistant to breakdown, and thus persist and accumulate in the marine environment. Many are ingested by marine animals, and remain in the stomach and accumulate, eventually causing starvation. Plastics are also a potential source of toxic chemicals (Lavers et al. 2014). These chemicals, once leached out of the ingested material and transferred into the blood and tissues of individuals, may cause sublethal health effects in wildlife, even at very low contamination levels (Tanaka et al. 2013). Marine life as small as plankton is affected by debris in the form of ‘microplastics’ (less than 5 millimetres in diameter), which are a widespread and ubiquitous contaminant of marine ecosystems across the globe. This form of marine debris has been reported as causing decreased feeding because of ingestion, and reduced mobility because of adherence to the external carapace and appendages of exposed zooplankton (Cole et al. 2013). Microplastics have also been linked to the degradation of molecular, cellular, physiological and, ultimately, ecological processes within the marine environment (Browne et al. 2015).
Data on the distribution of debris at sea are scarce, largely because of the expense of collecting these data, which requires use of aircraft or vessels. Some progress has been made in predicting the distribution of marine debris at sea (see Wilcox et al. 2013, 2015; Eriksen et al. 2014), which was surveyed for the first time in Australian waters in 2013 (Hardesty et al. 2014).
Despite initiatives banning the use of some plastic products, the use of plastics continues to grow globally. As a result, the pressures being placed on the marine environment by dumped, discarded and lost debris continue to increase (Jambeck et al. 2015). This was reflected in a review of the threat abatement plan for the effects of marine debris on vertebrate marine life (DEWHA 2009a), which concluded that the objectives of the plan had not been met and the plan needed to be revised (DoE 2015b). Impacts on the marine environment associated with marine debris are high, with a deteriorating trend.