A global review of marine turtle entanglement in anthropogenic debris: A baseline for further action

Abstract

Entanglement in anthropogenic debris poses a threat to marine wildlife. Although this is recognised as a cause of marine turtle mortality, there remain quantitative knowledge gaps on entanglement rates and population implications. We provide a global summary of this issue in this taxon using a mixed methods approach including a literature review and expert opinions from conservation scientists and practitioners worldwide. The literature review yielded 23 reports of marine turtle entanglement in anthropogenic debris, which included records for 6 species, in all ocean basins. Our experts reported the occurrence of marine turtles found entangled across all species, life stages and ocean basins, with suggestions of particular vulnerability in pelagic juvenile life stages. Numbers of stranded turtles encountered by our 106 respondents were in the thousands per year, with 5.5% of turtles encountered entangled; 90.6% of these dead. Of our experts questioned, 84% consider that this issue could be causing population level effects in some areas. Lost or discarded fishing materials, known as ‘ghost gear’, contributed to the majority of reported entanglements with debris from land-based sources in the distinct minority. Surveyed experts rated entanglement a greater threat to marine turtles than oil pollution, climate change and direct exploitation but less of a threat than plastic ingestion and fisheries bycatch. The challenges, research needs and priority actions facing marine turtle entanglement are discussed as pathways to begin to resolve and further understand the issue. Collaboration among stakeholder groups such as strandings networks, the fisheries sector and the scientific community will facilitate the development of mitigating actions.

Full text

ENDANGERED SPECIES RESEARCH
Endang Species Res
Vol. 34: 431–448, 2017
https://doi.org/10.3354/esr00865 Published December 11
INTRODUCTION
Marine plastic pollution
Anthropogenic materials, the majority of them plas-
tic, are accumulating on the surface of the oceans, in
the water column and on the seabed (Thompson et al.
2004). The durability of plastic means that it may per-
sist for centuries (Barnes et al. 2009). It is estimated
that 4.8 to 12.7 million tonnes of plastic waste could be
entering the marine environment annually (Jambeck
et al. 2015). Over 700 marine species have been
demonstrated to interact with marine plastic pollution
(Gall & Thompson 2015), which presents a risk to ani-
© The authors 2017. Open Access under Creative Commons by
Attribution Licence. Use, distribution and reproduction are un -
restricted. Authors and original publication must be credited.
Publisher: Inter-Research · www.int-res.com
*These authors contributed equally to this work
**Corresponding author: b.j.godley@exeter.ac.uk
REVIEW
A global review of marine turtle
entanglement in anthropogenic debris:
a baseline for further action
Emily M. Duncan1,2, 3,*, Zara L. R. Botterell1,*, Annette C. Broderick1,
Tamara S. Galloway2, Penelope K. Lindeque3, Ana Nuno1, Brendan J. Godley1,**
1Marine Turtle Research Group, Centre for Ecology and Conservation, University of Exeter, Penryn TR10 9FE, UK
2Biosciences, College of Life and Environmental Sciences, Geoffrey Pope Building, University of Exeter, Stocker Road,
Exeter EX4 4QD, UK
3Marine Ecology and Biodiversity, Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth PL1 3DH, UK
ABSTRACT: Entanglement in anthropogenic debris poses a threat to marine wildlife. Although
this is recognised as a cause of marine turtle mortality, there remain quantitative knowledge gaps
on entanglement rates and population implications. We provide a global summary of this issue in
this taxon using a mixed methods approach, including a literature review and expert opinions
from conservation scientists and practitioners worldwide. The literature review yielded 23 reports
of marine turtle entanglement in anthropogenic debris, which included records for 6 species, in all
ocean basins. Our experts reported the occurrence of marine turtles found entangled across all
species, life stages and ocean basins, with suggestions of particular vulnerability in pelagic juve-
nile life stages. Numbers of stranded turtles encountered by our 106 respondents were in the thou-
sands per year, with 5.5% of turtles encountered entangled; 90.6% of these dead. Of our experts
questioned, 84% consider that this issue could be causing population level effects in some areas.
Lost or discarded fishing materials, known as ‘ghost gear’, contributed to the majority of reported
entanglements with debris from land-based sources in the distinct minority. Surveyed experts
rated entanglement a greater threat to marine turtles than oil pollution, climate change and direct
exploitation but less of a threat than plastic ingestion and fisheries bycatch. The challenges,
research needs and priority actions facing marine turtle entanglement are discussed as pathways
to begin to resolve and further understand the issue. Collaboration among stakeholder groups
such as strandings networks, the fisheries sector and the scientific community will facilitate the
development of mitigating actions.
KEY WORDS: Conservation · Entanglement · Ghost fishing · Marine debris · Plastic pollution ·
Sea turtle · Strandings
O
PEN
PEN
A
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Endang Species Res 34: 431–448, 2017
mals through ingestion, entanglement, degradation of
key habitats and wider ecosystem effects (Nelms et al.
2016). Megafauna such as marine turtles with complex
life histories and highly mobile behaviour are particu-
larly vulnerable to its impacts (Schuyler et al. 2014).
Entanglement in marine litter
Entanglement in plastic debris is recognised as a
major risk for many marine species (Laist 1987,
Vegter et al. 2014). This has become sufficiently high
profile that the European Union’s Marine Strategy
Framework Directive (MSFD) Technical Subgroup
on Marine Litter has announced that it will develop a
dedicated monitoring protocol for its next report
(MSFD GES Technical Subgroup on Marine Litter
2011). Entanglement has the potential to cause a
range of fatal and non-fatal impacts such as serious
wounds leading to maiming, amputation, increased
drag, restricted movement or choking (Votier et al.
2011, Barreiros & Raykov 2014, Lawson et al. 2015).
Types of marine debris causing entanglement
The debris causing this entanglement falls into 2
broad categories. Firstly, hundreds of tons of fishing
gear are lost, abandoned or discarded annually,
forming ‘ghost gear’ which passively drifts over large
distances, sometimes indiscriminately ‘fishing’ mar-
ine organisms (Macfadyen et al. 2009, Wilcox et al.
2013). This gear is commonly made of non-bio -
degradable synthetic material that will persist in the
marine environment, potentially become biofouled
by marine organisms and act as a fish aggregating
device (FAD), attracting both grazers and predators
such as marine turtles (Filmalter et al. 2013, Wilcox et
al. 2013). It is important to distinguish here between
‘entanglement’ and ‘bycatch’. Bycatch can be de -
fined as unselective catch of either unused or unman-
aged species during fishing, with a particular focus
on ‘active’ gear, whereas ghost gear can be defined
as equipment of which the fisher has lost operational
control (Smolowitz 1978, Davies et al. 2009). There-
fore, here we consider animals caught in passive
ghost fishing gear as entangled, not bycaught. Sec-
ondly, there have also been reports of entanglement
in litter from land-based sources (Chatto 1995, Ben-
tivegna 1995, Santos et al. 2015). In this review we do
not include bycaught turtles — only those that have
become entangled in passive anthropogenic debris
such as ghost gear or land-based debris.
Current knowledge gaps regarding
turtle entanglement
Despite turtle entanglement being recognised as
one of the major sources of turtle mortality in north-
ern Australia and the Mediterranean, there is a quan-
titative knowledge gap with respect to the entangle-
ment rates and possible implications in terms of global
populations (Casale et al. 2010, Wilcox et al. 2013,
Camedda et al. 2014, Gilman et al. 2016). A re cent lit-
erature review by Nelms et al. (2016) returned only 9
peer-reviewed publications on marine debris entan-
glement and turtles (Bentivegna 1995, Chatto 1995,
López-Jurado et al. 2003, Casale et al. 2010, Santos et
al. 2012, Jensen et al. 2013, Wilcox et al. 2013, 2015,
Barreiros & Raykov 2014). Of these, 7 were focused
on ghost fishing gear, highlighting the distinct lack of
knowledge of entanglement in debris from land-
based sources. Even fewer of these studies focused
on the potential variable susceptibility among life
stages or species, with only one paper, Santos et al.
(2012), reporting that the majority of entangled olive
ridley turtles Lepidochelys olivacea on the Brazilian
islands of Fernando de Noronha and Atol das Rocas
were sub-adults and adults.
Research rationale in terms of marine turtles
and pollution
In terms of global research priorities for sea turtle
conservation and management, understanding the
impact of pollution is considered of high importance
(Hamann et al. 2010, Rees et al. 2016). To evaluate
this effectively, the impact of anthropogenic debris,
specifically, must be considered at a species and pop-
ulation level. Additionally, it is important to under-
stand the variation in entanglement rates among
species and life stages to better evaluate vulnerabil-
ity and the frequency of interactions with different
debris types (Nelms et al. 2016). Once these have
been established, opportunities for delivering effec-
tive education and awareness can be given or other
mitigation planned (Vegter et al. 2014).
Here, we define marine turtle entanglement as
‘the process under which a marine turtle becomes
entwined or trapped within anthropogenic materi-
als.’ We sought to include discarded fishing gear
(ghost fishing) as well as land-based sources. The
aim of this study was to (1) review existing, and
obtain new, reports of the occurrence and global spa-
tial distribution of marine turtle entanglement; (2)
gain insights into patterns of species, life stage and
432

Duncan et al.: Marine turtle entanglement
debris type involved across entanglement cases; and
(3) glean an insight into the change in prevalence of
marine debris entanglement over time. To address
these, a mixed methods approach was employed, in -
volving a literature review and an elicitation of ex -
pert opinions. Given the difficulty of acquiring robust
standardised data, this review is intended to high-
light the value of mixed methods as a first step to
understand complex conservation issues, and to pro-
vide suggestive yet relevant indications as to the
scale of the threat of entanglement to marine turtles.
MATERIALS AND METHODS
Literature review
In January 2016 and again in June 2017 (during the
manuscript review process), all relevant literature
was reviewed that may have contained records of
marine turtle entanglement. ISI Web of Knowledge,
Google Scholar and the Marine Turtle Newsletter
(www.seaturtle.org) were searched for the terms ‘en -
tanglement’, ‘entrapment’, ‘ensnare’ or ‘ghost fishing’
and ‘turtle’. The first 200 results were viewed, with
results very rarely fulfilling the criteria after the first
20; spurious hits were ignored and all relevant refer-
ences were recorded and investigated.
Elicitation of expert opinions
During the period 1–30 April 2016, an online ques-
tionnaire survey was conducted to investigate 3 main
topics of interest: (1) the occurrence and global spa-
tial distribution of sea turtle entanglement; (2) spe-
cies, life stage and debris type involved; and (3) the
change in entanglement prevalence over time. A
total of 20 questions requiring both open and closed
responses from a range of experts were used to
obtain insight into the scale of marine turtle entan-
glement. We clearly explained to the respondents the
definition of ‘marine turtle entanglement’ specific to
this study. Grid-like responses and Likert scales,
offering potential answers from a range of ordinal
options, were used to aid in achieving a quantitative
assessment of the issues (Elaine & Seaman 2007) (see
Box S1 in the Supplement at www. int-res. com/
articles/ suppl/ n034 p431 _ supp. pdf).
Potential participants for this questionnaire were
identified from lead authorship of papers compiled in
the recent review on the effects of marine plastic
debris on turtles from Nelms et al. (2016), and our
review due to their involvement in research into
marine debris. From reviewing the few published
reports, it was apparent that governmental stranding
networks, sea turtle rescue and rehabilitation centres
and conservation projects may also hold many un -
published records of entanglement occurrence. A
comprehensive list of such organisations from sea -
turtle.org (www.seaturtle.org/groups/; accessed 24
March 2016) was used to find more expert contacts to
participate in the questionnaire. Additionally, con-
sidering the aim of attaining an appropriate number
of respondents while avoiding potential sampling
biases due to researchers’ personal networks and per-
ceptions about the issue (Newing 2011), we employed
respondent-driven sampling; this purposive sam-
pling approach involves requesting those directly
contacted to recruit additional participants among
colleagues, peers and other organisations that may
have knowledge of additional records of marine
turtle entanglement.
From this first questionnaire, an initial report was
produced and sent to the expert respondents (n =
106) to share the results and thoughts that arose from
the first questionnaire. This included 8 initial figures
produced from the data given by respondents in the
original questionnaire to aid feedback of our results
(these were draft versions of Figs. 2, 3 & 4). Following
this, during the period 24 May to 30 June 2016, a fol-
low-up questionnaire survey was conducted with the
expert participants of the first questionnaire survey
who were then invited to comment and answer 10
open and closed questions (see Box S2 in the Supple-
ment). This aimed to further understand the chal-
lenges, future requirements (both research and prior-
ity actions) and perceptions of the likelihood of
population level effects of marine turtle entangle-
ment. In this second questionnaire, respondents
were asked to comment on our initial results and to
provide suggestions on future knowledge gains and
actions. Their answers were categorised using an
inductive approach; summary themes were identi-
fied through the process of directly examining the
data (Elo & Kyngäs 2008), instead of having pre-
defined categories.
RESULTS
Literature review
Our literature search yielded 23 reports regarding
entanglement in multiple species of marine turtles,
the majority of which were peer-reviewed publica-
433

Endang Species Res 34: 431–448, 2017
tions (n = 17) with additional grey literature reports
(n = 6). Species included loggerhead Caretta caretta
(n = 7), green Chelonia mydas (n = 7), leatherback
Dermochelys coriacea (n = 5), hawksbill Eretmo -
chelys imbricata (n = 5), olive ridley Lepido chelys oli-
vacea (n = 9) and flatback Natator depressus (n = 2).
There were no records for Kemp’s ridley Lepi-
dochelys kempii (Table 1). Of these publications, 18
reported entanglement due to ghost fishing or fish-
eries materials and 7 recorded entanglement in land-
based plastic debris; 7 publications reported the size
range and life stage of the entangled turtles. These
publications highlighted a range of impacts of entan-
glement, such as serious wounds leading to maiming,
amputation or death, increased drag, restricted
movement or choking that were further illustrated by
photographs from collaborating experts (Fig. 1).
Elicitation of expert opinions
Survey response rates and demographics
From an estimated pool of ca. 500 potential contacts,
the ‘Marine Turtle Entanglement Questionnaire’ was
received and completed by a total of 106 expert re-
spondents from 43 countries. However, due to the
anonymous nature of the survey and the potential
augmentation from the use of respondent-driven sam-
pling, it is not possible to determine how many of
those initially contacted took part in the survey. All
ocean basins were covered; the respondents’ main
oceanic region of work was given as: Atlantic (34.8%;
n = 39), Pacific (18.9%; n = 20), Caribbean (25.5%;
n = 27), Mediterranean (9.4%; n = 10) and Indian
(9.4%; n = 10). Respondents experienced a wide
434
Fig. 1. Impacts of marine turtle entanglement: (a) live leatherback turtle entangled in fishing ropes which increases drag,
Grenada 2014 (photo: Kate Charles, Ocean Spirits); (b) drowned green turtle entangled in ghost nets in Uruguay (photo:
Karumbe); (c) live hawksbill turtle entangled in fishing material constricting shell growth, Kaeyama Island, Japan 2001 (photo:
Sea Turtle Association of Japan); (d) live hawksbill turtle with anthropogenic debris wrapped around front left flipper con-
stricting usage of limb which could lead to amputation and infection, Kaeyama Island, Japan 2015 (photo: Sea Turtle Associa-
tion of Japan). All photos used with express permission

Duncan et al.: Marine turtle entanglement 435
Ocean basin/ Study area Reference Year of N CCL Pelagic Neritic Adult Debris
Species study range juvenile juvenile type
Loggerhead turtle Caretta caretta
Atlantic Ocean Northeastern (Boa Vista, López-Jurado et al. (2003) 2001 10 62.0−89.0 X ✓✓Fishing
Cape Verde Islands)
Northeastern Barreiros & Raykov (2014) 2004 −2008 3 37.3−64.1 X ✓✓Fishing/land-based
(Terceira Island, Azores)
Northeastern Orós et al. (2016) 1998−2014 945 Unknown ✓✓✓Fishing/land-based
(Gran Canaria, Spain)
Mediterranean Tyrrhenian sea Bentivegna (1995) 1994 1 48.5 X ✓X Land-based
Sea (Island of Panarea, Sicily)
Central Mediterranean (Italy) Casale et al. (2010) 1980−2008 226 3.8−97.0 ✓✓✓Fishing/land-based
South Tyrrhenian sea Blasi & Mattei (2017) 2009−2013 5 Unknown na na na Fishing/land-based
Global Balazs (1985) 1967−1984 5 Unknown ✓✓✓Fishing
Green turtle Chelonia mydas
Indian Ocean North (Maldives) Stelfox & Hudgins (2015) 2013−2015 2 Unknown na na na Fishing
Northeastern (Darwin, Australia) Chatto (1995) 1994 1 35 X ✓X Fishing
Northeastern (Australia) Wilcox et al. (2013) 2005−2009 14 Unknown na na na Fishing
Global Balazs (1985) 1967−1984 24 Unknown ✓✓✓Fishing (21), land-based (3)
Pacific Ocean Central (Hawaii) Francke et al. (2014) 2013−2014 51 Unknown ✓✓✓Fishing
Chaloupka et al. (2008) 1982−2003 43 20.0−100.0 ✓✓✓Fishing
Caribbean Sea Southeastern (Venezuela) Barrios-Garrido et al. (2013) 2013 1 Unknown na na na Fishing
Leatherback turtle Dermochelys coriacea
Indian Ocean North (Maldives) Stelfox & Hudgins (2015) 2013−2015 1 Unknown na na na Fishing
Pacific Ocean Northeastern (USA) Moore et al. (2009) 2001−2005 1 Unknown na na na Fishing
Atlantic Ocean Northwestern (USA) Hunt et al. (2016) 2007−2013 8 Unknown na na na Fishing
Northwestern (USA) Innis et al. (2010) 2007−2008 7 Unknown na na na Fishing
Global Balazs (1985) 1967−1984 5 Unknown X X ✓Fishing
Hawksbill turtle Eretmochelys imbricata
Indian Ocean North (Maldives) Stelfox & Hudgins (2015) 2013−2015 6 Unknown X ✓X Fishing
Northeastern (Darwin, Australia) Chatto (1995) 1994 1 32.5 X ✓X Fishing
Northeastern (Australia) Wilcox et al. (2013) 2005−2009 35 Unknown na na na Fishing
Northeastern White (2006) 2004 2 Unknown X ✓X Fishing
(Northern Territory, Australia)
Global Balazs (1985) 1967−1984 9 Unknown ✓✓✓Fishing (8), land-based (1)
Olive ridley turtle Lepidochelys olivacea
Indian Ocean North (Maldives) Anderson et al. (2009) 1998−2007 25 10.0−61.0 ✓✓X Fishing (22), land-based (3)
North (Maldives) Stelfox & Hudgins (2015) 2013−2015 163 Unknown ✓✓✓Fishing
Northeastern Jensen et al. (2013) Unknown 44 Unknown na na na Fishing
(McCluer Island, Australia)
Northeastern (Australia) Wilcox et al. (2013) 2005−2009 53 Unknown na na na Fishing
Northeastern (Darwin, Australia) Chatto (1995) 1994 2 64 X X ✓Fishing
Northwestern (Seychelles) Remie & Mortimer (2007) 2007 1 Unknown X ✓X Unspecified
Atlantic Ocean Southwestern (Brazil) Santos et al. (2012) 1996−2011 18 2.01−80.0 X ✓✓Fishing
Global Balazs (1985) 1967−1984 7 Unknown ✓✓✓Fishing
Pacific Ocean Central (Hawaii) Francke et al. (2014) 2013−2014 1 Unknown na na na Fishing
Flatback turtle Natator depressus
Indian Ocean Northeastern (Darwin, Australia) Chatto (1995) 1994 1 25.5 X ✓X Land-based
Northeastern (Australia) Wilcox et al. (2013) 2005−2009 3 Unknown na na na Fishing
Multiple
Indian Ocean Northeastern (Australia) Wilcox et al. (2015) 2005−2012 336 Unknown na na na Fishing
Pacific Ocean Southwestern (Australia) Meager & Limpus (2012) 2011 5 Unknown na na na Fishing
Table 1. Summary of all studies on entanglement of marine turtles in plastic debris. CCL: curved carapace length (cm); na: not available

Endang Species Res 34: 431–448, 2017
range in the number of annual stranding cases in their
respective study sites (annual maxima given in the
survey; mean ± SE = 239.9 ± 71.7, range = 0 to 4100,
n = 97) but in total, through addition of the respon-
dents’ answers, they are responsible for at tending an
estimated 23 000 stranded turtles yr−1. Respondents
also generally had many years of ex perience dealing
with and reporting marine turtle strandings (range = 2
to 42 yr, mean ± SE = 15.6 ± 1.1, n = 98), confirming
them as having relevant ex perience to answer the
survey. The second follow-up questionnaire sent to all
respondents (n = 106) received 63 responses with re-
spondents from 31 countries.
Rates of entanglement
A majority of respondents (84.3%; n = 101) had
encountered cases in which turtles were entangled in
anthropogenic debris. When broken down by spe-
cies, the proportion of stranded turtles that were
entangled did not differ significantly (Kruskal-Wallis:
χ2= 4.59, df = 6, p = 0.59) (Fig. 2a). There was a low
percentage incidence for all species, with the grand
median rate of 5.5%, although there was consider-
able inter- and intraspecific variation, with incidences
in different responses ranging from 0 to 95.5%. In
terms of the proportion of marine turtles alive when
found entangled, there were significant interspecific
differences (Kruskal-Wallis: χ2= 19.62, df = 6, p =
0.003). The proportion found alive (grand median =
9.4%) was significantly higher in green (25.5%) and
loggerhead (15.5%) turtles than in all other species
(5.5%) (Fig. 2b).
Entanglement rates also differed amongst life stages
for each species. Whilst respondents indicated that
all life stages of each species had been affected by
entanglement, the results suggested adults were
most impacted in leatherback and olive ridley turtles,
whereas for the remaining species respondents indi-
cated a higher rate of entanglement in juveniles
(pelagic and neritic; Fig. 3).
When considering this issue over time (over the last
10 yr), a similar proportion of respondents (35.8 % of
106) thought the prevalence of entanglement had
increased or remained the same, while the remainder
thought it had decreased (8.5%) or were unsure
(19.8%). Among those respondents that noted an
increase, some (n = 4) suggested that this may be
caused by an increase in reporting and awareness,
while others (n = 9) indicated the development of
coastal fishing activities might be a factor. When
asked to consider a shorter time period (the last 5 yr),
the majority of respondents believed that the pre -
valence of entanglement they had experienced had
remained stable (51.9%), whilst the others thought it
had increased (29.2%), decreased (3.8%) or were not
sure (15.1%).
Entanglement materials
The majority of entanglements recorded were with
lost/discarded fishing gear (Fig. 4). A clear distinc-
tion was made between ‘active’ and ‘lost/discarded’
436
0
20
40
60
80
100
CC CM DC EI LK LO ND
Species
% alive
0
20
40
60
80
100
a
b
CC CM DC EI LK LO ND
% entangled
Fig. 2. Inter-species comparison of the proportion of: (a)
stranded individuals found entangled and (b) individuals
found alive when discovered entangled. Violin plots show
the kernel density of data at different values. Median (black
dot) with interquartile range boxplot (black/white) and
grand median (black dashed line). Turtle species abbrevia-
tions: CC: loggerhead Caretta caretta; CM: green Chelonia
mydas; DC: leatherback Dermochelys coriacea; EI: hawks-
bill Eretmochelys imbricata; LK: Kemp’s ridley Lepidochelys
kempii; LO: olive ridley Lepidochelys olivacea; ND: flatback
turtle Natator depressus

Duncan et al.: Marine turtle entanglement
fishing gear to try and separate incidents due to
bycatch and subsequent stranding from those caused
by ghost fishing. The number of responses on the
occurrence of ghost fishing (GF) through discarded
fishing debris (rope, net and line) was generally
slightly higher than for bycatch (BC) through active
gear.
A smaller percentage of respondents specified
cases of turtle entanglement in land-based sources,
from polythene sheeting (n = 71), woven sacks (n =
72) and non-fishing rope/twine (n = 68). But in only a
few incidences were these said to be common oc -
currences (polythene sheeting [n = 3], woven sacks
[n = 4], non-fishing rope/twine [n = 7]). Respondents
were asked to comment on the occurrence of ‘other’
entangling materials (n = 54) and to provide exam-
ples (n = 20) that caused turtle entanglement. This
included debris from land-based sources (plastic -
balloon string, canned drink ‘6-pack’ rings, kite
string, plastic chairs, plastic packaging straps, wood -
en crates and weather balloons) and debris from
other maritime activities (boating mooring line, an -
chor line and discarded seismic cable).
Scale of issue
In order to obtain further insights into the potential
scale of this issue, respondents to the second survey
were asked whether they thought entanglement in
anthropogenic debris is causing population-level ef-
fect in marine turtles. Of the 63 respondents, 84.1%
thought that this was probable, very likely or definite
(see Fig. S1 in the Supplement). There was no signif-
icant difference in scaled responses by ocean basin
(Kruskal-Wallis: χ2= 1.82, df = 4, p = 0.77). In order to
assess the relative importance of different threats ac-
cording to experts, we also sought the experts’ opin-
ions on how they thought entanglement in anthro-
437
Fig. 3. Inter-specific comparison of the breakdown of entan-
gled sea turtle species by life stage. Black: pelagic juveniles
(PJ); white: neritic juveniles (NJ); light grey: juveniles (JV);
dark grey: adults (AD); see Fig. 2 for species abbreviations.
Flatback turtles were only categorised into juvenile or adult
classes with advice from species experts. Sea turtle skull fig-
ures used with permission of WIDECAST; original artwork
by Tom McFarland
0 20 40 60 80 100
L/DF net
L/DF line
A/F line
L/DF rope
A/F net
A/F rope
Other
Poly sheet
Woven sacks
Non-F rope/twine
n = 54
n = 71
No. of responses
n = 72
n = 68
n = 75
n = 81
n = 78
n = 79
n = 86
n = 81
Fig. 4. Entangling materials. L/DF: lost/discarded fishing;
A/F: active fishing; Non-F: non fishing; Poly sheet: poly -
ethylene sheeting. Black: common (10% or more of cases);
grey: sometimes (less than 10% of cases); white: never. Not
all participants categorised each material; total number of
responses for each material shown on the right of the graph

Endang Species Res 34: 431–448, 2017
pogenic debris compared to other threats to marine
turtles (i.e. ‘plastic ingestion’, ‘oil pollution’, ‘fisheries
bycatch’, ‘direct exploitation’ and ‘climate change’).
Although between 6.35 and 25.4% were unsure,
there was a strong opinion that plastic ingestion and
fisheries bycatch were greater threats, and that oil
pollution, climate change and direct exploi tation
were less severe threats than entanglement (Fig. 5).
Challenges, priority actions and research needs
Respondents to the second survey converged on a
limited number of themes when considering the
challenges, research needs and priority actions with -
in marine turtle entanglement. The challenges to
addressing the issue (115 suggestions) could be
grouped into 5 major categories: law and enforce-
ment (23.5%; n = 27); sources and spatial extent of
entanglement materials (24.3%; n = 28); education
and innovation (24.3%; n = 28); understanding the
full extent of the threat (18.3%; n = 21); and human
response to entangled turtles (9.6%; n = 11) (Table 2).
Seven major research areas were suggested by re -
spondents (91 suggestions): more specific reporting
and monitoring or a common database (23.1%; n =
21); mapping the threat/spatio-temporal hotspots
(31.9%; n = 29); identifying entanglement materials
and sea turtle inter actions (24.2%; n = 22); under-
standing post-release mortality and physical effects
(3.3%; n = 3); socio-economic impacts (4.4%; n = 4);
innovation of new replacement materials (6.6%; n =
6); and demographic risk assessments (6.6%; n = 6)
(Table 3). Priority actions (n = 121 suggestions) that
respondents believe would help re duce turtle entan-
glement were grouped into 5 major areas: educa-
tion/stakeholder engagement (31.4%; n = 38); fish-
eries management and monitoring (26.4%; n = 32);
research (5%; n = 6); law and enforcement (20.7 %;
n = 25); and development of alternative materials and
methods (16.5%; n = 20) (Table 4).
DISCUSSION
Global distribution
Our review and elicitation of expert opinions de -
monstrate that marine turtle entanglement is an issue
operating at a global scale, occurring in all species,
throughout their geographic range. We sought to
answer key knowledge gaps surrounding the issue of
turtle entanglement in marine debris as previously
highlighted by Vegter et al. (2014) and Nelms et al.
(2016). Difficulties in investigating these knowledge
gaps are in part due to a lack of robust data. This
highlights the importance of using mixed methods to
access expert opinion to gain an insight into this
global threat. The growing use of expert knowledge
in conservation is driven by the need to identify and
characterise issues under limited resource availabil-
ity, and the urgency of conservation decisions (Mar-
tin et al. 2012).
Acknowledging the incomplete coverage of our es-
timates, given the mean estimated number of strand-
ings and mortality rates, in the order of 1000 turtles
die annually as a result of entanglement in the areas
monitored by our respondents. These levels are
likely a profound underestimation of the scale of this
issue as the coverage of these actors is far from com-
prehensive. Second, it is well known that not all dead
turtles strand (Epperly et al. 1996, Sasso & Epperly
2007), especially small and pelagic animals, and
there can also be decay of entangled animals. Addi-
tionally, some of our respondents commented that
detection of stranded animals may be further con-
founded due to take of stranded animals for human
consumption.
438
Fig. 5. Responses to comparison of other threats faced by
marine turtles compared to entanglement (n = 63). Black:
greater than entanglement; grey: similar threat; white: less
than entanglement; striped: unsure

Duncan et al.: Marine turtle entanglement 439
Challenge category % of Challenges described Direct quotes from respondents
suggestions
(n = 115)
Law and enforcement 23.5 Management of both industrial ‘Under-resourced fisheries management of small-scale fisheries’
and small-scale artisanal fisheries
The issue of discarded fishing gear at sea ‘Trawlers should file a report anytime they lose netting’
Ineffectiveness of Marine Protected Areas ‘Shifting climate may render Marine Protected Areas as ineffective’
Source of entanglement 24.3 Estimating the amount and durability ‘Entangling material tends to be durable, so even if management scheme is
materials and extent of entangling material entering the sea put into place, have to deal with historic material already in the ocean’
Retrieving lost fishing gear ‘In my region, lost/discarded fishing lines are a big issue’
Lack of accountability ‘Inability to determine source of entanglement debris (no accountability)’
Education and innovation 24.3 Fisherman education and awareness ‘Engagement/education/enticement to bring artisanal fishers in developing
countries to a want to reduce turtle mortality’
‘Figuring out how to reach out to boaters/fishermen with making them want
to support sea turtle friendly habits’
Developing a discipline to avoid ‘Addressing amateur/recreational fishers is really hard. In my opinion, most
abandonment of fishing gear of the discarded fishing lines are left by this group’
Sourcing alternative materials ‘Creation of degradable nylon’
Understanding the full 18.3 Lack of stranding networks’ ability to ‘It is hard to estimate the total amount of entangled turtles, since these
extent of the threat measure the impact of the extent of animals are highly migratory and tend to be scattered over wide areas.
the threats in multiple areas Additionally turtles that become entangled may quickly die and be
predated. Scavengers, predators, wind and currents may prevent carcasses
from coming ashore’
‘Most entanglement records rely on land-based sampling and stranding
do not represent total deaths at sea’
‘It is hard to distinguish marine debris from active and ghost fishing gears’
Difficulty in determining if entanglement ‘Difficulty in determining if entanglement occurred pre- or post-mortem
occurred pre- or post-mortem (for some entanglement types, such as discarded nets/line)’
Survivorship of turtle found entangled alive ‘Limited post-release monitoring of live entangled turtles’
Response to entangled turtles 9.6 Detangle permits ‘Very few people are trained and permitted to disentangle them’
Discovery times need to be quick ‘Discovering entangled turtles quickly’
‘Entangled turtle can be challenging to disentangle especially if they are
not anchored and instead are free swimming’
Ineffectiveness of reporting systems ‘Having a good system in place that stranding will be reported (people that
see an entangled turtle have to be able to notify the correct organization)’
Lack of rehabilitation resources ‘Lack of rehabilitation resources for turtles hurt in incidents of entanglement’
for entanglement incidents
Table 2. Summary of major challenges regarding marine turtle entanglement as listed by respondents

Endang Species Res 34: 431–448, 2017
440
Research need category % of Research needs described Direct quotes from respondents
suggestions
(n = 91)
More specific reporting 23.1 Creation of a common database ‘A common database, long lasting surveys and a programme on a national
and monitoring/common base for monitoring of the state of debris in the sea’
database An increase in specificity of reporting ‘Better monitoring/reporting of entanglement cases by species, life stage,
of entanglement cases region’
‘Establish a protocol for sea turtle strandings networks for identify
entanglements and report these’
Collaboration of resource users in the ‘More collaboration with resource users in the marine environment in
marine environment respect to reporting cases of entanglement’
‘Getting information from fishermen when turtles get entangled. Support to
Fisheries Division who can provide accurate information on net damage
from reports by fishermen. Only a small percentage of stranded turtles will
wash up ... carcasses may become destroyed prior to reaching those coasts’
Mapping the threat/spatio- 31.9 Using stakeholder knowledge ‘Surveys to fishermen (industrial, artisanal and sport) to understand where
temporal hotspots and when they discard nets or lines and in water monitoring programs in
coastal areas with high pressure of artisanal and sport fishing’
Identifying and mapping the ‘Understanding where the event occurs, such as targeting if the problem is
entanglement rates due to different more from floating debris versus debris in water column’
gear types and materials
Modelling/mapping patterns of debris ‘Understanding overlap between sea turtle habitats (e.g. nesting and feeding
distribution, patterns of marine turtle grounds) with areas of high debris concentration (e.g. convergence zones)’
migrations and the characterization of
fisheries distributions ‘Spatio-temporal scales. Hotspots’
Entanglement materials and 24.2 Studying sea turtle and debris behaviour ‘Behavioural (foraging or sheltering) traits in different turtle species or
sea turtle interactions and their interactions populations that may them more vulnerable to entanglement’
‘Investigate the behavioural characteristics of the turtles that lead to their
entrapment in fishing gear with a view to improving mitigation actions’
Post-release mortality and 3.3 Understanding true post-release mortality ‘The effects of flipper amputations on survival’
survival/physical effects and morbidity
Socio-economic impact 4.4 Special focus on the fisher community ‘What are the opportunities and barriers to intervention?’
Innovation of new replacement 6.6 Innovation of biodegradable alternatives ‘Alternative materials for fishing and other things/activities’
materials & methods to commonly used plastic materials
Demographic risk assessments 6.6 Development of demographic risk ‘Develop the appropriate population demographic models for marine turtles
assessments for threatened populations to allow for assessment/identification of those mortality factor that are
of turtles not detrimental to maintaining robust non threatened population of turtle’
Table 3. Summary of research needs regarding marine turtle entanglement as listed by respondents

Duncan et al.: Marine turtle entanglement 441
Priority actions category % of Priority actions described Direct quotes from respondents
suggestions
(n = 121)
Education/stakeholder 31.4 Fisher involvement/education ‘Develop questionnaire for fishermen for their recommendations on how
engagement it would be possible to reduce turtle entanglement’
‘Partnership with local fishermen to locate and remove abandoned or lost
fishing gear (ghost gear). Financial incentives to return discarded gears to shore’
Community/public awareness campaigns ‘Organizing campaigns with scuba divers to clean sea bottom from the man
on marine litter debris and ghost nets/discarded fishing lines’
‘Implement an environmental stewardship certificate system among ocean
users and create a global open access database of entanglements to
facilitate research efforts’
Fisheries management and 26.4 The development of traceable gear ‘Developing/using traceable gear in combination with introducing
monitoring a fining policy’
Stricter regulations ‘Increased collaborations with commercial fisherman and recreational
fisherman to better understand their needs and the needs of the
turtles....and how these can be combined’
Research/knowledge 5 The implementation of the research needs ‘We cannot say before understanding the main reasons, main sources and
stated in Table 3 main habitats or localities in which entanglement occurs’
Law and enforcement on 20.7 Banning at-sea disposal of entangling ‘Enforcement of laws banning at-sea disposal of entangling material’
entanglement material materials
Better waste management and increased ‘Reduction of manmade debris, better waste management, more
recycling efforts biodegradable products’
Development of alternative 16.5 Development of alternative materials/ ‘Development of less environmentally persistent materials to be used in nets,
materials/methods methods fishing line, etc.’
Shifting gear type/increasing the use ‘Different strategies to different fishing gear; from the coastal sport
of biodegradable materials fishermen to high seas industrial fishermen’
‘Introduce biodegradable chord into selected net fisheries with high loss
to ghost nets’
Table 4. Summary of priority actions regarding marine turtle entanglement as listed by respondents

Endang Species Res 34: 431–448, 2017
Species differences
Although there was no interspecific difference in
the incidence of entanglement, most peer-reviewed
publications featured olive ridley turtles, with some
experts reporting high incidences of entanglement
for this species. Stelfox et al. (2016) noted that olive
ridley turtles accounted for the majority of sea turtles
identified as entangled (68%; n = 303), and this could
be for the following reasons. Firstly, this species,
which often exhibits mass nesting in the hundreds of
thousands of individuals, is highly numerous, and at
particularly high densities in some areas, leading to
entanglement hotspots (Jensen et al. 2006, Koch et
al. 2006, Wallace et al. 2010a). Secondly, the olive
ridley forages along major oceanic fronts which are
known to aggregate marine debris (Polovina et al.
2004, McMahon et al. 2007). Finally, their generalist
feeding behaviour potentially attracts them to feed
opportunistically on biofouled marine debris such as
ghost gear (Stelfox et al. 2016).
Life stages
Entanglement was reported to occur in all life
stages (pelagic juveniles, neritic juveniles and adults)
across all species (the exception being flatback tur-
tles which have no pelagic juveniles; Hamann et al.
2011). Perhaps of greatest concern is the signal of
high entanglement incidence in the pelagic juvenile
stage: despite the general inaccessibility of sampling
this life stage, they are still appearing as stranded
entangled. The currents that transport hatchlings to
oceanic convergence zones are also now recognised
as concentrating floating anthropogenic debris, cre-
ating the capacity for an ecological trap for these
young turtles, whether it be through ingestion or
entanglement (Nelms et al. 2016, Ryan et al. 2016).
Many respondents considered that entanglement
could be having a population level effect; a distinct
possibility if this there is a large impact on this cryptic
life stage and on pelagic foraging adults (Mazaris et
al. 2005).
Entangling materials
Respondent data highlighted that the majority of
entanglements were the result of fishery-based
material and other maritime activities. The issue of
ghost fishing featured highly, with numerous res -
ponses reporting entanglement within lost/discarded
gear. This gear is often lost, abandoned or discarded
when it becomes derelict, attracting scavengers and
acting as FADs (Gilman 2011). Subsequently, species
such as marine turtles become entangled within the
gear, perhaps encouraged by this process of ‘self-
baiting’ (Matsuoka et al. 2005).
Change in fishing practice
The issue of ghost fishing appears to have wors-
ened since the 1950s, as the world’s fishing indus-
tries have replaced their gear, which was originally
made of natural fibres such as cotton, jute and
hemp, with synthetic plastic materials such as
nylon, poly ethylene and polypropylene. Manufac-
tured to be resistant to degradation in water means
that once lost, it can remain in the marine environ-
ment for decades (Good et al. 2010). Furthermore,
there has also been a shift in the type of synthetic
nets being selected; for example, fishers in part of
Southeast Asia now increasingly favour superfine
nets. Al though this can help increase catches, the
twine thinness means that they break easily and are
difficult to repair once damaged (Stelfox et al.
2016). The incidences of entanglement caused by
this form of pollution in our expert surveys indicates
that this source of mortality for marine turtles mir-
rors that in marine mammals and sea birds, which
has increased substantially over the last century
(Tasker et al. 2000, Good et al. 2010, McIntosh et al.
2015).
Differentiation from bycatch
It is quite plausible that ghost fishing may be work-
ing synergistically alongside bycatch, but because of
its more cryptic nature this means that understand-
ing its role in marine turtle mortality is much more
difficult. Bycatch is better understood. For example,
the analysis of catch rates in the Mediterranean
allowed for the estimation of 132 000 captures and
44 000 incidental deaths per year (Casale 2011). Like-
wise, cumulative analysis of catch rates in US fish-
eries estimated a total of 71 000 annual deaths prior
to the establishment of bycatch mitigation methods.
Since these measures were implemented, mortality
estimates are ~94% lower (4600 deaths yr−1) (Fink -
beiner et al. 2011). This highlights the importance of
informed estimates to monitor the success of mitiga-
tion methods. In addition to bycatch mortality esti-
mates, spatial and temporal patterns of bycatch inci-
442

Duncan et al.: Marine turtle entanglement
dences can be identified. Using onboard observer
data, Gardner et al. (2008) found seasonal changes in
catch distributions of loggerhead and leatherback
turtles in the North Atlantic, with patterns of spatial
clustering from July to October. Analysed on a global
scale, Wallace et al. (2010b) were able to highlight
region− gear combinations requiring urgent action
such as gillnets, longlines and trawls in the Mediter-
ranean Sea and eastern Pacific Ocean. Generating
such estimates of catch rates and spatial/temporal
patterns for entanglement are not yet possible due to
the lack of quantitative information.
Land-based plastic entanglements
The domination of fisheries-based materials in the
results does not mean that land-based plastics are
not a source of entanglement. The increased input of
plastic debris from terrestrial run-off means that
these interactions are only likely to increase (Jam-
beck et al. 2015). Our literature search and ‘other’
materials stated by respondents contained a variety
of items causing entanglement that could be de -
creased by reduction of use, replacement with more
degradable alternatives and better waste manage-
ment and recycling. The prevalence of these materi-
als in the marine environment will very much de -
pend on future waste governance, especially in those
countries that generate the most plastic waste (Jam-
beck et al. 2015). A future technological solution
which is currently being investigated or adopted in
high plastic-generating countries such as Thailand
and India is the pyrolysis of plastics. This process
produces fuel from waste plastic, a better alternative
to landfill and a partial replacement of depleting
fossil fuels (Wong et al. 2015).
Caveats
It is important to recognise the biases associated
with using stranding animals for data collection.
Within and between stranding sites there are differ-
ences in turtle foraging ecology, life stages and
proximity to human habitation (Bolten 2003, Rees et
al. 2010), and therefore they are exposed to different
levels and types of potential entangling materials.
Individual turtles therefore may not represent a
homogeneous group in terms of entanglement oc -
currence within that population (Casale et al. 2016).
Additionally, recovered carcasses represent an un -
known fraction of at-sea mortalities, with physical
oceanography (e.g. currents) and biological factors
(e.g. decomposition) affecting the probability and
location of carcass strandings (Hart et al. 2006).
However, examining reports of stranded animals
represents a vital opportunity for research and can
provide insights into the impacts of anthropogenic
threats which would otherwise go undetected
(Chaloupka et al. 2008, Casale et al. 2010). In addi-
tion, stranding information aids with the assessment
of harder-to-access life stages, yielding key infor-
mation on the risk to specific resident populations
and contributing to building a worldwide perspec-
tive for conservation issues (Chaloupka et al. 2008,
Casale et al. 2016). Indeed, this was the aim of our
study: using stranding data from expert respondents
to gain an initial indication of the estimated magni-
tude of this threat.
Surveying experts can be a powerful tool for ob -
taining insights on particular topics not widely
known by others (Martin et al. 2012). Expert knowl-
edge and opinions may be the result of training,
research, skills and personal experience (Burgman et
al. 2011a). In this study, we sought the opinions of
conservation scientists and practitioners with experi-
ence in marine turtle entanglement and strandings.
Due to the purposive sampling nature of our ap -
proach, we aimed to identify people with relevant
experiences instead of focusing on obtaining a ran-
dom selection of representatives; this is a widely
used practice when undertaking social surveys that
focus on particular subgroups or specialists (Newing
2011). Nevertheless, expert knowledge and opinions
are also known to be subject to biases, including
overconfidence, accessibility and motivation (see e.g.
Burgman et al. 2011b and Martin et al. 2012). In the
absence of empirical data to validate our findings,
this remains as simply suggestive but nevertheless
relevant information in terms of identifying a poten-
tially important conservation issue and providing rel-
ative indications of the scale of entanglement as a
threat to sea turtles.
Future actions and recommendations
Ghost fishing
Issue and policy. Presently, a large knowledge
gap exists regarding effects of ghost fishing. While
there has been some progress in documenting the
frequency of loss from passive gear such as gillnets,
little is known about loss from active gears; effective
methodology to estimate the persistence of types of
443

Endang Species Res 34: 431–448, 2017
gear such as trawl nets has yet to be developed
(Gilman et al. 2013). While it would be optimal to
switch all gear to more biodegradable materials,
synthetic materials will continue to be used within
fisheries for the foreseeable future. This is an issue
that has been highlighted in policy by the Food and
Agriculture Organization (FAO), who recommend
the identification, quantification and reduction of
mortality caused by ghost fishing by implementing
this into fisheries management plans, increasing sci-
entific information and developing mitigation strate-
gies; but this appears still to be in its infancy
(Gilman et al. 2013). This is also reflected in man-
dates within the International Maritime Organisa-
tion (IMO) and International Convention for Preven-
tion of Pollution from Ships (MARPOL Annex V)
(Stelfox et al. 2016).
Need for a global database and spatial hotspot
identification. Undoubtedly a common global meta-
database recording the spatial distribution and abun-
dance of possible entangling ghost gear as well as
incidences of marine turtle entanglement incorporat-
ing a unit of effort metric would assist in quantifying
the mortality due to ghost gear that is needed to in -
form policy (Nelms et al. 2016). A recent global re -
view (dominated by the Atlantic and Pacific oceans)
on marine megafauna by Stelfox et al. (2016) re -
ported a total of 5400 individuals of 40 species that
had been associated with ghost gear between 1997
and 2015. They suggested this was a great underesti-
mate due to lack of capacity to record incidence.
Such data could feed into one of the major research
priorities emphasised by respondents; modelling
spatio-temporal hotspots of entanglement. An inno-
vative study by Wilcox et al. (2013) used beach clean
data and models of ocean drift to map the spatial
degree of threat posed by ghost nets for marine tur-
tles in northern Australia and map areas of high risk.
With the input of more specific marine location data
on ghost gear and the advocacy of the use of ever
improving modelling, this could provide a powerful
tool in the future.
Education and stakeholder engagement
Local initiative to reduce debris causing entangle-
ment. On a more local and regional scale, many ini-
tiatives are being brought into place to encourage a
reduction in the amount of ghost gear/plastic debris
entering the ocean and combat discarding at sea by
working closely with community education and
engagement; another highlighted topic by our re -
spondents. There are numerous examples: the sea
turtle conservation program in Bonaire has started a
‘Fishing Line Project’ (www.bonaireturtles. org/ wpp/
what-we-do/fishing-line-project) working with vol-
unteers to train them on how to remove discarded
line and nets from coral reefs, and the Zoological
Society of London’s ‘Net-works’ (www.net-works.com)
initiative has established a supply chain for discarded
fishing nets from artisanal fishing communities in the
Philippines to a carpet manufacturing company. With
further replication of such community-based projects
and stakeholder engagement, especially with arti-
sanal fisheries awareness, the potential exists to start
targeting hotspots of marine vertebrate entangle-
ment directly.
Stranding networks training. Another set of stake-
holders which will be important to engage are
stranding networks. Responses to entangled turtles
can often be slow, and respondents commented that
many are not trained in the correct protocols to
safely remove entangling materials. If stranding
networks were fully trained in a standardised proto-
col for removal, the techniques could then be
passed on through educational training programmes
to the fishing community, quickening the response
to such incidences. This is already beginning to
happen for bycatch cases; Sicilian fisherman now
actively volunteer to take part in the rescue of tur-
tles in difficulty and are trained in contacting the
competent authorities for the transfer of turtles to
the nearest recovery centres. This level of involve-
ment by workers in the fishery sector was stressed
and encouraged through both effective education
activity and specific targeted study campaigns
(Russo et al. 2014).
Future research avenues into marine turtle
entanglement
Respondents raised the issue of post-release mor-
tality and the importance of behavioural research
into the interactions between marine turtles and
potential entangling materials present in the marine
environment. The prominence of this has been em -
phasised within other taxa; for example, post-
release mortality can result from long-term chronic
effects of injuries in pinnipeds even after the entan-
glement has been removed (McIntosh et al. 2015).
Furthermore, it has been argued that some colonial
seabirds released from entangling plastic would not
survive without human intervention (Votier et al.
2011).
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Duncan et al.: Marine turtle entanglement
To validate the success of release protocols after
entanglement incidents (as mentioned above), tech-
niques could be employed from other areas of mar-
ine turtle research. Satellite telemetry has already
been used in a multitude of ways to provide infor-
mation on conservation issues facing marine turtles;
a number of studies have used this technique to
consider post-release mortality after bycatch fish-
eries interactions (reviewed in Jeffers & Godley
2016). Deploying tagged turtles that have been
involved in entanglements could aid in the under-
standing of survival after these events as well as
simultaneously providing information on the loca-
tion of sea turtles, feeding into information on en -
tanglement hotspots to target mitigation actions.
The benefits of utilising such techniques have been
illustrated in other endangered species facing en -
tanglement, such as studying mortality of silky sharks
Carcharhinus falciformis in the Indian Ocean; esti-
mates derived from satellite tracking showed that
mortality due to entanglement was 5 to 10 times
that of known bycatch mortality and provided evi-
dence for a call advising immediate management
intervention (Filmalter et al. 2013).
Other research methods and ideas could be modi-
fied from the study of plastic debris ingestion by sea
turtles. Studies are currently underway to under-
stand the selective mechanisms that lead to ingestion
of plastic pieces (Schuyler et al. 2014, Nelms et al.
2016). For instance, a study by Santos et al. (2016)
used Thayer’s law of countershading to assess differ-
ences in the conspicuousness of plastic debris to infer
the likelihood that visual foragers (sea turtles) would
detect and possibly ingest the plastic fragments. Sim-
ilar studies could be conducted to comprehend the
underlying behavioural and physiological mecha-
nisms that influence turtles to approach potential
entangling materials when encountering them with -
in the marine environment.
Similarly, comprehending how important the level
of biofouling on this synthetic debris is in contribut-
ing to the likelihood of entanglement will be impor-
tant. Total fish catches by monofilament gillnets in
Turkey was lower, as a result of accumulating detri-
tus and biofouling increasing the visibility of the nets
in the water column (Ayaz et al. 2006). Furthermore,
the level of biofouling could indicate the age of ghost
gear entangling marine turtles. Retrieved lost/dis-
carded fishing gears are usually found fouled by
macro-benthic organisms, so if a relationship be -
tween soak time and biofouling level could be
established, these organisms could provide a valid
methodology to age the gear and enable better esti-
mates of ‘catches’ made by the respective net (Sal-
danha et al. 2003).
Finally, it will be important to undertake demo-
graphic studies, calculating rates of entanglement,
especially for specific populations that are known to
be particularly vulnerable to a combination of other
anthropogenic threats. For species such as pinni -
peds, which are less elusive (hauling out on land)
than marine turtles, the literature describes different
methods. For example, a proportion de rived from a
count of entangled individuals from a sub-sample or
an estimate of the total population (Raum-Suryan et al.
2009, McIntosh et al. 2015), or more recently, the use
of mixed-effects models to obtain a prediction of the
total number of seals entangled per year, by examin-
ing changes in entanglement rates over time and the
potential drivers of these detected trends (McIntosh
et al. 2015). However, this can only be achieved if
reporting and recording such incidences in marine
turtles improves in efficacy and standardisation.
CONCLUSIONS
Further research may show that the issue is more
one of animal welfare than of substantive conserva-
tion concern to many marine turtle populations. It is
clear, however, that entanglement with anthro-
pogenic plastic materials such as discarded fishing
gear and land-based sources is an under-reported
and under-researched threat to marine turtles. Col-
laboration among stakeholder groups such as strand-
ings networks, fisheries and the scientific community
will aid in providing mitigating actions by targeting
the issue of ghost fishing, engaging in education and
producing urgently needed research to fill knowl-
edge gaps.
Acknowledgements. The authors thank all respondents of
the questionnaires for their invaluable knowledge and
insights regarding this issue. We are grateful to Karen Eck-
ert of WIDECAST for granting access to turtle graphics.
E.M.D. received generous support from Roger de Freitas,
the Sea Life Trust and the University of Exeter. B.J.G. and
A.C.B. received support from NERC and the Darwin Initia-
tive, and B.J.G. and P.K.L. were funded by a University of
Exeter — Plymouth Marine Laboratory collaboration award
which supported E.M.D. We acknowledge funding to T.S.G.
from the EU Seventh Framework Programme under Grant
Agreement 308370, and P.K.L. and T.S.G. received funding
from a NERC Discovery Grant (NE/L007010/1). This work
was ap proved by the University of Exeter, CLES ethics com-
mittee (Ref. 2017/1572). The manuscript was greatly
improved by the input of the editor and 2 anonymous
reviewers.
445

Endang Species Res 34: 431–448, 2017
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Editorial responsibility: Rory Wilson,
Swansea, UK
Submitted: February 22, 2017; Accepted: September 22, 2017
Proofs received from author(s): November 28, 2017