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ICES Journal of Marine Science , 2023, 80 , 1567–1579
DOI: 10.1093/icesjms/fsad115
Advance access publication date: 19 July 2023
Food for Thought
Evaluating the sustainability and environmental impacts of
tr a wling compared to other food production systems
R. Hilborn
1 ,*
, R. Amoroso
1
, J. Collie
2
, J. G. Hiddink
3
, M. J. Kaiser
4
, T. Mazor
5 ,6
,
R. A. McConnaughey
7
, A. M. P ar ma
8
, C. R. Pitcher
5
, M. Sciberras
3 ,4
, and P. Suuronen
9 ,10
1
School of Aquatic and Fishery Sciences, University of Washington, Seattle, WA 98195, USA
2
Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, USA
3
School of Ocean Sciences, Bangor University, Menai Bridge LL59 5AB, UK
4
The Lyell Centre, Heriot-Watt University, Edinburgh EH14 4AS, UK
5
Oceans and Atmosphere, Commonwealth Scientic and Industrial Research Organisation, Brisbane, QLD 4067, Australia
6
Biodiversity, Environment and Climate Change, Department of Environment Land Water and Planning, East Melbourne, VIC 3002, Australia
7
Alaska Fisheries Science Center, National Oceanic and Atmospheric Administration, Seattle, WA 98115, USA
8
Centro para el Estudio de Sistemas Marinos Centro Nacional Patagónico-CONICET , Puerto Madryn, Chabut 9120, Argentina
9
Fisheries and sh resources, Natural Resources Institute Finland (Luke), Helsinki 00790, Finland
10
International Seafood Consulting Group (ISCG), Helsinki 00100, Finland
∗Corresponding author : tel : + 206-883-5049; e-mail: hilbornr@gmail.com .
Mobile bottom contact gear such as trawls is widely considered to have the highest environmental impact of commonly used shing gears,
with concern about impact on benthic communities, b y catch, and carbon footprint frequently highlighted as much higher than other forms of
shing. As a result, the use of such gears has been banned or se v erely restricted in some countries, and there are many proposals to implement
such restrictions elsewhere. In this paper, we review the sust ainabilit y of bottom trawling with respect to t arget-species sust ainabilit y, impact
on benthic communities, b y catch and discards, carbon footprint from fuel use, and impact on carbon sequestration. We compare the impact to
other forms of shing and other food production systems. We show that bottom-trawl and dredge sheries have been sustained, and where
well managed, stocks are increasing . Benthic sediment ary habit ats remain in good condition where shing pressure is well managed and where
VME and species of concern can be protected by spatial management. Bycatch is intrinsically high because of the mixed-species nature of
benthic communities. The carbon footprint is on average higher than c hic ken or pork, but much less than beef, and can be much lo w er than
c hic k en or pork. T he impact on carbon sequestration remains highly uncertain. Overall, the concerns about trawling impacts can be signicantly
mitigated when existing technical gear and management measures (e.g. gear design changes and spatial controls) are adopted by industry and
regulatory bodies and the race-to-sh eliminated. When these management measures are implemented, it appears that bottom trawling would
ha v e a lo w er en vironmental impact than liv estoc k or fe d aquaculture, whic h w ould lik ely replace tra wl-caught sh if tra wling w as banned. A total
of 83 bottom-trawl sheries are currently certied by the Marine Ste w ardship Council, which is the most widely accepted measure of o v erall
sust ainabilit y.
Keywords: Bottom trawling, bycatch, carbon footprint, discards, environmental impacts of shing.
Introduction
Bottom trawls (such as beam trawls, otter trawls, and shellsh
dredges, which we will refer to as bottom trawls) are designed
to catch target species that live close to, in, and on the seabed.
The use of bottom trawls as a means of catching sh has met
with increasing opposition due to its impact on seaoor habi-
tats and biological communities (Watling and Norse, 1998 ;
Watling, 2013 ), its high bycatch rates (Pérez Roda et al., 2019 ;
Gilman et al., 2020 ), CO
2 release from fuel use (Tyedmers,
2004 ; Sala et al., 2022 ), and, lately, its potential contribution
to greenhouse emissions through the release of stored carbon
from disturbed seabed sediments (Sala et al., 2021 ). Although
the magnitude of those impacts remains the subject of in-
tense scientic debate (Pitcher et al., 2022 ), concerns about
the environmental impacts of trawling have fueled strong
public campaigns, resulting in bottom trawling being demo-
nized (Willer et al., 2022 ), severely restricted, or effectively
banned in some countries and regions (McConnaughey et al.,
2020 ).
However, bottom trawling accounts for 26% of global ma-
rine sheries catches (Steadman et al., 2022 ), providing food
and employment for millions of people at a time when the
contributions of marine sheries towards the United Nations
Sustainable Development Goals (United Nations, 2002 ) and,
specically, to meet the food and nutrient needs of a grow-
ing population, are increasingly recognized. While alternative
shing gears and methods may be available and economi-
cally viable in some cases, many benthic and demersal target
species would be difcult to catch without some form of bot-
tom trawling (Ziegler and Valentinsson, 2008 ; Suuronen et al.,
2012 ).
From this perspective, bottom trawling needs to be consid-
ered as one form of food production, and its sustainability and
environmental footprint should be compared to footprints of
other ways of producing food, including other capture sh-
eries, aquaculture, livestock, and crop production.
The purpose of this paper is to summarize the cur-
rent knowledge about the sustainability and environmental
Received: 24 October 2022; Revised: 29 June 2023; Accepted: 4 July 2023
©The Author(s) 2023. Published by Oxford University Press on behalf of International Council for the Exploration of the Sea. This is an Open Access
article distributed under the terms of the Creative Commons Attribution License ( https:// creativecommons.org/ licenses/by/ 4.0/ ), which permits unrestricted
reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
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1568 R. Hilborn et al.
1970 1980 1990 2000 2010
0.0 0.5 1.0 1.5 2.0 2.5
Year
Stock abundance/target
Figure 1. The abundance trend in global groundsh stocks relative to management targets (solid black line). In most cases, management t argets are
based on achieving maximum sustainable yield. Vertical bars show the range of 50% of the stocks, with 25% being below and 25% above. The thin grey
horizontal line shows where the stock abundance is equal to the management target. Redrawn from Hilborn et al. (2021) .
impacts of bottom trawling, to compare trawling impacts to
other forms of food production, to identify important infor-
mation gaps, and to suggest the best ways to minimize the
environmental impacts of trawling.
Certications as sustainable
At present, 83 bottom-trawl sheries representing 252
bottom-trawl-caught species/sheries combinations have been
certied by the Marine Stewardship Council (personal com-
munication, Mike Melnychuk, MSC staff) as sustainable.
These include 122 units of certication from Europe, 63 from
the United States, 19 from Canada, 15 from Australia, 12 each
from Chile and New Zealand, 5 from Africa, and 2 from Ar-
gentina. Many are recommended by the Seafood Watch pro-
gramme of the Monterey Bay Aquarium ( www.seafoodwatch
.org ). These are the two best-known international standards
for sheries sustainability, and the fact that bottom-trawl sh-
eries meet their standards is evidence that bottom-trawl sh-
ing can be sustainable. These sustainability evaluations con-
sider not only the status of the target stock but also the ma-
rine environmental impacts of the shing method and have
specic criteria regarding the management of bottom-trawl
impacts on benthic communities (Monterey Bay Aquarium,
2023 ) (Marine Stewardship Council, 2023 ).
Sustainability of target species
Bottom trawling is the primary method used to harvest many
demersal species known as groundsh, which include cod,
haddock, pollock, hake, and multiple species of atsh and
rocksh. Globally, almost all the catch of groundsh comes
from sh stocks whose trends in abundance are scientically
assessed (Hilborn et al., 2021 ). Groundsh populations are
increasing overall and above the target levels for sustainable
exploitation ( Figure 1 ) (Hilborn et al., 2021 ). Arguments that
bottom trawling is incompatible with sustaining a shery for
the target species are contradicted by the trends in the abun-
dance of groundsh stocks. The mixed-stock nature of all bot-
tom shing methods (trawl, longline, Danish seine, gillnet)
poses challenges to sustainable exploitation of mixed species
of differential productivity, but the increasing trend of ground-
sh in many regions of the world shows that even in mixed-
species sheries, good management can lead to sustainabil-
ity (Fernandes and Cook, 2013 ; Zimmermann and We r n er,
2019 ).
There are of course many stocks that are overexploited with
bottom trawls, but this is a failure of sheries management
to control shing pressure rather than a direct consequence
of the shing gear used, as it has been clearly demonstrated
that well-regulated bottom-trawl sheries can avoid oversh-
ing (Hilborn et al., 2021 ). Bottom trawling and related mobile
bottom-contact gear like dredges are also commonly used for
many invertebrates, but there has been no global summary of
the trends in abundance of these species.
Impact of tr a wling on benthic ecosystems
The magnitude of the effect of the trawl disturbance on ben-
thic communities depends on the frequency of trawling, the
impact (or depletion rate) per trawl pass, and the individual
recovery rates of biota exposed to trawling (Hiddink et al.,
2017 ). The effects of trawling on the commonly shed sedi-
mentary habitats, such as muddy and sandy seabeds, are much
less severe than on the more sensitive habitats, such as oys-
ter reefs in shallow waters and vulnerable marine ecosystems
(VMEs) (Parker et al., 2009 ), such as sponge gardens or cold-
water coral reefs (Clark and Rowden, 2009 ; Clark et al., 2015 ;
Kaiser et al., 2018 ), in deeper waters. For sedimentary habi-
tats, average depletion rates (the percentage of benthic inverte-
brates killed per passage of the gear) range from 4.7 to 26.1%
depending on trawl type, gear penetration depth, and habitat
type, with otter trawls causing the lowest depletion, followed
by beam trawls and towed dredges causing the most impact
(Sciberras et al., 2018 ). Depletion rates are lower in sand than
in gravel and mud (Collie et al., 2017 ; Pitcher et al., 2022 ).
Recovery rates are related to the longevity of the affected
species (Hiddink et al., 2019 ). Meta-analysis of studies report-
ing how the biomass of the benthic community declines with
increasing trawling intensity produced estimates of recovery
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Sustainability of trawling 1569
rates that ranged from 29 to 68% per year along a gravel-to-
mud continuum (Pitcher et al., 2022 ). Slower recovery with
increasing gravel reects the greater proportions of longer-
lived species found in more stable gravel habitats. Epiben-
thic megafauna and biogenic habitats are the most sensitive to
all forms of trawling, and recovery rates are often measured
in decades (Kaiser et al., 2018 ). However, complex habitats
like coral reefs and rocky bottoms are generally avoided by
trawlers because of the threats to their nets. When these habi-
tats are trawled, they are heavily impacted (Parker et al., 2009 ;
Williams et al., 2020 ), and a consensus is growing that the best
practise is to close such areas to mobile bottom contact gear
(McConnaughey et al., 2020 ).
A global modelling assessment of trawl impacts on macro
epifauna and infauna in sedimentary habitats showed that
the status of benthic populations relative to an untrawled
state differs greatly among regions and was related to the to-
tal amount of trawling (Pitcher et al., 2022 ). The model in-
cluded 24 regions worldwide and used ne-scale data on the
frequency of trawling and recovery rates of biota estimated
from meta-analysis ( Figure 2 ). The measure used, relative ben-
thic status (RBS), reects the extent to which the macrofauna
have been numerically reduced and is an aggregated measure
across many species (Pitcher et al., 2017 ). A status of 0.9, for
instance, would mean that the abundance of benthic macro-
fauna averaged across taxa would be 90% of the abundance
in the absence of trawling. Even with a RBS of 0.9, some more
sensitive species would be reduced more than that and more
resilient species less. The RBS for a region will reect the aver-
age across untrawled, lightly trawled, and heavily trawled ar-
eas in the region, weighted by the area of each level of trawl in-
tensity. Mazor et al. (2021) were able to look at the impacts on
specic species where data were available. There are no estab-
lished targets for this index, and as in discussions of changes
in biodiversity, multiple measures are potentially usable. RBS
allows us to compare widely across benthic habitats in many
different regions.
Overall impacts are low in most regions examined, and
much of the seabed is untrawled in many regions. Regional
average status relative to an untrawled state (status = 1.0) was
high ( > 0.9) in 15 regions (mostly outside Europe) but < 0.7
in three European regions and only 0.25 in the Adriatic
Sea. Across all regions, 66% of the seabed area was not
trawled, 1.5% was depleted (status = 0), and 93% had sta-
tus > 0.8 ( Figure 2 ) (Pitcher et al., 2022 ).
The RBS is calculated for each region in the most recent
range of years where trawl effort data were available (mostly
2010–2014), and reects the expected status of benthos at that
intensity of trawling. RBS depends on habitat type (reecting
both the taxa found and the sensitivity to trawling) and the
intensity of trawling. In most areas where we have trawl-effort
data, there is declining shing pressure (see a later section on
trends in trawl footprint), so we would expect that in general
RBS would be improving.
Mazor et al. (2021) provide more detail on impacts within
different taxonomic groups. The status of populations of
benthic-invertebrate groups was examined for 13 of the 24
regions for which suitable invertebrate distribution data were
available and ranged between 0.86 and 1 (mean = 0.99), with
78% of benthos-groups having a status > 0.95 (Mazor et al.,
2021 ). Again, mean benthos status was lower in European re-
gions than regions elsewhere, which accords with the intensity
and history of shing in Europe.
Assessing the status of sedimentary habitats (the habitat
types where most trawling occurs) is critical to ensuring the in-
tegrity of the seabed ecosystems because sedimentary habitats
constitute most of the continental shelves. Nevertheless, much
concern surrounds rarer, more sensitive habitat types that can
characterize VMEs and biogenic habitats (F AO , 2009 ). These
habitats are not well mapped over large scales in most regions,
and while impact rates are known to be high in many cases,
there are few quantitative estimates of the impact that bottom
trawling has on them because few studies have been carried
out because it is hard to justify trawling over such sensitive
habitats for a scientic experiment (Hall-Spencer and Moore,
2000 ). Even the most resilient of these VMEs cannot with-
stand trawling more than once every three years (Thompson
et al., 2016 ). A preliminary assessment conducted by Pitcher
et al. (2022) calculated the percentage of each of the 24 regions
in their study where trawl intensity exceeded that frequency,
which was used as a local extinction threshold for highly sen-
sitive biota. The percentage of seabed trawled at least once ev-
ery three years ranged from 0.2% in southern Chile to 82% in
the Adriatic Sea and was > 20% for 10 regions (all European
regions and northern Benguela) (Pitcher et al., 2022 ). In those
regions, we would expect the sensitive species in VMEs to be
eliminated in proportion to the amount of area trawled three
times or more. Because of the high sensitivity of the habitat-
forming biota types that characterize VMEs, sheries manage-
ment should seek to prevent signicant adverse impacts on
them, according to the Deep-Sea Fisheries Guidelines (F AO ,
2009 ).
The data on trawl intensity in Pitcher et al. ( 2022 ) cov-
ers almost all European waters, Australia, New Zealand,
South Africa, Namibia, Argentina, Chile, the western US,
and Alaska. There is no coverage of Asia, where trawling is
thought to be quite intense (Suuronen et al., 2020 ), and Africa
with the exception of Namibia and South Africa.
Indirect impact of tr a wling on productivity of
target species
Intense bottom trawling causes a high level of local mortal-
ity to benthic fauna, and for sh species that depend on ben-
thic fauna for food, shelter, productivity, and hence sustain-
able harvest may decline with increasing levels of bottom
shing disturbance. Indirect effects of bottom shing have
been demonstrated experimentally and with dynamic mod-
els in which trawling affects the target species, their benthic
prey, and the habitat-forming epifauna (Collie et al., 2017 ;
Pitcher et al., 2022 ). Ultimately, the response of sh produc-
tivity to bottom shing depends on the interplay between re-
duced benthic prey abundance and reduced competition for
benthic food as sh density declines (Hiddink et al., 2011 ;
2016 ). Historically, trawling may have modied habitat and
reduced the carrying capacity of sh stocks, but these ef-
fects are difcult to distinguish empirically because shing
and other factors may impact the abundance of target species.
Over large areas of the continental shelf with sandy sedi-
ments, these indirect effects are estimated to be small com-
pared with the direct mortality caused by shing target species
(Collie et al., 2017 ; Pitcher et al., 2022 ). A possible expla-
nation for this small effect is that the distribution of shing
effort is very patchy—small fractions of shing grounds are
heavily shed, while large fractions are lightly shed or un-
shed (Amoroso et al., 2018 ). Therefore, the indirect effects of
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1570 R. Hilborn et al.
Figure 2. Depletion le v el (RBS) of benthic ora and fauna in different regions of the world where data on trawl effort and sediment type are available.
Data from Pitcher et al. ( 2022 ).
bottom shing are also likely to be localized, for example,
where target species live on vulnerable habitats.
By catc h and discards
Bycatch is generally dened as the “unintended, non-targeted
organisms caught while shing for particular species (or sizes
of species),” including “landed bycatch,” which is retained to
be eaten or sold (Pérez Roda et al., 2019 ). Discards are the
portion of the catch that are returned to the sea whole, alive
or dead. Fishers are discarding in response to numerous and
continuously changing factors, including market conditions,
regulations, and the size and quality of the catch.
Using Food and Agriculture Organization (FAO) databases
on country-specic landings, Pérez Roda et al. (2019) esti-
mated the discard rate and magnitude for the period 2010–
2014 for global marine capture sheries using shery-specic
discard rates derived from direct observations and global gear-
specic discard rates. Discard rates for trawl sheries and se-
lected other gear types are shown in Table 1 .
Table 1 shows that the dominant determinant of discard
rate is whether the shing occurs on the bottom or surface or
Ta b l e 1. Mean discard rates and 95% condence bound (CI) for different
shing gears from Pérez Roda et al. , 2019 (Table B1).
Gear Category
Mean percent
discarded 95% CI
Purse seine 5% 3 .9–5.6%
Longline, pelagic 7% 5 .8–9.4%
Pole-and-line 9% 6 .4–14.4%
Handline 10% 1 .9–44.2%
Gillnet, pelagic (driftnet) 12% 7 .4–19.0%
Otter trawl, midwater 12% 8 .2–18.2%
Longline, bottom and pelagic 13% 11 .0–16.4%
Pots 17% 12 .1–22.2%
Gillnet, surface and bottom 17% 8 .8–32.9%
Tra wl, pair, midwater 19% 3 .3–73.0%
Trolling lines 20% 6 .8–49.8%
Longline, bottom 24% 18 .0–31.1%
Gillnet, bottom 26% 19 .8–33.8%
Otter trawl, bottom 31% 28 .5–60.0%
Tra wl, otter twin 44% 28 .5–60.0%
Tra wl, beam 46% 37 .7–53.8%
Tra wl, pair, bottom 48% 14 .1–87.8%
Tra wl, shrimp 55% 50 .0–59.6%
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Sustainability of trawling 1571
Ta b l e 2. The average, minimum, and maximum amount of fuel used to capture one MT (litres per MT) of sh for different gear types and the amount of
carbon released per kilogramme (Kg) of sh wet weight landed (Kg CO
2
per kg landed).
Liters of fuel per MT landed Kg CO2 per Kg landed
Gear Average Minimum Maximum Average Minimum Maximum
Surrounding nets 252 8 659 0.68 0.02 1 .78
Dredges 506 15 1 822 1.37 0.04 4 .92
Pelagic trawls 667 36 2 475 1.80 0.10 6 .68
Gillnets 604 199 2 162 1.63 0.54 5 .84
Divers 951 585 1 472 2.57 1.58 3 .97
Hooks and lines 1 032 47 4 985 2.79 0.13 13 .46
Bottom trawls 1 722 65 17 300 4.65 0.18 46 .71
Pots and traps 3 014 331 9 474 8.14 0.89 25 .58
Data source is Parker and Tyedmers (2015) .
midwater. Bottom trawls generally have the highest discard
rate and account for an estimated 46% of all discards, with
shrimp trawls having particularly high discards (Pérez Roda
et al., 2019 ). In many trawl sheries (and most other sheries),
most of the discarded catch will not survive, but this depends
largely on species, size of organisms, handling practises (e.g.
sorting time), environmental conditions (e.g. air temperature),
and haul duration and depth (Broadhurst et al., 2006 ). For in-
stance, many crustaceans typically incur < 50% discard mor-
talities, whereas small pelagic sh may suffer very high mor-
tality (reviewed by Broadhurst et al., 2006 ).
When comparing the FAO discard estimates covering four
decades (Alverson et al., 1994 ; Kelleher, 2005 ; Pérez Roda
et al., 2019 ), it is obvious that there has been a declining trend
from the late 1980s, as the latest discard estimate is less than
half of the initial estimate. The estimates from the current as-
sessment are consistent with the ndings of Zeller et al. (2018) ,
who found that annual discards peaked at around 19 million
tonnes in 1989 and gradually declined to under 10 million
tonnes by 2014.
Improved gear selectivity and reduction of shing effort
have contributed to the reduction of discards in many trawl
sheries in Europe, North America, and Australia (Kennelly
and Broadhurst, 2021 ). A major change has also been the in-
creased utilization of all species in trawl sheries of SE Asia,
where trawling has been largely non-selective and thus has re-
sulted in large volumes of juvenile sh, small-sized sh species,
and other organisms in the landings (Funge-Smith et al., 2012 ;
Suuronen et al., 2020 ). Most of these sh are now used in SE
Asia both for local markets and for aquaculture feed, and dis-
carding is uncommon. Increased use of trawl “bycatch” is also
growing in Africa and Latin America, leading to reduced dis-
cards.
The capture of endangered, threatened, or protected
species, such as rays, sharks, and sea turtles, as well as juveniles
of target species, remains a cause of concern in some trawl
sheries (Gray and Kennelly, 2018 ). They estimated that 19%
of sea turtles discarded globally at sea were taken by trawls
(both pelagic and bottom), that the extensive Alaska bottom-
trawl shery annually discarded 534 seabirds, the Argentine
factory trawl eet discarded 8500 seabirds and suggest that
the global trawl impact on seabirds may be on the same order
as the longline eets.
Carbon footprint of fuel use
The majority of the carbon footprint of capture sheries
comes from the fuel used, and Parker and Tyedmers (2015) as-
Ta b l e 3. Kg CO
2
per kg of processed product from life cycle analysis.
Food type
Kg
CO2/kg
Corn 0 .10
Wheat 0 .23
Rice 0 .33
Tofu 0 .60
Potatoes 0 .80
Alaska pollock shery 0 .83
Alaska bottom-trawl shery 1 .17
Isle of Man scallop shery 1 .73
New Zealand hoki and ling 2 .24
Chicken 2 .28
Pork 2 .92
Impossible Burger 3 .50
Bottom-trawl sheries average 4 .65
Farmed Salmon Norway 5 .50
Beef 19 .20
Data sources: crops and livestock from Poore and Nemecek (2018) ; Pollock
from Zhang et al. (2022) ; Alaska bottom trawl converted by ratio of fuel
used in pollock shery (Fissel et al., 2016 ); scallop shery (Bloor et al., 2021 );
Impossible Burger (Khan et al., 2019 ); New Zealand (Mazzetto and Ledgard
2023, ); Norwegian farmed salmon (Ziegler and Hilborn, 2023 ).
sembled an impressive collection of 878 studies of fuel use in
sheries since 1990, measured as litres of fuel used per metric
tonne (MT) landed. The data are predominantly from Europe,
North America, and Oceania, with few studies from Africa or
Asia. For bottom trawl gear, Europe had a fuel consumption
per MT landed that was 1.8 times as high as North America
and Oceania. Table 2 shows the fuel use and carbon released
by fuel use for different shing gears.
The most important feature of these data is the high vari-
ability within and among different sheries, indicating that
almost any shing gear type can catch sh with a much lower
carbon footprint than the average, and no method is consis-
tently best. Nevertheless, bottom trawls are among the least
fuel-efcient gear types. Two-thirds of the bottom trawl data
set is from Europe, and many of the data are from the 1990s,
a time of low stock status and highly competitive sheries (i.e.
greater shing effort was required to catch the same amount of
sh relative to when stock status was more abundant). In con-
trast, trawl sheries for stocks at high abundance and where
the race-to-sh has been eliminated by the allocation of quota
to cooperatives have much lower fuel use and carbon foot-
print (Fissel et al., 2016 ). Tw o Alaskan trawl sheries have
quite low carbon footprint per unit of edible product (0.83
and 1.17 kg CO
2
/kg; see Tab l e 3 ) and exemplify how the car-
bon footprint of trawling can be reduced by maintaining high
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1572 R. Hilborn et al.
stock size and eliminating the race-to-sh and sets a standard
for other trawl sheries to aspire to. The New Zealand deep-
water trawl eet has a carbon footprint of 2.24 kg CO
2
/kg
(Mazzetto and Ledgard, 2023 ). Similarly, a well-managed ter-
ritorial use rights-based scallop dredge shery in the Isle of
Man (Irish Sea) resulted in emissions of 1.73 kg CO
2
/kg of
scallop meat, compared with up to 4.07–13.61 kg CO
2
/kg
scallop meat in the adjacent open access scallop shery (Bloor
et al., 2021 ). At present, both the Alaskan and Isle of Man
sheries are dominated by older vessels, and it would be ex-
pected that newer, more fuel-efcient vessels could reduce the
carbon footprint further.
Impact of tr a wling on carbon sequestration
Carbon stocks in seabed sediments are a large natural as-
set (e.g. 0.52 Pg of organic and 2 Pg of inorganic carbon in
UK waters) (Parker et al., 2020 ; Smeaton and Austin, 2022 ),
and bottom-trawl shing is the most extensive anthropogenic
physical disturbance to these sediments (Legge et al., 2020 ).
The impacts of shing on carbon stocks are currently unquan-
tied and unregulated. The available evidence suggests that
the seabed disturbance could result in greenhouse gas release
(CO
2
, CH
4
, and others) from the seabed into the water col-
umn (Epstein et al., 2022 ). A global extrapolation by Sala et al.
(2021) suggested that seabed disturbance with mobile shing
gears releases 0.16–0.4 Pg carbon per year to the ocean, but
this estimate has been widely criticized and is likely to be two
orders of magnitude too high (Epstein et al., 2022 ) (Hiddink
et al., 2023 ), meaning that mineralization of benthic carbon
stores comes primarily from natural processes.
This controversy has highlighted major uncertainties in the
magnitude and even the direction of the response of sediment
carbon stores due to sediment mixing, resuspension, and a re-
duction in the bioturbation activity as a result of the loss of
benthic fauna following trawling disturbance (Smeaton et al.,
2021 ; Epstein et al., 2022 ). Knowledge about how these effects
translate into changes in carbon storage and uxes into or out
of seabed sediments and across the air-sea interface showed
that of 49 investigations reporting the effect of bottom trawl-
ing on seabed carbon, 61% of studies showed no signicant
effect, 29% reported lower organic carbon after shing, and
10% reported higher seabed organic carbon after shing (Ep-
stein et al., 2022 ). Only ve studies have estimated changes
in carbon mineralization and O
2
uptake, and the majority of
these recorded a decrease rather than an increase in CO
2
pro-
duction with trawling (e.g. Polymenakou et al., 2005 ). With
respect to potential impacts on climate change, even if trawl-
ing does signicantly increase the mineralization of seabed
carbon, only a fraction of it would make it into the atmo-
sphere (Collins et al., 2022 ). We conclude that there is little
evidence that trawling increases sediment carbon mineraliza-
tion signicantly, even less that it impacts atmospheric CO
2
levels, but uncertainty certainly remains.
Int er action of bottom trawling and hypoxia
Marine benthic habitats in continental shelf regions are
increasingly impacted by hypoxia [dissolved oxygen (DO)
≤2 mg L
−1
] caused by the combination of eutrophication
and climate warming. Environmental hypoxia has been doc-
umented in over 400 marine systems globally and affects
> 240000 km
2
of coastal habitat (Diaz and Rosenberg, 2008 ;
Breitburg et al., 2018 ). The combined effects of trawling
and hypoxia on benthic community biomass and seabed pro-
cesses may be synergistic and disproportionally impact ben-
thic fauna, or trawl impacts may be smaller in hypoxic ar-
eas. Despite the high annual trawling intensities in the south-
ern Baltic Sea (each square metre of bottom is trawled seven
times per year on average), van Denderen et al. (2022) found
that the benthic community was predominantly impacted by
low oxygen concentrations (DO at sites studied ranged be-
tween 0.8 and 5.8 ml O
2 L
−1
) and found neither an effect of
trawling nor a synergistic effect of trawling and hypoxia. In
such cases, benthic communities may be expected to benet
most from management actions targeting reductions of nu-
trient loads and reversing eutrophication and hypoxia. Con-
versely, management efforts for regulating trawling are better
targeted to regions that are not in a prolonged state of hy-
poxia.
Hypoxia has also been demonstrated to alter catch and
effort patterns. Purcell et al. (2017) showed that hypoxia-
induced changes in the distribution of shrimp also alter the
spatial dynamics of the Gulf of Mexico shrimp eet, with po-
tential consequences for harvest interactions and the economic
condition of the shery. Bio-economic simulations of the Gulf
shrimp trawl shery suggest that hypoxia can lead to both
short-term increases or decreases in catch, depending on the
effects of hypoxia on components of shrimp production (e.g.
growth, mortality) and the behaviour of the shery (e.g. catch-
ability) (Smith et al., 2014 ).
Is the tr a wl f ootprint expanding?
A common perception of trawling is that it is expanding
worldwide and new areas are being impacted each year. Some
have compared trawling to forest clear cutting and stated that
the area trawled each year, estimated from trawl effort, speed,
and width of trawl nets, is 150 times the area of forest clearcut
(Watling and Norse, 1998 ). The obvious aw in this analogy
is that, for the most part, the same areas are trawled each year,
and indeed, in some cases, many times each year, but you can-
not clearcut the same area twice.
Amoroso et al. (2018, SM) calculated the increase in the cu-
mulative area impacted by trawling as a function of the num-
ber of years considered using data from 32 regions of the con-
tinental shelf. They found that the trawling footprint tended to
be rather stable, especially in mid-to-highly impacted regions.
For example, in regions where > 30% of grid cells were annu-
ally impacted by trawling, the cumulative number of cells im-
pacted over a three-year period was at most 40% larger than
the annual impact, indicating a substantial overlap in shing
areas from year to year. Using detailed tow-by-tow data by
individual vessel in the British Columbia bottom trawl eet,
Branch et al. (2005) showed that each vessel shed over a lim-
ited number of standard locations (an average of 26 per ves-
sel), where the vessel had previously shed, and exploration
of new shing grounds was uncommon.
Certainly some new areas have been explored, particularly
in deeper waters as gear technology has permitted deeper
tows, and as species distributions shift shing effort may also
shift. For the major bottom-trawl sheries on groundsh (cod,
pollock, haddock, hake, and atsh), the annual harvest rates
and catches have been declining, the total effort declining, and
hence the area trawled is presumably also declining (Hilborn
et al., 2021 ). However, without a longer time series of spatial
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Sustainability of trawling 1573
data on trawl effort, it is difcult to determine if the extent of
bottom trawl footprints is expanding.
Conicts with other shing gears and ocean
uses
Bottom-trawl sheries have a long history of conict with
static shing gears that lie on the bottom, such as longlines,
gillnets, and pots, and when shing grounds overlap, interfer-
ence may result in xed gear losses and hazards for the trawls.
This has led, in some circumstances, to formal or informal
zoning or rotational arrangements. In many cases, inter-gear
conicts reect competition for the same target resources be-
tween small and large-scale eets, which has led to the estab-
lishment of exclusive coastal zones for artisanal or small-scale
sheries where trawling is banned (McConnaughey et al.,
2020 ). An example of this is the Inshore Potting Agreement
(IPA), a voluntary shery management system designed and
operated by shers of south Devon, England to reduce con-
ict between static-gear (pot and net) and towed-gear (trawl
and dredge) shers. The IPA is regarded as a successful sh-
eries management regime by shers and managers because it
has effectively allowed shers from both sectors to operate
protably on traditional shing grounds (Hart et al., 2002 ).
Oil and gas pipelines and communication cables laid on the
seaoor are also typically in conict with sheries, and new
demands on the seaoor, such as wind farms (Rodmell and
Johnson, 2002 ; Stokesbury et al., 2022 ), tidal power, and
seabed mining, have added to the competition for space. On
the West Coast of the United States, communication compa-
nies negotiated nancial arrangements with trawl eets, pro-
viding research funds administered by the trawl-shing orga-
nizations ( https:// bandoncable.org/ history.asp ).
Management actions to reduce impacts
A variety of management measures reduce the impacts of bot-
tom trawling on benthic biota and habitats, minimize bycatch,
and reduce fuel usage to address sustainability goals. These
measures, voluntary industry actions, and their interactions
with existing management systems address conicting soci-
etal, environmental, and economic objectives, often requiring
trade-offs. They broadly consist of technical measures related
to gear and operations, spatial controls, impact quotas, and
shing-effort controls. Their efcacy and practicality, alone or
in combination, depend on the characteristics of the shery,
the management capacity, and the local tradeoffs between en-
vironmental effects, food security, income, and employment.
Guidance has been proposed to evaluate potential best prac-
tises for a region (McConnaughey et al., 2020 ). In most cases,
compliance and performance are predicated on stakeholder
engagement (Suuronen et al., 2020 ; Suuronen, 2022 ).
Direct impacts on the benthos can be signicantly reduced
by gear modications that reduce contact with the seaoor
and/or penetration depth while maintaining or increasing the
catchability of the target species. Impacts have been reduced
with otter trawl doors that do not touch the bottom, ele-
vated footropes, and the use of electricity to cause the sh to
swim into a net that is not making bottom contact (Delaney
et al., 2022 ). An absolute prohibition of bottom trawling is the
most comprehensive measure of protection and typically pro-
vides additional shing opportunity to alternative gears and
thus has been advocated for reasons other than conservation
(Blyth-Skyrme et al., 2006 ). At the same time, absolute pro-
hibition directly affects those employed in the trawl indus-
try and may cause redistribution of effort if the prohibition
is localized. Alternative trawl restrictions include freezing the
trawling footprint to prevent expansion into previously un-
trawled areas, but this limits a eet’s adaptability to changing
sh distributions.
Particularly sensitive habitats, such as coral, sponge, and
nearshore nurseries, can be effectively protected when their
locations are known and closures are implemented prior to
signicant disturbance. Substantial invertebrate bycatch can
be mitigated by voluntary or regulated movement to other
areas with real-time reporting and closures; however, such
“move-on” rules displace effort to similar areas, thereby ex-
panding the overall footprint and its effects. When move-on
rules were combined with tradable quotas, detailed maps of
sensitive areas, and onboard observers, a substantial reduc-
tion in invertebrate bycatch was achieved in British Columbia,
Canada without affecting overall eet performance (Groen-
baek et al., 2023 ). Perhaps the simplest change is to reduce
shing effort when overshing occurs. This reduces impacts
on benthic biota and increases shery yield (Amoroso et al.,
2018 ; McConnaughey et al., 2020 ), which may confer eco-
nomic benets due to trip reductions and lower fuel usage but
would normally have short-term negative economic impacts.
Fuel consumption is the primary source of the carbon foot-
print for all shing vessels. Gear modications that reduce
contact with the seaoor reduce fuel consumption and ex-
tend gear life, which improves overall protability if target-
species catchability is maintained or nearly so. However, in
some sheries, there is a trade-off between the catchability of
the target species and bycatch reduction. Gear that reduces by-
catch may require more effort (and fuel) to achieve the same
landings. Management measures that increase target-species
abundance will normally be expected to increase catch rates
and thus lower fuel use per tonne captured. Newly constructed
vessels tend to have reduced fuel use as a major design crite-
rion.
Many of the same measures that reduce benthic impacts
and reduce fuel use are also used to manage bycatch and re-
duce discards. Technical, administrative, and economic mea-
sures include modications to shing gear or shing practises,
time and area restrictions, bycatch limits, effort restrictions,
and discard bans (i.e. landing obligations), and may also lead
to active avoidance of high bycatch areas and involve coop-
erative eet communications, awareness raising, and training
(Pascoe, 1997 ; Suuronen and Gilman, 2020 ; Suuronen et al.,
2020 ). Technical measures to manage trawling bycatch are
based on a large body of empirical experiments intended to
improve species- and size-selectivity by modifying gear and
operations (Kennelly and Broadhurst, 2021 ), with attention
paid to unobserved mortality rates (Rose et al., 2013 ). Real-
time closures involving move-on protocols may be effective in
dynamic situations where the bycatch level is unpredictable.
Bycatch quotas or limits on “choke species” are incentives
to avoid premature closures of target sheries before quota
uptake is achieved. Measures to limit effort are based on the
simple rationale that less effort equates to less bycatch (Alver-
son et al., 1994 ). An outright discard ban, where all catches
of species or stocks with an established TAC or covered by
minimum landing size regulations must be kept on board,
landed, and deducted from established quotas, was imple-
mented by the EU Common Fisheries Policy and represents a
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1574 R. Hilborn et al.
fundamental regulatory shift from landings to catches (Karp
et al., 2019 ), but has proven ineffective because of numerous
exceptions and the difculty in implementation and enforce-
ment (Uhlmann et al., 2019 ; Borges, 2021 ).
Management measures that minimize the footprint of sh-
ing have been shown in one study to lead to higher yields than
measures that spread shing activity more widely and evenly
across the seabed (Bloor et al., 2021 ). This was demonstrated
in a case study in the Isle of Man, where a territorial use rights-
based shery ring-fenced vulnerable habitat from shing while
demarcating a shing zone within the management system.
Pre-open season shery surveys directed shing activity specif-
ically to high-density aggregations of target species (scallops),
thereby increasing the efciency with which the total allow-
able catch was taken and reduced the amount of seabed im-
pacted to a negligible level (3% of the available area for sh-
ing; Bloor et al., 2021 ). Using such approaches or regulating
the overall