By Dr. Samuel Georgian, Marine Biogeographer at Marine Conservation Institute
We’re all too familiar with the horrible images of once pristine beaches that are now covered with trash, threatening a wide array of charismatic animals including sea turtles, seabirds, and marine mammals. What about our ocean’s most remote habitat though – the deep sea? You may think that the deep sea is largely safe from man-made trash and pollution due to its isolated nature. Unfortunately, that is not the case.
The deep sea is subject to a constant flow of anthropogenic pollution ranging from large debris such as fishing gear , legally and illegally discarded ship waste , microplastics , and sewage, dredging, drilling, and mining waste products  . A wide suite of chemical waste has also reached its depths, including pharmaceuticals , toxic metals , pesticides , and PCB’s (harmful chemicals used as coolants and banned in the US since 1978) . Oil and natural gas drilling operations are also chronic polluters in the deep sea, resulting in the release of sediments and drilling fluids, and small oil and gas leaks . Recent incidents – such as the Deepwater Horizon oil spill – have highlighted the damage that large-scale oil spills can have on deep-sea biota . The deep sea has also been discussed as a viable future dumping ground for unwanted carbon dioxide, litter, and even radioactive waste . Out of sight, out of mind – right?
Marine litter is often the most readily observable form of pollution – not surprising given that over 6.4 million tons is dumped into the ocean every year . On our recent cruise to the Cordell Bank region in conjunction with NOAA and Marine Applied Research & Exploration (MARE), we witnessed numerous examples of discarded or abandoned debris including ropes, nets, and fishing lures. The cruise was focused on studying deep-water coral and sponge habitats across three National Marine Sanctuaries off the coast of California, in many cases visiting unexplored sites. The presence of trash in these remote, protected, and never-before-seen habitats was a stark reminder of the wide-ranging impacts of our society on the ocean.
Not-so-fun fact: the deepest recorded instance of litter is from an astonishing depth of 7,216 meters (over 23,000 feet) off the coast of Japan . It seems, therefore, that no habitats on the planet are truly removed from human disturbance. While the occasional rope draped across the seafloor may not seem like a significant threat, this debris can physically damage seafloor ecosystems in the same way that bottom trawl fisheries do, and may continue to cause damage long after being discarded. The habitat structures created by deep-water corals and sponges in these ecosystems are extremely fragile and highly sensitive to disturbance – once damaged, these areas may not recover within our lifetimes.
Of particular concern is lost or abandoned fishing gear, which has given rise to a phenomenon known as ghost fishing. Fishing gear – especially traps, nets, and lures – can continue to catch fish and other organisms long after being discarded, in addition to the physical damage they have on deep-water reefs. In many cases, trapped fish attract predators or scavengers that are then caught as well. Lost fishing gear makes up 10% of all marine litter  and, with the increasing use of synthetic materials, may not degrade for considerable time periods. In some cases, lost fishing gear has continued to catch fish with 50-90% efficiency , and have been found to continue to operate for years after being discarded . Alarmingly, ghost fishing may catch a large percentage of species that were not originally targeted by the fishery, including marine mammals  and endangered species .
The deep sea is already threatened by a large number of global-scale anthropogenic impacts ranging from warming, deoxygenation and ocean acidification to overfishing and habitat destruction. Marine debris and pollutants pose additional risks to these sensitive habitats, further damaging already stressed ecosystems. While we are unlikely to solve these global stressors overnight, reducing the stream of trash that finds its way into our deep oceans may slow the damage being done to these fragile habitats.
 Humborstad OB, Løkkeborg S, Hareide NR, Furevik DM. 2003. Catches of Greenland halibut (Reinhardtius hippoglossoides) in deepwater ghost-fishing gillnets on the Norwegian continental slope. Fisheries Research 64(2-3): 163-70.
 Goldberg ED. 1975. Assessing potential ocean pollutants – a report of a Panel. Study Panel on Assessing Potential Ocean Pollutants. Woods Hole, Mass, Washington, D.C., USA, 1972. National Academy of Sciences – National Research Council. Ocean Affairs Board, Washington, D.C., USA. 438 p.
 Van Cauwenberghe L, Vanreusel A, Mees J, Janssen CR. 2013. Microplastic pollution in deep-sea sediments. Environmental Pollution 182: 495-499.
 Van Dover CL, Grassle JF, Fry B, Garit RH, Starczak VR. 1992. Stable isotope evidence for entry of sewage derived organic material into a deep-sea food web. Nature 360: 153-156.
 White HK, Hsing PY, Cho W, Shank TM, Cordes EE, Quattrini AM, Nelson RK, Camilli R, Demopoulos AW, German CR, Brooks JM. 2012. Impact of the Deepwater Horizon oil spill on a deep-water coral community in the Gulf of Mexico. Proceedings of the National Academy of Sciences 109(50): 20303-20308.
 Simpson DC, O’Connor TP, Park PK. 1981. Deep-ocean dumping of industrial wastes. In: Geyer RA, ed. Marine Environmental Pollution, Vol. 2, Dumping and Mining, Elsevier Scientific, New York, pp 379–400.
 Mormede S, Davies IM. 2001. Heavy metal concentrations in commercial deep-sea fish from the Rockall Trough. Continental Shelf Research 21(8-10): 899-916.
 Storelli MM, Losada S, Marcotrigiano GO, Roosens L, Barone G, Neels H, Covaci A. 2009. Polychlorinated biphenyl and organochlorine pesticide contamination signatures in deep-sea fish from the Mediterranean Sea. Environmental Research 109(7): 851-856.
 Froescheis O, Looser R, Cailliet GM, Jarman WM, Ballschmiter K. 2000. The deep-sea as a final global sink of semivolatile persistent organic pollutants? Part I: PCBs in surface and deep-sea dwelling fish of the North and South Atlantic and the Monterey Bay Canyon (California). Chemosphere 40(6): 651-60.
 Larsson AI, Purser A. 2011. Sedimentation on the cold-water coral Lophelia pertusa: cleaning efficiency from natural sediments and drill cuttings. Marine Pollution Bulletin 62(6): 1159-68.
 Fisher CR, Hsing PY, Kaiser CL, Yoerger DR, Roberts HH, Shedd WW, Cordes EE, Shank TM, Berlet SP, Saunders MG, Larcom EA. 2014. Footprint of Deepwater Horizon blowout impact to deep-water coral communities. Proceedings of the National Academy of Sciences 111(32): 11744-9.
 Ramirez-Llodra E, Tyler PA, Baker MC, Bergstad OA, Clark MR, Escobar E, Levin LA, Menot L, Rowden AA, Smith CR, Van Dover CL. 2011. Man and the last great wilderness: human impact on the deep sea. PLoS One 6(8): e22588.
 UNEP. 2009. Marine litter: a global challenge. Nairobi: UNEP. pp 232. www.unep.org/pdf/UNEP_Marine_Litter-A_Global_challenge.pdf
 Miyake H, Shibata H, Furushima Y. 2011. Deep-sea litter study using deep-sea observation tools. In: Omori K, Guo X, Yoshie N, Fujii N, Handoh IC, Isobe A, Tanabe S, eds. Interdisciplinary Studies on Environmental Chemistry – Marine Environmental Modeling and Analysis Terrapub. pp 261–269.
 Macfadyen G, Huntington T, Cappell R. 2009. Abandoned, lost or otherwise discarded fishing gear. Food and Agriculture Organization of the United Nations (FAO).
 Revill AS, Dunlin G. 2003.The fishing capacity of gillnets lost on wrecks and on open ground in UK coastal waters. Fisheries Research 64(2-3): 107-113.
 Tschernij V, Larsson PO. 2003. Ghost fishing by lost cod gillnets in the Baltic Sea. Fisheries Research 64(2-3): 151-162.
 Stelfox M, Hudgins J, Sweet M. 2016. A review of ghost gear entanglement amongst marine mammals, reptiles and elasmobranchs. Marine Pollution Bulletin 111(1-2): 6-17.
 Bettoli PW, Casto‐Yerty M, Scholten GD, Heist EJ. 2009. Bycatch of the endangered pallid sturgeon (Scaphirhynchus albus) in a commercial fishery for shovelnose sturgeon (Scaphirhynchus platorynchus). Journal of Applied Ichthyology (1): 1-4.
Marine Conservation Institute is a team of experienced marine scientists and policy advocates dedicated to saving ocean life. The organization’s goal is to create an urgently-needed worldwide system of strongly protected areas—the Global Ocean Refuge System—to protect marine biodiversity. Marine Conservation Institute also maintains the most accurate marine protected area database, the Atlas of Marine Protection.