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TOPICAL REVIEW • OPEN ACCESS
The quest for seafloor macrolitter: a critical review of background
knowledge, current methods and future prospects
To cite this article: Miquel Canals et al 2021 Environ. Res. Lett. 16 023001
View the article online for updates and enhancements.
This content was downloaded from IP address 46.4.80.155 on 08/02/2021 at 17:58
Environ. Res. Lett. 16 (2021) 023001 https://doi.org/10.1088/1748-9326/abc6d4
TOPICAL REVIEW
The quest for seafloor macrolitter: a critical review of background
OPEN ACCESS
knowledge, current methods and future prospects
RECEIVED
17 August 2020 Miquel Canals1, Christopher K Pham2, Melanie Bergmann3, Lars Gutow3, Georg Hanke4, Erik van Sebille5,
REVISED Michela Angiolillo6,7, Lene Buhl-Mortensen8, Alessando Cau9, Christos Ioakeimidis10, Ulrike Kammann11,
16 October 2020
Lonny Lundsten12, George Papatheodorou13, Autun Purser3, Anna Sanchez-Vidal1, Marcus Schulz14,
ACCEPTED FOR PUBLICATION
2 November 2020 Matteo Vinci15, Sanae Chiba16, François Galgani17, Daniel Langenkämper18, Tiia Möller19,
PUBLISHED
Tim W Nattkemper18, Marta Ruiz20, Sanna Suikkanen21, Lucy Woodall22, Elias Fakiris13,
19 January 2021 Maria Eugenia Molina Jack15 and Alessandra Giorgetti15
1
GRC Geociències Marines, Departament de Dinàmica de la Terra i de l’Oceà, Universitat de Barcelona, Martí i Franquès s/n, 08028
Original content from Barcelona, Spain
this work may be used 2
Okeanos/MAR—Instituto do Mar, Departamento de Oceanografia e Pescas, Universidade dos Açores, Rua Prof. Doutor Frederico
under the terms of the
Creative Commons Machado, 9901-862 Horta, Faial, Ilhas dos Açores, Portugal
3
Attribution 4.0 licence. Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar-und Meeresforschung, Am Handelshafen 12, D-27570 Bremerhaven,
Any further distribution Germany
4
of this work must European Commission, Joint Research Centre (JRC), Via Enrico Fermi 2749, I-21027 Ispra, Italy
maintain attribution to 5
Utrecht University, Institute for Marine and Atmospheric Research, Princetonplein 5, 3584 CC Utrecht, The Netherlands
the author(s) and the title 6
Istituto Superiore per la Protezione e Ricerca Ambientale (ISPRA), Area per la tutela della biodiversità, degli habitat e speciemarine
of the work, journal
citation and DOI. protette (BIO-HBT), Via Vitaliano Brancati 60, 00144 Roma, Italy
7
Stazione Zoologica Anton Dohrn (SZN), Via Po 25, 00198 Rome, Italy
8
Institute of Marine Research, P.O. Box 1870 Nordnes, N-5817 Bergen, Norway
9
Università di Cagliari, Dipartimento di Scienze della Vita e dell’ambiente, Via Tommaso Fiorelli 1, 09126 Cagliari, Italy
10
UN Environment/Mediterranean Action Plan Coordinating Unit, Barcelona Convention Secretariat, Vas. Konstantinou 48, Athens
11635, Greece
11
Thünen Institute of Fisheries Ecology, Herwigstr. 31, 27572 Bremerhaven, Germany
12
Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, United States of America
13
Department of Geology, University of Patras, 26504 Patras, Greece
14
Universität Osnabrück, Fachbereich Mathematik/Informatik, Albrechtstraße 28a, 49076 Osnabrück, Germany
15
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Borgo Grotta Gigante 42/c, 34010 Sgonico, Trieste, Italy
16
Research Institute for Global Change (RIGC), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushi-
macho, Yokosuka 237-0061, Japan
17
IFREMER, Laboratoire LER/PAC, immeuble Agostini, ZI Furiani, 20600 Bastia, la Corse, France
18
Biodata Mining Group, Faculty of Technology, Bielefeld University, PO Box 100131, D-33501 Bielefeld, Germany
19
University of Tartu, Estonian Marine Institute, Mäealuse 14, Tallinn 12618, Estonia
20
Baltic Marine Environment Protection Commission, Katajanokanlaituri 6 B, FI-00160 Helsinki, Finland
21
Finnish Environment Institute (SYKE), Marine Research Centre, Latokartanonkaari 11, 00790 Helsinki, Finland
22
Department of Zoology, University of Oxford, Oxford OX1 3SZ, United Kingdom
E-mail: miquelcanals@ub.edu and georg.hanke@ec.europa.eu
Keywords: seafloor, marine litter, trawl surveys, visual surveys, deep sea, modelling, data harmonisation
Abstract
The seafloor covers some 70% of the Earth’s surface and has been recognised as a major sink for
marine litter. Still, litter on the seafloor is the least investigated fraction of marine litter, which is
not surprising as most of it lies in the deep sea, i.e. the least explored ecosystem. Although marine
litter is considered a major threat for the oceans, monitoring frameworks are still being set up. This
paper reviews current knowledge and methods, identifies existing needs, and points to future
developments that are required to address the estimation of seafloor macrolitter. It provides
background knowledge and conveys the views and thoughts of scientific experts on seafloor marine
litter offering a review of monitoring and ocean modelling techniques. Knowledge gaps that need
to be tackled, data needs for modelling, and data comparability and harmonisation are also
discussed. In addition, it shows how research on seafloor macrolitter can inform international
protection and conservation frameworks to prioritise efforts and measures against marine litter
and its deleterious impacts.
© 2021 The Author(s). Published by IOP Publishing Ltd
Environ. Res. Lett. 16 (2021) 023001 M Canals et al
List of acronyms MITgcm Massachusetts Institute of Technology
General Circulation Model
ALDFG abandoned, lost or otherwise dis- MSFD Marine Strategy Framework Directive
carded fishing gear Mt Million metric tons
AUV Autonomous underwater vehicle mwd metres of water depth
AWARE Action with awareness regarding edu- NASA National Aeronautics and Space
cation (AWARE Foundation) Administration (USA)
AWI Alfred Wegener Institute NS-IBTS North Sea International Bottom Trawl
BIIGLE BioImage Indexing, Graphical Survey (from table 1)
Labeling and Exploration OGCM Ocean General Circulation Model
BITS Baltic International Trawl Survey OSPAR Convention for the Protection of the
CATAMI Collaborative and Automated Tools Marine Environment of the North-
for Analysis of Marine Imagery East Atlantic (formerly Oslo and Paris
CNR Consiglio Nazionale delle Ricerche Conventions)
(National Research Council) PANGAEA Data Publisher for Earth & Environ-
DSV Deep Submergence Vehicle mental Science (a digital data library
EC European Commission and a data publisher for Earth system
ECOTAXA A tool for the taxonomic classification science, which services are generally
of images of plankton open for archiving, publishing, and
EMODnet European Marine Observation and re-usage of data)
Data Network PAPARA(ZZ)I Program for Annotation of Photo-
EU European Union graphs And Rapid Analysis (of Zillions
G7 Group of Seven, an international and Zillions) of Images
forum including the seven countries RecoMIA Recommendations for marine image
with the largest advanced economies annotation
in the world according to the Inter- RF random forests (a type of machine-
national Monetary Fund: Canada, learning algorithm)
France, Germany, Italy, Japan, the ROI region of interest (in imagery)
United Kingdom, and the United ROMS Regional Ocean Modeling System
States ROV Remotely Operated Vehicle
G20 Group of 20, an international forum RSC Regional Sea Convention
for the governments and central bank SCUBA Self-Contained Underwater Breathing
governors from 19 countries and the Apparatus (equipment used to breathe
EU underwater)
GES Good environmental status (within SI International System of Units
the MSFD) SOV Soviet (from table 1)
GOC Grande Ouverture avec Côtés (a newly SQUIDLE A centralised web-based framework
designed single trawling gear pro- for management, exploration and
moted by MEDITS) annotation of marine imagery
GOV Grande Ouverture Verticale (kind of TUC Towed Underwater Camera
trawl net) TV Trawl vessel (from table 1)
GRID Global Resource Information Data- TVL Trawl vessel large (from table 1)
base TVM Trawl vessel medium (from table 1)
GUI graphical user interface TVS Trawl vessel small (from table 1)
HOV Human occupied vehicle UN United Nations
IAS Institute of anthropic impacts and UNEA United Nations Environmental
sustainability in the marine environ- Assembly
ment UNEP United Nations Environment Program
IBTS International Bottom Trawl Survey US United States (of America) (from
ICES International Council for the Explora- table 1)
tion of the Sea USBL ultra-short baseline transponders
IUCN International Union for Conservation VARS Video Annotation and Reference Sys-
of Nature tem
IFREMER Institut Français de Recherche pour WHOI Woods Hole Oceanographic Institute
l’Exploitation de la Mer
INSPIRE INfrastructure for SPatial InfoRma-
tion in Europe 1. Introduction
JAMSTEC Japan’s Agency for Marine-Earth Sci-
ence and Technology The leakage of human waste into the ocean, whether
JRC Joint Research Centre directly from ships and other maritime platforms or
LITTERBASE Online portal for marine litter from from land, is affecting the ocean and its ecosystems.
AWI
Large quantities of waste, including an estimated 8
MBARI Monterey Bay Aquarium Research
Institute Mt of plastics from land alone in 2010 (Jambeck et al
MEDITS Mediterranean International Trawl 2015), enter the oceans every year. The overall figure
Survey for plastic waste leakage has been recently updated
2
Environ. Res. Lett. 16 (2021) 023001 M Canals et al
by Borrelle et al (2020) to 19–23 Mt for all aquatic and discarded unless intercepted by beaches (but see
ecosystems in 2016. Yet, global annual waste gener- Lebreton et al 2019, Pham et al 2020). Indeed, 8000
ation is projected to jump from 2.0 billion tons in litter items km−2 (Tubau et al 2015) and 20 000 items
2016 to ca. 3.4 billion tons over the next 30 years, km−1 (Pierdomenico et al 2019) have been estimated
with plastics accounting for ∼12% (≈ 242 Mt) of this to sit on the deep seafloor in the Mediterranean Sea.
refuse (Kaza et al 2018). Other modelled predictions These figures could be even larger in other areas, for
range from 20–53 to 90 Mt a year, depending on scen- which no data exist. Vast areas of the seafloor remain
arios, of plastic emissions to the world’s aquatic eco- to be mapped and many species have yet to be dis-
systems by 2030 (Borrelle et al 2020). The amount covered (Ramirez-Llodra et al 2010). The assessment
of plastics entering the environment is thus likely to of the abundance, distribution, and effects of marine
increase significantly (Geyer et al 2017), even under litter on the ocean floor is therefore challenged by our
ambitious system change management scenarios (Lau limited knowledge of this environment.
et al 2020). Still, we currently do not know where 99% In general terms, sources of seafloor litter are
of the plastic debris that made its way into the oceans commonly classified as either land- or sea-based.
is (van Sebille et al 2015). Given that approximately Sea-based sources of marine litter include mostly
half of the plastics produced is heavier than seawa- merchant shipping and fisheries-related activities but
ter (Engler 2012), a large proportion can be expec- also cruise and leisure shipping, aquaculture, milit-
ted to sink directly to the ocean floor, which would ary fleets and oil and gas exploration and production.
constitute a major sink (Woodall et al 2014). Even Land-based sources involve those related to activit-
initially buoyant litter is expected to eventually settle ies along the shoreline and riverbanks, such as land-
to the seafloor after suffering alterations through a fills, littering and inefficient waste management. They
variety of physical, physicochemical, biochemical and may also include wind transport of light litter. Land-
biological processes (Kukulka et al 2012, Tubau et al sourced litter can be a major fraction of total lit-
2015, Besseling et al 2017, Katija et al 2017, Kooi et al ter in coastal waters (figures 1(a)–(d)), which flow
2017, Porter et al 2018, de Haan et al 2019, Wieczorek from distant inland sources through riverine and
et al 2019, Tekman et al 2020, van Sebille et al 2020). aeolian transport. Extreme events such as heavy rain-
In the absence of light, a steady low temperature falls, river floods, sewage overflow, coastal storms,
and comparatively low-energy regime (except in shal- hurricanes and tsunamis can carry large amounts of
low coastal areas), plastic items on the seafloor can debris to beaches and coastal waters in a matter of a
be assumed to be stable, though breakdown may few hours or days, part of which subsequently spreads
occur at poorly known rates (Napper and Thompson seawards and settles to the seafloor (Galgani et al
2019). Thus, there is mounting evidence that quant- 2000, Thiel et al 2013, Galgani 2015, Maximenko et al
ities of litter on the seafloor are growing continuously 2018, Murray et al 2018). Seafloor litter can harm
(Tekman et al 2017, Gerigny et al 2019). marine organisms of all sizes by various mechanisms,
Marine litter (or marine debris) is described including entanglement, smothering and ingestion
as ‘any persistent, manufactured or processed solid (Kühn et al 2015). Although many observations have
material discarded, disposed of or abandoned in the been made, the pathways, distribution and true extent
marine and coastal environment’ (UNEP 2005). This of the harm of litter on biota on the seafloor are largely
includes plastic material leakage from diverse path- unknown.
ways including those of terrestrial (e.g. rivers, coastal Targets and standards such as those of the MSFD
landfills) or marine (e.g. loss of fishing gear, direct (European Commission 2008) of the EU, resolutions
garbage dumping) origins. While marine litter occurs and decisions of the UNEA (several years), declara-
in a continuous size spectrum from nanoparticles to tions of the G7 and G20 assemblies, and agreements
items many metres in dimension, size categories have of the RSCs, have recognised the need for action to
been defined for monitoring purposes. The size class minimise marine litter. Resulting activities include
>25 mm is referred to as macrolitter, being the frac- the EU Directive on the Reduction of the Impact
tion that can be monitored by visual census and col- of Certain Plastic Products on the Environment
lected in bottom trawls. However, litter also enters (European Union 2019), the revision of the EU Port
the environment as very small particles, the so-called Reception Facilities Directive (European Commis-
microlitter or microplastics (≤ 5 mm), which is emit- sion 2018), the UN Global Partnership against Mar-
ted in this size or derived from fragmentation of lar- ine Litter (www.unenvironment.org/explore-topics/
ger plastic items. This review focuses on large debris oceans-seas/what-we-do/addressing-land-based-poll
(i.e. macrolitter) on the seafloor. ution/global-partnership-marine), an aggravation
Litter on beaches raises awareness of the extent of of multiple-polymer plastic exports under the Basel
litter pollution to the general public, however the sea- Convention, and Regional Action Plans against Mar-
floor is out of sight to most of society, even at short ine Litter. For planning, prioritisation and monitor-
distances from the shore. Reports of benthic accu- ing of the effectiveness of measures, these schemes
mulation areas support the hypothesis that the sea- rely on scientifically sound quantitative assess-
floor represents an ultimate sink for all materials lost ments of litter and its impacts on different marine
3
Environ. Res. Lett. 16 (2021) 023001 M Canals et al
Figure 1. Land-sourced litter paving erosive channels in the Messina Strait, Central Mediterranean Sea (a)–(d). Plastics are the
dominant component amidst a noticeable diversity of litter types and objects shown in the pictures. The images were taken
during fall 2016 at 415 (a), 550 (b) and (c), and 575 (d) mwd. All scale bars are 20 cm. Photographs taken with POLLUX III ROV,
courtesy of M Pierdomenico and D Casalbore from the CNR, and F Chiocci from the University of Rome La Sapienza, Italy.
environments and their inhabitants. In this respect, from national programs for MSFD implementation,
it is important to keep in mind that seafloor litter Regional seas monitoring programs, project-related
is often investigated opportunistically, and relies on surveys, and research activities at large. The number
methodologies used to address research questions of studies specifically focussing on seafloor litter is,
that do not focus on marine litter (e.g. fish stock however, still rather minor. In 2019 it represented less
assessments, which yield physical samples as by-catch than 1/4 of all studies on marine litter, microplastics
in bottom trawls). Similarly, camera footage obtained excluded (figure 2(a)), and about 1/7 of all studies
by (un-)manned vehicles and other devices, originally on marine litter, microplastics included (figure 2(b)).
designed for other ecological or geological purposes, Furthermore, not all information is readily accessible,
is also used to quantify benthic pollution. In opera- as at present there is no common reporting template
tional terms, different tools are employed at different for seafloor litter.
depths (Galgani et al 2013b). Attempts to compile available information on
The paragraphs above underline the need for lar- marine litter at the global scale, including seafloor lit-
ger and more standardised datasets and procedures ter, have been made on the AWI’s online portal LIT-
providing more information for more robust decision TERBASE (https://litterbase.awi.de/; Bergmann et al
making on the protection and management of mar- 2017), which pools data from peer-reviewed publica-
ine ecosystems. One major reason for focusing on sea- tions and provides analyses and maps of litter distri-
floor marine macrolitter is that there are fewer stud- bution and impacts. They show that all ocean basins
ies on this litter class compared to studies on smaller contain litter (figure 3(a)–(d)). While plastics account
size classes and on floating or beached debris. Marine globally for 75% of marine debris, on the seafloor
litter seafloor research should move towards a har- they comprise 62%. LITTERBASE also shows that
monised monitoring approach at large spatial and despite the recently increasing research effort, the spa-
temporal scales. Given the widespread and cumulat- tial coverage of data is still limited and highly unbal-
ive character of seafloor macrolitter and the distinct anced. For example, until 2019 there was only one
observation and collection methodologies involved, study available for the whole of the Indian Ocean
this paper focuses on (a) providing a synthesis of cur- seafloor (Woodall et al 2015) and less than a hand-
rent knowledge on seafloor macrolitter, and (b) per- ful of studies published on the South Atlantic and
forming a methodological review from where lessons Southeast Pacific. The degree of pollution in the vast
for future work could be extracted. expanse of the open oceans is also largely unknown as
most research focuses on coastal areas, European seas,
2. Marine litter: boosted scientific North America, and the Western Pacific.
production and databases JAMSTEC established the Deep-sea Debris Data-
base (www.godac.jamstec.go.jp/catalog/dsdebris/e/;
With the rising awareness of marine litter, an increas- Chiba et al 2018), which provides access to geo-
ing number of studies have been undertaken spurring referenced imagery of litter mostly from the deep
a surge of publications in the last decade (figure 2). Pacific Ocean taken during the institute’s many sub-
Data on marine litter have been derived mostly mersible dives since the 1980s. Regional initiatives
4
Environ. Res. Lett. 16 (2021) 023001 M Canals et al
Figure 2. (a) Number of peer-reviewed papers on marine litter and microplastics (left axis, grey bars, from 0 to 120) either
beached, floating or on the seafloor, and on seafloor litter (right axis, red curve, from 0 to 25) published between 1980 and 2019
extracted from Scopus. Search terms in title, abstract or keywords included ‘marine litter’ or ‘macrolitter’ or ‘marine debris’, and
‘sea surface’ or ‘beach’ or ‘float∗ ’ or ‘∗ floor’ or ‘deep sea’, but not ‘microplastic∗ ’. Amongst the latter, only the terms ‘∗ floor’ or
‘deep sea’ are considered in the right axis. (b) Same searches including ‘microplastic∗ ’ (left axis, grey bars, from 0 to 400; and right
axis, red curve, from 0 to 50). The asterisks (∗ ) mean that every word including the search term with asterisk has been considered
in the search (e.g. ‘∗ floor’ vs. ‘seafloor’ or ‘sea-floor’). Beware of differential scaling of the two ordinates.
and project-based databases provide useful data mass concentrations of microplastics using probabil-
portals. For example, the EMODNet Chemistry ity density functions. It would be worth exploring if
portal for marine data is currently compiling such a method could also work for marine macrolitter
project data on benthic litter at the European or, at least, for some macrolitter types (e.g. macro-
scale (www.emodnet.eu/emodnet-chemistry-litter- plastics).
data-maps-first-release). Another good example is Size classes and the terms used to refer to them
the MBARI video annotation database including are not currently standardised. The terms, macro-,
records from 1989 onwards (Schlining et al 2013). meso- and microlitter are defined respectively as
Citizen science is also a growing component, as objects larger than 25 mm, which are visible from
shown by the Dive Against Debris program of the greater distances; items between 25 and 5 mm, requir-
AWARE Foundation, which relies on SCUBA divers ing specific methods for standardised sampling; and
(AWARE Foundation 2015; Consoli et al 2020a). particles smaller than 5 mm (MSFD GES Technical
While expanding the spatial and temporal scales of Subgroup on Marine Litter 2011) and were applied
knowledge, initial instruction and quality control of in scientific papers (e.g. Haseler et al 2018, Cowger
such data is essential (see section 6.2) et al 2020) and citizen science protocols (e.g. Baleta
et al 2017). Megalitter has also been used for very large
items such as shipwrecks or containers, however oth-
3. Litter typology ers such as Mordecai et al (2011) proposed a semi-
quantitative size scale for litter objects: (a) small for
Litter can be classified according to a variety of criteria objects 50 cm.
The classification of litter results from a number of For trawl-sampled seafloor litter (see section 6.2),
conventions made for practical reasons. While there six size classes based on the litter item area have
is need to report litter in SI units, i.e. through weight been proposed with upper limits at 25, 100, 400,
and number, associated information on the precise 2500, 10 000 and >10 000 cm2 (Galgani et al 2013b),
material composition is rarely available. The longest where the small class size is determined by the mesh
dimension of items is often used to describe litter size, size of the net. It will be important to select and
but often does not provide a complete picture. The agree on litter classes that are compatible across dif-
size of some litter objects can be inferred from their ferent types of environments and sampling meth-
identification (e.g. 0.5 l beverage bottle), and often the odologies. The direct weighing of litter sampled on
use of size-range classes can provide information as a the seafloor is impractical due to its wet weight,
rough estimate to litter mass. However, depending on while litter monitored by visual observation can-
the size ranges used, it can result in large uncertain- not be weighed. In addition to standardised size
ties and non-comparable data if standardised proto- classes, it may be useful to follow a common scheme
cols are not used, as is often the case. Very recently, allowing the semi-quantification of litter masses,
Koelmans et al (2020) have provided a method for enabling comparable survey data across locations and
accurate interchange between number, volume, and habitats
5
Environ. Res. Lett. 16 (2021) 023001 M Canals et al
Figure 3. Global distribution of seafloor litter (a) after assessment from different methods (see legend in the upper map lower left
corner). Regional maps display seafloor litter densities and the share of plastics (red sections of pie charts) in (b) the NE Atlantic
Ocean and the Mediterranean Sea, (c) the Gulf of Mexico, and (d) the Western Pacific Ocean. Seafloor litter densities in (b)–(d)
are given as items km−2 . Scale for (b) and (c) in the lower left corner of (c). Maps are based on data extracted from LITTERBASE
as of September 2019.
While there are only rough size categories (2.5– habitats, surveys, and geographic locations, including
50 cm as one class) in current use for several beach the seafloor.
litter monitoring schemes, for an updated beach mac- Litter types are described according to categor-
rolitter protocol and for floating macrolitter it is ies related to their constituent material (i.e. syn-
recommended to be classified in size ranges of 2.5– thetic polymer, metal, glass/ceramic, paper, pro-
5, 5–10, 10–20, 20–30 and 30–50 cm, and larger than cessed wood, rubber, chemicals), their use and
50 cm (Galgani et al 2013b, Fleet et al 2020). So source (fisheries, construction, packaging), and
far, this situation prevents data comparison across individual description (e.g. bottle, bag, sheet, net,
6
Environ. Res. Lett. 16 (2021) 023001 M Canals et al rope), which may be specified further (e.g. beverage is especially true for the light portion, mainly made bottle, oil bottle, rope diameter
Environ. Res. Lett. 16 (2021) 023001 M Canals et al
for dense water masses from the continental shelf impact assessment such as habitat alteration or leak-
to the deep sea (Canals et al 2006, Pusceddu et al age of associated chemicals (e.g. Missiaen and Henriet
2013, Schlining et al 2013) behave as vectors for 2002, Czub et al 2018, Bełdowski et al 2019). The
low-density marine litter, which then accumulates removal of seafloor litter, a management option cur-
in the deepest canyon reaches (Tubau et al 2015). rently under debate, needs to be assessed on a case-
More generally, irrespective of their origin, dimen- by-case basis, judging the benefits of cleaning against
sion and location, seafloor depressions promote lit- the damages caused by such interventions, which also
ter accumulation (Galgani et al 2000). Rocky sub- have to be safe and cost-effective (Da Ros et al 2016,
marine canyons and seamounts are habitats where Madricardo et al 2020).
fishing gear easily becomes entangled (Galgani et al
2018b). An aggravating circumstance is that most 5. Interaction of marine biota with
modern fishing gears, irrespective of the size and seafloor litter
species targeted, can persist in the marine environ-
ment for decades to centuries whilst ghost fishing Because of its wide distribution, durability and low
(see section 5) (Kaiser et al 1996, Brown and Mac- biodegradability, marine litter can affect marine life
fadyen 2007). Materials such as nylon, a polyamide, in numerous ways, thus threatening biodiversity and
and, especially, a new generation of ultra-resistant the natural environment (Laist 1987, 1997, Yoshikawa
polymers such as Dyneema, an ultra-high molecular and Asoh 2004, Lee et al 2006, Bo et al 2014). Dir-
weight polyethylene (Dyneema 2008, Vlasblom 2018; ect and indirect interactions between marine biota
https://usarope.net/why-choose-dyneema-rope-vs-s and seafloor litter (figures 4(a)–(d)), such as entan-
teel-wire-rope-for-heavy-duty-rigging/) used in fish- glement, formation of artificial substrata for set-
ing gear and other marine applications, are suspected tling, covering/smothering of biota, transport vec-
to degrade very slowly where oxygen is scarce and tor of xenobionts, ingestion as well as pathogenic
light absent, such as the deep marine environment, and toxicological impacts have been recorded from
even though robust data on degradation rates of syn- the world’s ocean. Wounding, secondary infections,
thetic polymers in the natural environment are still amputation, movement restrictions, impaired pred-
lacking (Ward and Reddy 2020). When specific land- ator avoidance, exhaustion, suffocation and reduced
forms (i.e. canyons and seamounts) occur close to the foraging ability seriously affect entangled animals
coastline, they generally become spots for (artisanal) (Laist 1997). Charismatic animals such as cetaceans,
fisheries. This leads to the deployment of longlines, seals, sea turtles, and marine birds have received most
trammel nets, cages, fish aggregation devices and attention by researchers and the media, as they are
other gear, and their inevitable loss (Lastras et al clearly hindered in their ability to move, feed, and
2016; Consoli et al 2020b). Also, fishermen delib- breathe when entangled. On the seafloor, sessile erect
erately use deep sections of submarine canyons to invertebrates, such as fragile and long-lived corals
dump useless fishing gear and other items such as oil and sponges, and motile benthopelagic species likely
drums or ropes, damaging vulnerable habitats (Tubau are most affected by entanglements (Colmenero et al
et al 2015). In turn, large litter items and piles of litter 2017, de Carvalho-souza et al 2018, Galgani et al
have a strong trapping capability for other litter types, 2018b). Nearly a third of the specimens of two sponge
such as plastics, resulting in the formation of ‘litter species in the deep Arctic Ocean had entangled plastic
hotspots’ (Tubau et al 2015). Man-made structures debris in them, which might have effects at the pop-
emerging from the seabed, including wrecks, also ulation level (Parga Martínez et al 2020). ALDFG
favour litter trapping (Brown and Macfadyen 2007, may cause damage to species of conservation concern
Mordecai et al 2011). and to structuring species as well as sensitive habit-
Once on the seabed or in the sub-seabed, litter ats for a long time after reaching the bottom (Dono-
is subject to a variety of human activities, such as hue et al 2001, Yoshikawa and Asoh 2004, Chiap-
dredging or bottom trawling. These activities may pone et al 2005, Bauer et al 2008, Heifetz et al 2009,
remobilize litter items (secondary dispersal), likely Orejas et al 2009, Madurell et al 2012, Pham et al
enhancing further degradation and fragmentation. 2013, 2014, Bo et al 2014, Fabri et al 2014, Angiolillo
Bottom trawling, for instance, may remobilise low- et al 2015, Consoli et al 2018). Abrasion due to fric-
density litter, which is then also subject to burial by tion caused by moving or entangled gears on organ-
sediment flows and resettling sedimentary particles isms like corals causes a progressive removal of their
(Tubau et al 2015). Although this has yet to be con- tissue, making them more vulnerable to parasite or
firmed, our current knowledge on bottom trawling- bacterial infections and fouling organisms, eventu-
induced deep-water sediment transport is indicative ally leading to the death of colonies (Bavestrello et al
of the potential magnitude of the phenomenon (Puig 1997, Bo et al 2014, Angiolillo et al 2015, Deidun et al
et al 2012). Also waste-dumping sites, including war- 2015). A recent study in reef areas of the Gulf of Man-
fare material, toxic substances, wrecks and other types nar, in southeast India, found that 34% of the cor-
of anthropogenic material on the seafloor, should als in contact with debris, mainly made of abandoned
be located and mapped for proper accounting and fishing nets, suffered from tissue loss, and other
8Environ. Res. Lett. 16 (2021) 023001 M Canals et al
47.56% were fragmented (Patterson Edward et al life on the seabed, altering the natural environment
2020). Based on skeletal characteristics, morphology and community structure (figures 4(a)–(d) Saldanha
and size, colonies can be found with broken branches, et al 2003, UNEP 2009, Ioakeimidis et al 2015,
entirely removed, crushed or with the soft tissue Gündoğdu et al 2017, Angiolillo 2019). Seafloor waste
scraped off the branches, and rarely overgrown com- may also favour the settlement of non-indigenous
pletely (Bo et al 2014, Angiolillo 2019). The finding of species (Mordecai et al 2011), which can use lit-
isolated and damaged or entangled colonies or spe- ter objects as a transport vehicle for dispersal by
cies usually forming dense populations may indeed bottom currents (Kiessling et al 2015, Kühn et al
be considered an indication of a lost ecosystem (Bo 2015). Once settled in a new habitat, populations
et al 2014, 2015). Derelict traps and nets attract and of alien species may grow, subsequently outcompet-
entrap mobile organisms such as crabs, octopuses and ing resident species (Barnes et al 2009, Kühn et al
fish that die from stress, injury or starvation, as they 2015).
cannot escape. The odour of their carcasses in turn Litter items covering the body surface of emer-
attracts further organisms starting a process called gent epifauna could interfere with filter-feeding,
ghost fishing (Matsuoka et al 2005, Erzini et al 2008, decrease water exchange, respiration, and reproduc-
Kühn et al 2015). tion (Kühn et al 2015). It has been shown that plastic
At least 693 marine species are known to inter- debris smothering corals can lead to shading, dam-
act with marine debris globally (Gall and Thompson age, anoxic conditions, transfer of adhered toxins
2015), 17% of which are included in the IUCN Red and pathogens and thereby providing a foothold for
List. Shortly after UNEP and GRID-Arendal (2016) disease (Lamb et al 2018). Litter lying on the sea-
reported that 92 species of invertebrates, 89 of fish, floor hinders biogeochemical exchange processes at
83 of birds, 38 of mammals and all species of sea the sediment-water interface leading to anoxia and
turtles suffered wounding or death due to entangle- food-deprivation of sediment inhabiting biota and
ment in marine debris. In a more recent review focus- alters benthic community composition (Uneputty
ing on shallow and deeper water rocky reef habit- and Evans 1997, Mordecai et al 2011, Smith 2012,
ats, de Carvalho-souza et al (2018) recorded 418 reef Green et al 2015).
species belonging to various taxa (Porifera, Cnidaria, A growing number of marine organisms are
Platyhelminthes, Mollusca, Annelida, Arthropoda- known to ingest litter of almost all sizes (de Stephanis
Crustacea, Echinodermata, Chordata) and more than et al 2013, Wright et al 2013, Kühn et al 2015, Werner
36 389 individuals associated with marine litter, also et al 2016). Organisms ingest litter deliberately when
noticing that entanglement and ghost catches by they mistake it for prey (Cole et al 2011, Wright
derelict fishing gear represented the main impact on et al 2013, Romeo et al 2015, 2016), accidentally
marine biota in these environments. The portal LIT- while they are feeding on other prey (Thompson et al
TERBASE even recorded a total of 2248 taxa (status 2004, Van Cauwenberghe et al 2013, Fossi et al 2014),
2018) that were globally affected by entanglement, when they are hunting on shoals (Romeo et al 2015,
ingestion, colonisation and other interactions with Battaglia et al 2016), or as a result of secondary inges-
litter. Benthic/demersal taxa accounted for 43% of tion (debris already ingested by prey). Seabirds, fish,
these. Lamb et al (2018) estimated that 11.1 billion and copepods are actually attracted to the odour of
plastic items could be entangled in coral reefs across plastics, probably as a result of infochemicals on litter
the Asia-Pacific region. Further, a recent review of acquired in the photic zone (Savoca et al 2016, 2017,
litter impact on reef systems of the Mediterranean Procter et al 2019). These processes may also play a
Sea identified 78 impacted taxa (Angiolillo and Forti- role for benthic communities but our hitherto scarce
buoni 2020). knowledge focuses largely on the ingestion of small
The most noticeable impacts of litter on soft- sized plastics (e.g. Renzi and Blašković 2020 and ref-
bottom environments are smothering and the intro- erences therein), and demersal fish (e.g. Kühn et al
duction of artificial substrata. Litter items such 2020 and references therein). Ingested items may be
as car tires (figure 4(b)), cans, glass bottles, nets fragmented, compacted and excreted or stored, but
(figure 4(d)), longlines, and bigger objects like bins in any case they can cause physical and mechanical
or wrecks increase habitat heterogeneity as they damage, such as abrasion and obstruction, inflam-
provide new hard substrata for encrusting and sessile mation and blockage of feeding appendages or filters
organisms and vagile fauna in environments oth- (Cole et al 2011, de Stephanis et al 2013, Wright et al
erwise dominated by soft sediments (Watters et al 2013, Li et al 2016, Pedà et al 2016, Cau et al 2020),
2010, Mordecai et al 2011, Bergmann and Klages or lead to malnutrition and pseudo-satiation result-
2012, Angiolillo 2019). Derelict nets and longlines ing in reduced food intake (Kühn et al 2015, Romeo
are frequently colonised by polychaetes, hydroids, et al 2016). However, crustaceans, for example the
encrusting sponges, tunicates, bryozoans, corals and Norway lobster Nephrops norvegicus, may be able to
zoanthids (Fabri et al 2014, Tubau et al 2015, Cau shed small plastic debris during moulting (Welden
et al 2017). Even if these artificial substrata seem to and Cowie 2016). Indigestible debris may accumulate
enhance local diversity, they actually interfere with in digestive tracts and affect individual fitness with
9Environ. Res. Lett. 16 (2021) 023001 M Canals et al
Figure 4. Litter interactions with marine life. (a) Lost bottom trawl net lying on bathyal mud at about 500 mwd at Cap de Creus
Canyon (Northwestern Mediterranean Sea) with organisms using it as shelter or hideout. The fish is a greater forkbeard (Phycis
blennoides), the cnidarian a coloured tube anemone (Cerianthus membranaceus), and the crustacean a Bathynectes maravigna.
There is also an unidentified dead fish. The green laser dots are 15 cm apart. Photograph taken with Liropus II ROV by GRC
Geociències Marines, Universitat de Barcelona, Spain (modified from Tubau et al 2015). (b) Car tire collected during MEDITS
experimental trawl surveys at 74 m depth, from the northern coast of Sardinia (Central Western Mediterranean Sea). The tire is
covered with fouling and encrusting fauna, with the presence of a highly visible squid egg laying (Loligo spp.), and also the
Porifera Clathria spp. and the bryozoan Miriapora truncata. Photograph by Università di Cagliari, Italy. (c) Brittlestar (Ophiura
sp.) and glove in a soft bottom covered up by these organisms at 502 mwd off Miyagi prefecture, in the Pacific Ocean, east of
Japan. Photograph taken with Hyper-Dolphin ROV by JAMSTEC, Japan. (d) Yellow gorgonian Eunicella cavolini colonies
entangled by lost ropes and net on the seafloor at Gulf of Naples (Mediterranean Sea) at 150 m depth. Photograph taken with
Pollux ROV by ISPRA, Italy. All scale bars are 15 cm.
consequences for reproduction and survival (Kühn this is still under debate (Koelmans et al 2016, Saley
et al 2015). This could have more pronounced effects et al 2019).
on deep-sea biota, which often inhabit food-limited Overall, the available data show different interac-
environments. tions and effects according to the characteristics of the
Marine litter may also represent a direct and seabed, associated organisms and the observational
indirect vehicle for the introduction and release of methods applied (figures 4(a)–(d)). Existing records
chemical substances into the marine environment provide an indication of relative sampling effort in
and the organisms inhabiting it. The degradation of different marine regions rather than an accurate view
metals, paints, plastics and other materials can res- of the number of affected species and their expos-
ult in the release of toxic chemicals (UN 2013). Some ure to marine litter, with the vast majority of records
xenobiotics, such as persistent organic pollutants, coming from the Pacific and Atlantic oceans, and the
heavy metals, radioactive substances, pesticides, herb- Western Mediterranean Sea (de Carvalho-souza et al
icides, and pharmaceuticals are resistant to degrada- 2018). The extent of litter effects on deep-sea habitats
tion and deep waters and sediments have been sugges- and their fauna is still poorly known, despite a grow-
ted as the final sink for such pollutants with long term ing number of studies in the last decade have shed
consequences on environmental health (Ramirez- some light on this issue (Ramirez-Llodra et al 2013, de
Llodra et al 2011, Salvadó et al 2013, Jamieson et al Carvalho-souza et al 2018, Angiolillo 2019) (figure 2).
2017). These chemicals could become available to This also highlights the need for a standard method-
organisms (Rochman et al 2014), enter cells and cause ology for assessments of litter–habitat–wildlife inter-
sub-lethal and chronic effects (Teuten et al 2009). actions and impacts to compare data across different
Transfer of pollutants from ingested plastic debris locations and time.
to benthic biota has been recently demonstrated for
the clam Scrobicularia plana, the amphipod Talitrus 6. Methods of seafloor litter monitoring
saltator, N. norvegicus, and the coral Stylophora pistil-
lata (Devriese et al 2017, O’Donovan et al 2018, Sco- Unlike beach litter, which is readily accessible, and
petani et al 2018, Aminot et al 2020). Moreover, they floating litter, which can be identified and monitored
can enter food webs and potentially lead to bioac- by simple and low-cost methods (Gutow et al 2018),
cumulation and biomagnification of toxins, although the quantification of seafloor litter comes with a range
10Environ. Res. Lett. 16 (2021) 023001 M Canals et al
of technical challenges, which rise with increasing 6.2. Trawl surveys: outcomes, strengths
water depth and remoteness of the study area. and weaknesses
Methodologies for the quantification of seafloor Numerous studies on the distribution and compos-
litter can be divided into those involving the collec- ition of seafloor litter have made use of standard-
tion of litter and those relying on in situ observations, ised trawl surveys designed for large-scale fish stock
either directly by humans or by using cameras moun- assessments (figure 4(b)). Many European countries
ted on a variety of platforms. Furthermore, differ- record litter items in catches as part of other envir-
ent methodologies are required for shallow-water and onmental monitoring activities, e.g. the ICES IBTS
deep-sea research. (Moriarty et al 2016) and the MEDITS (Bertrand
In this section we describe and discuss the three et al 2002a, Fiorentino et al 2013). For MEDITS, a
main methodologies for seafloor litter assessments, common protocol in line with the requirements of
data formats and management issues and end with a the MSFD was established in 2013 for the voluntary
discussion of current and future possibilities and lim- collection marine litter data (Fiorentino et al 2013).
itations to be overcome by model projections (see sec- Therefore, systematic litter data collection from bot-
tion 7). We critically consider achievements to date tom trawling monitoring is rather recent, especially
and the state-of-the-art technology for each main when compared to bottom fishing records (Kirby
methodology, identify needs and propose steps for 2004, Thurstan et al 2010, Sampson 2011) or even
further progress, and try to anticipate what the future some monitoring efforts on the status of specific
will bring in that field. benthic habitats (Ardizzone et al 2006).
While early studies of marine litter were mostly
6.1. Shallow-water surveys snapshots of occurrence on the seafloor (Jewett 1976,
In shallow waters, SCUBA divers or snorkelers can Feder et al 1978, Bingel et al 1987, June 1990, Galil
make direct observations or conduct still image or et al 1995), systematic spatially distributed investiga-
video transects using weighted lines or by mov- tions using trawls, which allowed the determination
ing directly between waypoints. These surveys can of sources and accumulation were first published in
cover small areas of seafloor in great detail, poten- 2000 (Galgani et al 2000, Moore and Allen 2000).
tially focusing on accumulations of litter in hydro- Driven by the cost-efficiency of on-going trawl pro-
dynamic traps associated with seafloor unevenness, grams, standardised monitoring protocols have there-
such as rocky outcrops or coral reefs (Katsaneva- fore produced time series allowing trend analyses for
kis and Katsarou 2004, Bauer et al 2008, Abu-Hilal the last ∼20 years (Schulz et al 2015, Maes et al 2018)
and Al-Najjar 2009). They enable direct observation and have helped to identify seafloor litter sources
of litter–biota interactions (Katsanevakis and Kat- not only generically but also in terms of geographic-
sarou 2004, Kipson et al 2015, Sheehan et al 2017, ally defined source regions (Wei et al 2012, Ramirez-
Nunes et al 2018, Patterson Edward et al 2020), col- Llodra et al 2013, Ioakeimidis et al 2015, Neves et al
lection of physical samples for inspection (Sheehan 2015, Lopez-Lopez et al 2017, García-Rivera et al
et al 2017), manipulation experiments (Parrish and 2017, 2018, Kammann et al 2018, Urban-Malinga et al
Kazama 1992, Katsanevakis et al 2007, Baeta et al 2018). Inspection of the physical samples caught in
2009) and spontaneous adaptation of surveys if inter- the nets enables source allocations in some cases. Lit-
esting processes or features are observed as well as ter densities reported from trawl surveys range from
the involvement of citizen scientists (Bauer-Civiello 50 to 289 items km−2 on the French continental shelf
et al 2018; Consoli et al 2020a). However, the depth and canyons of the Mediterranean Sea (Gerigny et al
and area that can be covered by such surveys are 2019), to 2840 items km−2 in the northern Gulf of
limited by air supply, field conditions (water tur- Mexico and 0–1835 items km−2 in the North and
bidity, temperature) and safety considerations. The Irish seas (Maes et al 2018).
typical maximum depth of such surveys is 20–40 m Despite all the progress reported above, some
and quantities recorded ranged from 11.0 to 24.2 issues in trawl surveys remain. For instance, some lit-
items km−2 in the North Sea and 3.1–10.4 items ter, especially that which is low-density could eas-
km−2 in the Baltic Sea (Kammann et al 2018). The ily be transported from one location to another by
integration of litter observation as additional para- the action of bottom currents or bottom fishing gear,
meter in on-going surveys for other purposes, such as leading to highly dynamic scenarios. The use of trawls
biodiversity assessments, may provide cost-effective originally designed for fish stock assessments likely
monitoring. As on the deeper seafloor, TUCs and underestimates the total litter abundance and distorts
(mini-)ROVs or underwater drones can be employed, these data, as small items can pass through the net
with the advantage that these can be low cost plat- mesh (Watters et al 2010). In addition, bottom trawls
forms and vehicles with a limited depth rating. are not always in constant contact with the seabed and
ROVs and TUCs are touched upon in more detail in can get temporarily stuck on bottom features and also
section 6.3.1. stay on the seafloor for some time prior to recovery.
11Environ. Res. Lett. 16 (2021) 023001 M Canals et al
Therefore, trawls are considered semi quantitative methods best suited to the observed frequency distri-
sampling gears for benthos (Eleftheriou and Moore butions are further questions of crucial importance.
2005), which should also apply to litter assess- Recent studies stepped forward to model-based pre-
ments. Acoustic devices can improve the accuracy of dictions of seafloor litter densities using trawl data
area swept estimates, helping to measure sampling (Lopez-Lopez et al 2017, Gutow et al 2018). Such
effort. approaches can be regarded as advantageous low-
Another aspect is that trawl surveys cannot be cost methods to identify litter hotspots and locations
used in several areas due to the risk of catching and representative of the background situation, however
bringing up unexploded munition dumped during these have to be groundtruthed and monitored on
and after wars in years 1918–80. Globally, it is estim- a long-term basis in order to produce time series
ated that around 1 Mt of chemical weapons lie on of seafloor litter and enable trend analyses. Further-
the ocean floor (Curry 2016) and these pose another more, the disturbance of seafloor habitats by trawl-
risk both for marine and human life (Greenberg ing is often considerable and therefore trawling to
et al 2015). Regionally, e.g. in the North Sea and monitor and survey litter must deserve appropriate
Baltic Sea, the amount of conventional and chemical consideration.
munitions is estimated to 1.3 millions and 385 000 Some of the limitations of trawl surveys are dif-
tons, respectively (Bełdowski et al 2019). Although ficult to overcome (e.g. the changing nature of the
dropped bombs, grenades, torpedoes, land mines seafloor, the low-catchability of highly mobile light-
and other ammunition types are now widely dis- weight litter) but there are many improvements that,
tributed across the seafloor (Bełdowski et al 2016), if implemented, would lead to more robust data-
specific dumpsites are generally known and can be sets. They range from agreeing on using specific
avoided (Szarejko and Namieśnik 2009, OSPAR Com- litter-catching gear following calibration tests; apply-
mission 2010, Noack 2018, Mie˛tkiewicz 2020). Thus ing standardised protocols when trawling for litter,
in unexplored depths of known war regions visual and also when counting, weighing and classifying lit-
(and acoustic) surveys should be the first option. ter catches; and reporting conventions. ICES (2011,
A wide range of types of trawling gears have 2012) did develop standardised monitoring protocols
been used to sample seabed litter (table 1) and of bottom trawl surveys for the North and Baltic seas,
it is also well established that different gear types and for the North-East Atlantic, which have been used
have a different catch efficiency and sample areas since 2011, however trawl gear differs between mar-
(Fiorentini et al 1999, Bertrand et al 2002b, Kam- ine regions, because of different substrate types and
mann et al 2018). EMODnet Chemistry registered fish stocks. Most importantly, the use of seafloor non-
13 different types of trawling gears used for data ploughing gear vs. ploughing gear deserves attention.
collection (www.emodnet-chemistry.eu/welcome), Dederer and Schulz (2016) found significant differ-
whereas ICES describes 52 bottom-touching gear ences in the catchability of benthic litter when com-
types (https://vocab.ices.dk/?ref=2) (table 1). These paring beam trawls, otter trawls and non-ploughing
different gears were designed to be efficient to sur- fishing gear used for cod. In Europe at least, this issue
vey different components of marine life and ground can be considered only in a top-down directive, pos-
types (e.g. muddy, sandy or rocky beds, shallower or sibly as a detailed protocol for Descriptor 10 (marine
deeper). litter) of the MSFD. Clearly, method harmonisation
The high diversity of gear types used among should be given urgent prioritisation.
regions, programs and studies seriously hampers
cross comparisons of litter quantity based on data
obtained opportunistically from such programs. 6.3. Visual surveys: outcomes, strengths
For instance, figures 5 and 6 illustrate how dif- and weaknesses
ferent countries in Europe have used different Significant advances in underwater image techno-
gears across their fleet for the 2006–2019 period, logy have provided new opportunities to observe and
and which gears are used in different geograph- study the marine environment in a wide range of
ical areas, respectively. In order to standardise this research disciplines, from geology to animal beha-
method, MEDITS promotes the use of a newly viour at different spatial and temporal scales (Dur-
designed single trawling gear, GOC 73, in all surveys den et al 2017, Parga Martínez et al 2020) and
(https://cobmedits2012.wordpress.com/goc-73/). for habitat mapping programmes (Bo et al 2011,
Although trawl-based litter data could be repor- Buhl-Mortensen and Buhl-Mortensen 2017). Sea-
ted as items km−2 , g km−2 , items haul−1 or g haul−1 , floor imagery is increasingly being used to study the
litter quantities are usually referred to in terms of haul abundance and distribution of debris on the sea-
or hour. This further renders comparisons between floor as well as its interactions with marine organ-
studies difficult. Commonly, weight is only provided isms (figures 4(a), (c) and (d), and figures 8(a)–(d))
when there are many small pieces too numerous to (see section 5).
count (figures 7(a)–(d)). How to deal with outliers There are strengths and weaknesses in using
(Fleet et al 2020) and identifying the geostatistical image-based systems for litter quantification.
12Environ. Res. Lett. 16 (2021) 023001 M Canals et al
Table 1. Gears used in IBTSs as identified by ICES (left column, https://vocab.ices.dk/?ref=2) and those in the EMODnet database (right
column, www.emodnet-chemistry.eu/welcome). The overall design, mesh size and operational/technical details for each gear type could
influence litter catchability. Most acronyms are spelt out in the corresponding row. Other acronyms are: GOC; GOV; SOV, from Soviet,
only used by the Russian Federation between 1976 and 1982 in the NS-IBTS survey; TV; TVL; TVM; TVS. See also the list of acronyms
at the end of the paper.
ICES (52) EMODNET (13)
1 AAL—Bottom trawl net Aalhopser –
2 ABD—Aberdeen 18 ft trawl –
3 BAK—Baka trawl Baka trawl
4 BMT—Beam trawl Beam trawl
5 BOT—Bottom trawl Bottom trawl
6 BOX—Boris ‘Goshawk’ box trawl –
7 BT2—Beam trawl 2 m –
8 BT3—Beam trawl 3 m –
9 BT4—Beam trawl 4 m –
10 BT4A—4 m beam trawl, aft 4 m beam trawl, aft
11 BT4AI—4 m beam trawl, aft -in Irish Sea q3 BTS 4 m beam trawl, aft—in Irish Sea q3 BTS
12 BT4P—4 m beam trawl, port –
13 BT4S—4 m beam trawl, starboard –
14 BT6—Beam trawl 6 m –
15 BT7—7 m beam trawl 7 m beam trawl
16 BT8—8 m beam trawl 8 m beam trawl
17 CAR—Bottom trawl FGAV019 –
18 CHP—Cod hopper
19 COM—Commercial trawl with sprat bag –
20 DBT—Danish bottom trawl –
21 DT—Russian bottom trawl –
22 ESB—Estonian small bottom trawl –
23 GOS—GOV small –
24 GOV—GOV trawl GOV trawl
25 GRT—Granton (otter) trawl –
26 H12—Herring bottom trawl 120 feet –
27 H18—Herring bottom trawl 180 feet –
28 H20—Herring ground trawl (HG20/25) –
29 HAK—Hake-4 m (bottom trawl) –
30 HOV—High opening bottom trawl –
31 HT—Herring bottom trawl –
32 INT—Industrial trawl –
33 JDT—Jackson trawl, modified otter trawl –
34 LBT—Latvian bottom trawl –
35 LCT—Lithuanian cod trawl –
36 NCT—Norwegian Campell trawl 1800/96 Norwegian Campell trawl 1800/96
37 P20—Herring bottom trawl (P20/25) –
38 PAR—Pair trawl –
39 PHT—Portuguese high headline trawl –
40 PORB—Porcupine baka Porcupine baka
41 RAPIA—Modified beam trawl with a rigid mouth –
42 ROT—Rock hopper otter trawl –
43 SON—Sonderborg trawl –
44 SOV—SOV-NET –
45 TV-3—Bottom trawl TV-3 930# –
46 TVL—Large TV trawl Large TV trawl
47 TVM—Bottom trawl TV-3-280 x 80 –
48 TVS—Small TV trawl Small TV trawl
49 US4—US 4 Seam 3 Bridle –
50 VIN—Vinge trawl –
51 W2A—Western IIA (otter trawl) –
52 Y36—Yankee #36 (bottom trawl) –
– – GOC 73 bottom trawl
Advantages depend on the specifications of the plat- with the capability to obtain precise geo-referencing
form used, are that in situ imagery can be obtained of each litter item. In addition, these methods
from all depths and landscapes, including those allow small-scale observations, which are import-
that are not accessible to bottom trawls, together ant, for example, to identify litter–biota interactions
13Environ. Res. Lett. 16 (2021) 023001 M Canals et al
Figure 5. Number of countries, gears and ships reporting data on seafloor litter in Europe from 2006 to 2019. Countries include
Belgium, Croatia, Denmark, Estonia, France, Germany, Greece, Ireland, Italy, Latvia, Lithuania, Montenegro, Norway, Poland,
Portugal, Romania, Slovenia, Spain, Sweden, The Netherlands, United Kingdom (England) and United Kingdom (Scotland). Data
for 2019 are still incomplete (worked out from www.emodnet-chemistry.eu).
Figure 6. Geographical distribution of operations of different trawling gears providing data on seafloor litter in Europe during the
2006–2019 period (modified from www.emodnet-chemistry.eu).
(figures 4(a), (c) and (d)) (Mordecai et al 2011, subsequently released to the sea surface for final
Parga Martínez et al 2020) and accumulation areas recovery.
(figures 8(a), (b)) (Tubau et al 2015). Furthermore, if On a general level, seafloor-imaging surveys may
properly operated, image-based systems are harmless easily underestimate litter items, especially when a
to benthic organisms. significant proportion of them are buried, blended
Two potentially significant disadvantages of the within accumulations of litter formed by mixtures
image-based approach to seafloor litter quantification of land- and marine-sourced litter items and natural
are, first, that the minimum size of litter that can be debris ‘hotspots’ (Tubau et al 2015). In addition, some
identified depends on the resolution achievable by objects could be hardly identified by conventional
the cameras and, second, that only seabed-exposed imagery, e.g. white fragments, which may easily be
litter can be observed (figure 8). Items covered by confused with shell fragments.
sediments, or entrained within seagrasses, coral reefs
or fine-scale rocky structures cannot be detected. 6.3.1. Platforms for image acquisition of marine litter
Physically sampling and bringing litter items for 6.3.1.1. Remotely operated vehicles (ROVs)
onboard examination also is severely limited, as it Early on, ROVs used for science were often work-
represents additional operations that are potentially class, industry vehicles that were retrofitted for sci-
complex (e.g. to minimise entanglement of the cam- entific purposes (Dawe et al 1998). Institutions
era platform) and volume-limited for most plat- such as IFREMER in France, JAMSTEC in Japan,
forms and strategies used for video surveys. One and MBARI and WHOI in the USA, were the
way of partially minimising this limitation is pla- first ocean research organisations to design and
cing litter samples within seabed lift boxes that are develop purpose-built scientific ROVs (Newman
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