Marine debris (or litter) is commonly defined as human-created persistent solid material that is manufactured or processed and directly or indirectly, deliberately or accidentally, disposed of or abandoned into the marine or coastal environment (UNEP 2016). This ranges from plastic bags, soda cans to ruined/old fishing gear and abandoned vessels. Marine litter is a major global environmental issue (ibid.). In case of cetaceans, 69 of the 89 species officially recognised by IUCN are reported to have been affected by marine debris pollution: 60 cetacean species have been impacted by entanglement, and 48 have ingested marine debris (Pierantonio et al. 2018).
Plastic items are the most abundant type of marine litter (from 60 to 80%) (Gregory and Ryan 1997). Between 5 and 13 million metric tons of plastic enter the oceans annually (Jambeck et al. 2015) and has been exacerbated as a result of COVID-19. You can find plastic in the ocean as bottles, bags, toys, six-pack rings, packaging bands and other waste. Only 14% of plastics are currently effectively collected for recycling, while another 14% is incinerated and/or recovered into energy on a global average (Hallanger and Gabrielsen 2018). 40% end up in the landfill while the resting 32% ends up in the ecosystems (ibid.), including the ocean. The total of plastic in the ocean is estimated to weight more than every blue whale left in the world’s ocean today (International Whaling Commission (IWC) 2020).
Currents and winds transport debris across oceans, and are therefore found everywhere, even in remote areas, far from human presence and obvious sources. According to the Norwegian Polar Institute (NPI) (2021), there is an average of 1914 pieces of litter per km² of ocean. Buhl-Mortensen and Buhl-Mortensen (2017) conducted the first large-scale mapping of seabed litter in arctic and subarctic waters in the Barents and Norwegian Seas covering approximately 3,735,900 m². Video transects allowed recording the present density, distribution and composition of litter. The mean density of items was 202 items/km² in the Barents Sea against 279 items/km² in the Norwegian Sea. It is worth noting that the highest density recorded exceeded 6000 items/km² alongside the coast near a major fishing bank in Sveinsgrunnen, located North of Senja in Northern Norway. The alarming conclusion was that the recorded litter density was higher than or similar to that recorded in the continental shelfs of European waters which is of 200 items/km² (Pham et al. 2014). The highest litter density in European waters, by a maximum value of 6620 items/km², was reported from the Lisbon canyon, Portugal (ibid.).
The Greenland Sea and the Barents Sea act as a cul-de-sac for this plastic conveyor belt. Scientists fear that the Barents Sea will host its own garbage patch – like the much-discussed great pacific garbage patch – by the year 2050 if plastic pollution continues increasing (NPI 2021).
Most plastic are non-biodegradable so that they just break down in ever smaller particles down to the size of micro- and nanoparticles.
Plastic items have become increasingly strong and resistant. They take a long time to degrade and disappear. The table below indicates the time needed for different types of debris to disappear in the marine environment.
Ingestion of plastic debris by marine mammals occurs often, but some species ingest more than others. For instance, due to their prey-capture strategy, filter-feeding megafauna, such as fin whales, are susceptible to high levels of plastic ingestion as they engulf water in the buccal cavity (IWC 2020). For a different reason, deep divers like sperm whales and Cuvier’s beaked whales also ingest large plastic fragments. These species may mistake plastic debris with the squids they mostly feed upon.
Health consequences tied to the ingestion of plastic by marine mammals are various. Marine mammals can die due to gastric impaction, occlusion, or perforation (IWC 2020). Besides lethal effects, plastic in marine mammals’ gastro-intestinal tracts (GIT) may reduce the individual’s fitness and space for food, therefore inducing malnutrition (ibid.). Ingested debris could also cause inflammatory changes to the GIT and/or induce pain and stress to the individual.
Plastic debris can also act as a sponge and accumulate contaminants. Plastic components contain chemicals of concern for the surrounding marine environment (brominated flame retardants (BFR), bisphenol A (BPA), nonylphenols (NPs), and stabilizers phthalate esters (PAEs)), but also absorbs hydrophobic substances, such as polychlorinated (PCBs), polycyclic aromatic hydrocarbons (PAHs) (World Economic Forum (WEF) et al. 2016, IWC 2020). Ingestion of such debris and contamination by plastic-associated toxins are dangerous for marine organisms and could impair immune function of the intestinal microbiota. Recent studies suggest that polystyrene microfibers induced intestinal microbiome dysbiosis, hepatic metabolism disorders and changes in the gut barrier function (IWC 2020). Also, once ingested these substances and chemicals would enter the food chain, so that the negative impact can reach mankind (WEF et al. 2016).
Entanglement in marine debris
Entanglement is another negative consequence tied to marine debris impacting marine mammals. It can be caused by abandoned, lost or discarded fishing gears (ALDFG), such as nets, ropes and monofilament lines, but also by six-pack rings and strapping bands.
Entanglement decreases swimming efficiency, and thereby efficiency in finding food and escaping predators. It also causes injuries such wounds, cuts and suppurative inflammation of the skin occasionally causing septicaemia, if not death by starvation, drowning, suffocation or strangulation (Unger et al. 2017). Research suggests that over 300,000 whales and dolphins die each year due to entanglement in fishing gear and marine debris (IWC Entanglement response). Of large whales, the northern right whale and the humpback whale are the most affected (Butterworth 2016). Entanglement is an obstacle to the recovery of some endangered whale populations, such as the North Atlantic right whale of Canada and the USA.
Pinnipeds, especially monk seals, fur seals and California seas lions, are particularly affected by entanglement (Butterworth 2016), probably due to their very inquisitive nature.
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Butterworth, A. (2016). A Review of the Welfare Impact on Pinnipeds of Plastic Marine Debris. Frontiers in Marine Science, 3, 149. https://doi.org/10.3389/fmars.2016.00149
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Hallanger, I.G. and Gabrielsen, G.W. (2018). Plastic in the European Arctic. Brief Report no. 45, Norwegian Polar Institute. Available at https://brage.npolar.no/npolar-xmlui/bitstream/handle/11250/2478285/Kortrapport45.pdf
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