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Marine Pollution Bulletin 201 (2024) 116141
Available online 23 February 2024
0025-326X/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Marine turtles as bio-indicators of plastic pollution in the
eastern Mediterranean
Emily M. Duncan
a
,
*
, Hasan Deniz Akbora
b
,
c
, Patrizia Baldi
a
, Damla Beton
d
, Annette
C. Broderick
a
, Burak Ali Cicek
b
,
c
, Charlotte Crowe-Harland
a
, Sophie Davey
d
, Tess DeSerisy
a
,
Wayne J. Fuller
a
,
d
,
e
, Julia C. Haywood
a
, Yu Jou Hsieh
a
, Ecem Kaya
d
, Lucy C.M. Omeyer
a
,
Meryem Ozkan
d
, Josie L. Palmer
a
, Emma Roast
a
, David Santillo
f
, M. Jesse Schneider
a
,
g
, Robin
T.E. Snape
a
,
d
, Katrina C. Sutherland
a
, Brendan J. Godley
a
a
Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Penryn, Cornwall TR10 9EZ, United Kingdom
b
Underwater Research and Imaging Centre, Biological Sciences Department, Eastern Mediterranean University, 99628 Famagusta, Cyprus
c
Department of Biological Sciences, Faculty of Arts and Sciences, Eastern Mediterranean University, 99628 Famagusta, Cyprus
d
Society for Protection of Turtles, Levent Daire 1, Ulus Sokak, G¨
onyeli, Nicosia, Cyprus
e
Faculty of Veterinary Medicine, Near East University, Nicosia, Cyprus
f
Greenpeace Research Laboratories, University of Exeter, Exeter, United Kingdom
g
Abess Center for Ecosystem Science and Policy, University of Miami, Coral Gables, FL, USA
ARTICLE INFO
Keywords:
Plastic pollution
Plastic litter
Bio-indicator species
Caretta caretta
Marine turtles
Marine debris
ABSTRACT
The loggerhead turtle (Caretta caretta) has been suggested as a bio-indicator species for plastic pollution.
However, detailed investigations in the eastern Mediterranean are limited. Here, we present data from logger-
head turtles (2012-2022; n =131) of which 42.7 % (n =57) had ingested macroplastic (pieces ≥5 mm). Fre-
quency of occurrence (%) was not found to have changed over time, with body size (CCL cm), between stranded
or bycaught turtles, or with levels of digesta present. The characteristics of ingested plastic (n =492) were
largely sheetlike (62 %), clear (41 %) or white (25 %) and the most common polymers identied were Poly-
propylene (37 %) and Polyethylene (35 %). Strong selectivity was displayed towards certain types, colours and
shapes. Data are also presented for posthatchling turtles (n =4), an understudied life stage. Much larger sample
sizes will be needed for this species to be an effective bio-indicator, with the consideration of monitoring green
turtles (Chelonia mydas) for the eastern Mediterranean recommended allowing a more holistic picture to be
gathered.
1. Introduction
High densities of marine plastic pollution are now present across
oceanic gyres, coastal waters and beaches (van Sebille et al., 2015),
putting long-lasting pressure on marine systems (Barnes et al., 2009). An
estimated 8 million metric tons of plastic enter the ocean each year
(Jambeck et al., 2015), which is predicted to increase 2.6-fold by 2040
(Lau et al., 2020). The widespread dispersion and mobility of plastic
pollution and its presence in all marine habitats allows it to interact with
a wide variety of biota (Gall and Thompson, 2015). Plastic represents a
threat to multiple marine vertebrate species through ingestion, entan-
glement and degradation of key habitats (Duncan et al., 2017;
Fackelmann et al., 2023; Fuentes et al., 2023; Nelms et al., 2016, 2019;
Schuyler et al., 2014a, 2014b; Wilcox et al., 2013), although population-
scale impacts have been more difcult to determine (Senko et al., 2020).
Records of plastic ingestion in marine turtles are now ubiquitous
across all species and ocean basins including; the Atlantic (Colferai et al.,
2017; Di Beneditto and Awabdi, 2014; Eastman et al., 2020; Machovsky-
Capuska et al., 2020; Mascarenhas et al., 2004; Pham et al., 2017a; Rice
et al., 2021; Rizzi et al., 2019; Santos et al., 2016; Witherington, 2002),
the Pacic (Clukey et al., 2017; Godoy and Stockin, 2018; Jung et al.,
2018; Ng et al., 2016; Wedemeyer-Strombel et al., 2015), the Indian
Ocean (Hoarau et al., 2014; Yaghmour et al., 2018, 2021), the western
Mediterranean Sea (Bruno et al., 2022; Camedda et al., 2014, 2022a,
* Corresponding author.
E-mail address: ed291@exeter.ac.uk (E.M. Duncan).
Contents lists available at ScienceDirect
Marine Pollution Bulletin
journal homepage: www.elsevier.com/locate/marpolbul
https://doi.org/10.1016/j.marpolbul.2024.116141
Received 14 December 2023; Received in revised form 5 February 2024; Accepted 6 February 2024
Marine Pollution Bulletin 201 (2024) 116141
2
2022b; Campani et al., 2013; Darmon et al., 2022; Digka et al., 2020;
Matiddi et al., 2017; Solomando et al., 2022; Tomas et al., 2002) and the
eastern Mediterranean sea (Duncan et al., 2019; Darmon et al., 2022).
Ongoing, long-term monitoring is important to observe trends in
ingestion, to understand potential future population level impacts
related to this threat, and also track concentrations of plastic pollution in
marine environments (Darmon et al., 2022; Senko et al., 2020). The
loggerhead turtle (Caretta caretta) is one of the most studied marine
species for this threat due to its widespread global distribution, with
frequency of occurrence (FO%; proportion of turtles assessed that con-
tained ingested plastic) being highly variable globally (0-90 %) (Lynch,
2018).
A major global plastic pollution hotspot is the Mediterranean Sea,
where between 873 and 2576 t of plastic debris is estimated to be
oating on the sea surface (C´
ozar et al., 2015; Suaria et al., 2016). The
loggerhead turtle has been designated as a bio-indicator species under
the EU Marine Strategy Framework Directive (MSFD) (2010) (2010/
477/EU) (Galgani et al., 2014; Matiddi et al., 2017), for evaluating the
Good Environmental Status (GES) at the Mediterranean and European
scales (Darmon et al., 2022). It has also been proposed as a bio-indicator
species for monitoring under The Convention for the Protection of the
Marine Environment of the North-East Atlantic (OSPAR) and the Bar-
celona Convention for the Protection of the Marine Environment and the
Coastal Region of the Mediterranean (Casale and Ceriani, 2020). How-
ever, despite inclusion within these conventions, records of ingestion in
loggerhead turtles within the eastern Mediterranean basin are limited
(Darmon et al., 2022). Therefore, collecting and reporting detailed in-
formation about the ingestion of plastic in this data poor region of the
Mediterranean Sea is crucial for contributing to the efforts of monitoring
GES (Galgani et al., 2014; Matiddi et al., 2017; Darmon et al., 2022).
Studies from the eastern Mediterranean have demonstrated the
presence of widespread post-pelagic foraging grounds for loggerhead
turtles around North Cyprus (Haywood et al., 2019; Palmer et al., 2021,
2024; Snape et al., 2013). Year-round strandings and bycatch occurs
with seasonal peaks during the summer months, providing opportunities
to directly assess gut content (Palmer et al., 2021; Snape et al., 2013).
Here we sought to augment knowledge of plastic ingestion by logger-
head turtles in the eastern Mediterranean region by: 1. Assessing
abundance and occurrence in loggerhead turtles while considering turtle
size, whether animals were stranded or bycaught, and according to
levels of digesta present; 2. Exploring temporal and spatial trends; 3.
Describing plastic ingested according to type, colour, shape and poly-
mer; 4. Investigating potential selectivity of plastic ingestion compared
to environmental baseline levels of beach debris. All data are presented
in a form to facilitate building extensive datasets and future meta-
analyses.
2. Material & methods
2.1. Study area and sample collection
This study was conducted in North Cyprus, in the eastern Mediter-
ranean basin. The island hosts important nesting beaches and foraging
grounds for loggerhead turtles (Haywood et al., 2019; Omeyer et al.,
2021). The coastline is regularly patrolled for nest monitoring and
stranded turtles, as well as having sheries-focused research and public
awareness activities that facilitate the discovery, reporting, and trans-
portation of stranded or bycaught turtles (fresh dead to moderately
decomposed) to the project base for necropsy (with permission obtained
from the North Cyprus Department of Environmental Protection, Animal
Husbandry and Veterinary Department). The vast majority of turtles
subjected to necropsy are considered to have died as a result of bycatch
incidents in coastal small-scale sheries, typically being drowned in
bottom-set trammel nets (Snape et al., 2016).
A total of 135 loggerhead turtles were sampled spanning a period of
11 years (2012–2022; Fig. 1.). Given the size distribution of the turtles
investigated, the majority (n =131; 97 %) were post-pelagic juveniles
and adults with CCL >35 cm (Casale et al., 2008). There was additional
access to a small number of stranded posthatchling turtles (n =4),
relatively rare for the region (Levy et al., 2015; Türkozan et al., 2013),
that were likely still in their pelagic life phase. These were considered
separately and presented to facilitate future collaborative analyses.
During necropsy, the entire gastrointestinal tract was removed and
initial contents weighed. The contents were grossly examined then
rinsed through a 1 mm mesh sieve to separate and remove the plastic for
classication and dietary items for storage and analysis (Palmer et al.,
2021). Turtles demonstrated varying levels of gut ll and were classied
as those with normal levels of digesta (NLD) present therefore exhibiting
recent feeding activity, i.e. digested and partially digested remains of
multiple prey items, and those with a distinct lack of recognisable di-
etary items with low levels of digesta (LLD), i.e. no discernible food
items present, containing only sediment and digestive uids.
To establish if there was a difference in plastic ingestion between
stranded turtles and those known to be freshly captured in sheries or if
Fig. 1. Size of turtles investigated Frequency histogram of loggerhead turtles
(Caretta caretta) according to curved carapace length (cm) where data were
available for a) stranded turtles (dark grey; n =80; n =4 of which were
posthatchlings: PH) and b) bycaught turtles (light grey; n =49). For 6 in-
dividuals CCL measurements were not obtained. Original artwork by
Emma Wood.
E.M. Duncan et al.
Marine Pollution Bulletin 201 (2024) 116141
3
varying levels of gut ingesta may correlate with differences in plastic
incidence, we considered post-pelagic turtles in four groups separately
in the rst instance: Stranded NLD (n =72), Stranded LLD (n =9),
Bycaught NLD (n =45), Bycaught LLD (n =5). Additionally, from
previous studies, beach plastics around North Cyprus are known to vary
spatially (Duncan et al., 2018; ¨
Ozden et al., 2021) so turtles with from
known stranding or bycaught locations were grouped according to the
areas of the coast from where they were obtained; East (n =68), North
(n =37), West (n =13), Unknown (n =14).
To normalise for turtle size the body burden of ingested plastic (g
plastic/kg turtle) was calculated following calculations outlined in
Clukey et al. (2017) and Lynch (2018). Body condition index (BCI)
(turtle mass kg/SCLˆ3) was also calculated following indices described in
Bjorndal et al. (2000) and Rice et al. (2021) to compare with measures of
plastic burdens.
2.2. Plastic classication
Ingested plastic was classied using a system outlined in Duncan
et al. (2019, 2021) which builds upon the Fulmar Protocol and MSFD
(Marine Strategy Framework Directive) Marine Litter Report 2011
(Descriptor 10) toolkits (Galgani et al., 2014; van Franeker et al., 2011).
This involves recording the type of plastic debris: industrial plastic
pellets or nurdles (IND) and user plastics (USE) which can be split into
several sub-categories; sheetlike plastic (SHE e.g., plastic bags),
threadlike/lamentous plastic (THR e.g., remains of rope), foamed
plastics (FOAM e.g., polystyrene), fragments (FRAG e.g., hard plastic
pieces) and other (POTH, e.g., rubber, elastics, items that are ‘plastic-
like’, not clearly tting into another category). Dry weight (g) was
recorded for each individual piece isolated. Additional recordings of
colour and three-dimensional measurements of each individual piece
were also taken. Colour was recorded within 11 categories: Clear, White,
Pink/Purple, Red, Orange, Yellow, Green, Blue, Brown, Black, Grey. Width
to length were calculated (W/L) for all pieces ingested. A ratio close to 1
indicated a square or round piece of debris with ratios leading to rect-
angular and progressively more linear shapes with decreasing ratio.
These were grouped according to ve quintiles of Shape Class: SC1 <
0.2, SC2 <0.4, SC3 <0.6, SC4 <0.8, SC5 <1.0.
2.3. Polymer identication
The polymer make-up of marine plastic debris may aid in identifying
possible sources of the material. A sub-sample of randomly selected 137
(28 % of total plastic pieces) retained items was subject to analysis using
Fourier-transform infrared (FT-IR) spectroscopy. This offers a simple,
efcient non-destructive method for identifying and distinguishing
polymers, based on infrared absorption bands representing distinct
chemical functionalities present in the material (Jung et al., 2018).
Analysis was carried out using a PerkinElmer Spotlight 400 universal
diamond – ATR (attenuated total reection) attachment, placing each
fragment or bre onto the diamond surface (after precleaning the sur-
face with analytical grade ethanol) and applying a consistent force using
the sample clamp. Spectra were collected over a broad range (630–4000
cm
−1
) from an average of four sample scans with a resolution setting of
4 cm
−1
. Spectra were corrected for background variation. The infrared
spectra were acquired, processed and analysed using PerkinElmer
Spectrum software (version 10.5.4.738) and compared against a total of
eight commercially available polymer libraries (adhes.dlb, Atrpolym.
dlb, ATRSPE~1.DLB, bres.dlb, IntPoly.spl, poly1.dlb, polyadd1.dlb
and POLYMER.DLB, as supplied by PerkinElmer), checking also against
an additional library compiled at the Greenpeace Research Laboratories
in order to exclude contaminants arising from materials commonly used
in the laboratory. Spectrum software allowed for the comparison of
spectra obtained for each sample against these nine libraries, reporting
the 10 most likely matches. In each case, matches were then checked by
the analyst to verify the quality of the match and the reliability of the
identication. Match quality scores were generated for each spectrum,
and only scores with >70 % match similarity and/or reliable spectra
were accepted.
2.4. Selectivity analysis
Potential selectivity was tested for using the package “adehabitatHS”
and graphical calculated selectivity ratios were obtained for debris type,
colour and shape according to the relative makeup of debris according to
these attributes in the environment (Duncan et al., 2019). A value of >1
or <1 indicates a positive or negative selectivity, respectively, compared
to what is available in the environment. All data processing and analysis
was performed in R version 4.4.0 (R Core Team, 2023), and the signif-
icance level for statistical tests was alpha =0.05 throughout.
3. Results
3.1. Overall plastic ingestion
Of the post-pelagic turtles analysed (n =131), 42.7 % (n =57) had
ingested plastic. The highest number of pieces ingested by a single in-
dividual, was 67 pieces weighing 9.66 g. Over the eleven-year period,
frequency of occurrence of plastic ingestion (FO%) ranged from 0 to 70
FO% (n =131), which did not show a signicant change over time
(Spearman’s Correlation: R =0.39, p =0.23) or with size of turtle (CCL)
(R =0.27, p =0.44; Fig. 2). When years or size classes with small sample
samples (n ≤3) were excluded, there were still no signicant correla-
tions in FO% (year: R =0.21, p =0.54; size: R =0.90, p =0.08).
All groups of turtles presented individuals that had ingested plastic
Fig. 2. Frequency of occurrence (FO%). The proportion of turtles showing
plastic ingestion (n =131) a) per year of stranding and b) according to size (n
=125). For 6 individuals CCL measurements were not known. Sample size for
each increment is shown above the respective bar. Original artwork by
Emma Wood.
E.M. Duncan et al.
Marine Pollution Bulletin 201 (2024) 116141
4
(Table 1). There was no signicant difference in incidence of plastic
ingestion between NLD and LLD stranded turtles (Fisher’s exact test
(FET): odds ratio =2.61, p =0.29, n =81), NLD and LLD bycaught
turtles (FET: odds ratio =1.42, p =1, n =50) and between stranded and
bycaught NLD turtles (Chi-squared Test:
χ
2 =0.18, p =0.67, n =117)
and stranded and bycaught LLD turtles (FET: odds ratio =0.45, p =0.58,
n =14).
For plastic positive turtles, there was no signicant difference in the
number of pieces (Wilcoxon Rank Sum Test: W
33,24
=419; p =0.71) and
mass of ingested plastic (W
33,24
=361; p =0.57) between stranded and
bycaught turtles. Additionally, there was no signicant difference be-
tween the number of pieces or mass of ingested plastic (g) between NLD
or LLD stranded turtles (number pieces: W
31,2
=17.5, p =0.32; mass:
W
31,2
=21, p =0.46), NLD or LLD bycaught turtles (number pieces:
W
22,2
=40, p =0.07; mass: W
22,2
=19, p =0.79), or stranded or
bycaught NLD turtles (number pieces: W
31,22
=331, p =0.87; mass:
W
31,22
=333, p =0.89) or LLD turtles (number pieces: W
2,2
=4, p =
0.22; mass: W
2,2
=3, p =0.67). Due to the lack of any signicant dif-
ferences among groups further analyses were undertaken for all plastic-
positive animals combined excluding posthatchling turtles. For future
reference and meta-analysis, however, data have been hosted in such a
way that these different groups can be examined separately (Duncan
et al., 2024a).
3.2. Patterns with body size
There was no signicant relationship between turtle size (CCL) and
the number of ingested pieces (Spearman’s Correlation: R = − 0.09, p =
0.49), mass (g) of plastic (R =0.01, p =0.95), and body burden index (R
= − 0.14, p =0.30) (Fig. S1). There was no signicant correlation be-
tween BCI and number of pieces (R = − 0.05, p =0.75), mass (g) of
ingested plastic (R = − 0.08, p =0.57) or body burden index (R =0.05, p
=0.74). Additionally, there was no signicant relationship between
turtle size (cm) and mean length of ingested plastic pieces (mm) (R =
−0.21, p =0.13) or maximum length of ingested plastic pieces (mm) (R
= − 0.11, p =0.41) (Fig. S2).
3.3. Temporal patterns of ingestion
There were no statistically signicant correlations between year and
the number of pieces ingested (R = − 0.13, p =0.34), mass of ingested
pieces (g) (R =0.10, p =0.45) or body burden index (g/kg) (R =0.07, p
=0.63) (Fig. S1.). However, there was a signicant, positive relation-
ship between year and the maximum mass of ingested pieces (g) (R =
0.75, p =0.01) and body burden index (g/kg) (R =0.75, p =0.02)
(Fig. S1). Maximum number of pieces ingested demonstrated a positive
non-signicant trend over time (R =0.50, p =0.12; Fig. 3.).
3.4. Spatial pattern of ingestion
There was no signicant difference between the coasts from which
animals originated and abundance measures for the number of pieces
(Kruskal Wallis (KW):
χ
2
3
=3, p =0.47) mass (g) (KW:
χ
2
3
=2, p =
0.53) or body burden (g/kg) (KW:
χ
2
3
=4, p =0.28) of ingested plastic.
Additionally, frequency of occurrence did not vary signicantly by
coastline (FO%: East =42.0 %; North =47.5 %; West =53.8 %; Chi-
squared Test:
χ
2 =0.58, p =0.75) (Table 2).
3.5. Ingested plastic description
The most abundant type of plastic ingested (n =492) was sheetlike
plastic (SHE: 62 %) followed by hard fragments (FRAG: 23 %; Fig. 4a).
The most numerous colour ingested was clear (41 %) followed by white
(25 %) and black (16 %, Fig. 4b). The majority of ingested plastic were
rectangular in shape; SC3 (31 %) followed by SC4 (23 %, Fig. 4c).
3.6. Polymer identication
The most common polymers identied were Polypropylene (PP: 37
%; 43 %-FRAG, 36 %-THR, 32 %-SHE), Polyethylene (PE: 35 %; 47
Table 1
Abundance and mass of plastic in affected turtles. Mean ±SE and range of number and mass (g) of ingested plastic pieces of stranded and bycaught loggerhead turtles
(Caretta caretta). For abbreviations see text.
Number of pieces Mass (g)
Stranded Bycaught Stranded Bycaught
NLD (n =31) LLD (n =2) NLD (n =22) LLD (n =2) NLD (n =31) LLD (n =2) NLD (n =22) LLD (n =2)
Mean ±SE 10.1 ±2.8 10.0 ±5.0 7.0 ±1.6 1.0 ±0.0 0.9 ±0.3 0.7 ±0.6 0.7 ±0.3 0.3 ±0.3
Range 1.0–67.0 5.0–15.0 1.0–27.0 1.0–1.0 <0.01–9.7 <0.01–1.9 <0.01–4.8 <0.01–0.6
Fig. 3. Temporal pattern of maximal incidence of plastic ingestion in loggerhead turtles (n =55). Relationship between year and a) annual maximum number of
ingested plastic pieces, b) annual maximum mass of ingested plastic pieces (g) and c) annual maximum plastic debris body burden index (mg plastic/g turtle). For 2
individuals CCL measurements were not known.
E.M. Duncan et al.
Marine Pollution Bulletin 201 (2024) 116141
5
%-SHE, 36 %-FRAG, 36 %-THR) and Polyamide (PA: 19 %; 27 %-THR,
21 %-FRAG, 21 %-SHE; Fig. 5.). Other polymers identied at lower
levels were Polyhexamethylene (PHMB), Polyundecanoamide (PAU),
Polyisoprene (PI) and Polyvinyl Chloride (PVC) (Duncan et al., 2024b).
3.7. Selectivity
There was signicant selectivity of the type (λ =0.152, p <0.001),
colour (λ =3.16, p <0.001) and shape (λ =0.742, p =0.002) of plastic
ingested. Calculated ratios suggest loggerhead turtles exhibited a very
strong selectivity towards both sheetlike (SHE) and threadlike (THR)
plastic (wi =6.97, wi =4.52, respectively) and slight selection towards
foamed (FOAM) plastic (wi =1.93), but appeared to not actively select
hard fragments (FRAG), “other pollutants” (e.g. rubber) (POTH) and in-
dustrial (IND) types (Fig. 4d). When considering the ingestion by colour
categories, loggerhead turtles showed strong selectivity for clear and
black debris (wi =1.99, wi =1.95, respectively) and also highly mar-
ginal selectivity for pink/purple (wi =1.01), while not showing active
selection of white, red, grey, orange, blue, brown, yellow, green plastics
(Fig. 4e). For shape, the strongest selectivity was towards thin, elon-
gated pieces (SC1) and a very slight selectivity towards more rectan-
gular shapes (SC3) (wi =2.74, wi =1.13, respectively; Fig. 4f).
3.8. Plastic ingestion in posthatchlings
Plastic ingestion in posthatchling turtles (n =4) was at a frequency of
occurrence (FO%) was 75 % with the mean number pieces being 7.0 ±
5.5 (mean ±SE; range: 1-18) and mass (g) ingested was 0.11 ±0.10
(range: 0.001-0.31). The most prevalent type of plastic ingested for
posthatchling turtles was sheetlike plastic (SHE: 77 %) followed by
foamed plastics (FOAM: 14 %) and hard fragments (FRAG: 9 %). The main
colours ingested were white (32 %), clear (32 %) and black (14 %). Most
of the ingested plastic were in SC4 (32 %) followed by SC3 (23 %). For
ingested pieces from one individual posthatchling of 18.8 CCL (cm) that
received polymer analysis all pieces were identied as Polyamide (n =
7).
4. Discussion
Here we make a signicant addition to the limited knowledge base
regarding plastic ingestion in loggerhead turtles within the data poor
region of the eastern Mediterranean basin with the high contribution of
fresh bycaught turtles. This allowed detailed comparisons between
stranded turtles and those bycaught directly from foraging grounds.
Loggerhead turtles in the study exhibited plastic ingestion with fre-
quency of occurrence (FO%) in the mid-range of values for those re-
ported previously in the Mediterranean (14-85 FO%; (Camedda et al.,
2014; Casale et al., 2008; Digka et al., 2020; Lazar and Graˇ
can, 2011;
Matiddi et al., 2017; Tomas et al., 2002). The majority of ingestion was
of sheet-like plastics, with potential sources being plastic bags and food
packaging, which is consistent with results from previous reports from
other regions and species of marine turtle (Lynch, 2018; Schuyler et al.,
2014b). Furthermore, colours were also similar to those previously re-
ported; clear, white and black being the majority of those found (Lynch,
2018; Schuyler et al., 2014b). The dominance of Polyethylene and
Polypropylene as polymers found also follows global trends (Bruno
et al., 2022; Camedda et al., 2022a; Solomando et al., 2022). For post-
pelagic turtles there was no relationship between turtle size and plas-
tic ingestion abundance measures which is in accord with previous work
in the western Mediterranean (Casale et al., 2016). This may, in part, be
due to size being a poor predictor of loggerhead turtle foraging ecology
in the Mediterranean due to the proximity of different habitats and
smaller turtles foraging benthically at comparably smaller sizes to others
globally (Casale et al., 2008).
Plastic ingestion FO% and abundance measures did not show a
marked increase over the 11-year time period of this study. This is
consistent with other recent long-term studies within the region (Dar-
mon et al., 2022). Plastic ingestion could depend on food and litter
availability, which varies spatially, seasonally and annually (Darmon
et al., 2022; Mansui et al., 2020) obfuscating temporal trends, with
relatively small sample sizes per year. Despite this, it is important to note
the maximum values of abundance estimates here did increase over the
study period, perhaps indicating the extreme cases of ingestion are
increasing temporally. Other regional studies on different species with
longer time series data have noticed a marked increase overtime from
minimal or no plastic ingestion in the 1970-90s to higher rates of
occurrence (Mrosovsky et al., 2009; Yaghmour et al., 2021). As envi-
ronmental plastic levels were already very high in the early to mid-
1990s in the eastern Mediterranean (Broderick and Godley, 1996),
detecting a similar increase in plastic ingestion in our dataset as
observed in the wider literature is likely difcult based on the time series
of our study (Broderick and Godley, 1996).
Previously the literature has called for only turtles assumed to have
had normal feeding behaviour to be taken in consideration as plastic
ingestion occurrence in stranded turtles may be biased as consequence
of illness or injury, with health status modifying the normal foraging
behaviour (Casale et al., 2016). A recent large-scale study, which
included twenty four animals also included in the current work, how-
ever, found that bycaught and stranded exhibited no difference in body
condition (Darmon et al., 2022). Within the current study we observed
no difference in ingestion incidence or abundance measures between
those turtles dened as stranded or bycaught. Indeed, there was also no
difference in recent dietary composition and feeding behaviour between
these two groups of loggerhead turtles in this study area (Palmer et al.,
2021). It is important to note within this study area that many strandings
are thought the result of sheries interactions (Snape et al., 2013), since
strandings are temporally correlated with setting of high bycatch shing
metiers during months when shers indicated bycatch is a problem and
because in the majority of cases, the only cause of death is circumstantial
evidence indicating drowning in sheries. The mortalities within this
area are thought to happen in shallow, near-shore waters due to inter-
action with small-scale/semi-industrial shing eets, with the greatest
proportion of sheries deaths occurring in set nets (Palmer et al., 2024;
Snape et al., 2013, 2016). Another consideration we investigated was
the inclusion of a limited number of turtles with low levels of dietary
digesta. Again, no marked differences in plastic ingestion incidence or
abundance were apparent according to this factor. Sample sizes for some
groups were, however, small, and ongoing consideration should be
given to these potential sources of bias.
Although differing classication systems of plastic prohibit detailed
Table 2
Geographic breakdown of abundance and mass of plastic in affected animals. Mean ±SE and range of number and mass (g) of ingested plastic pieces of loggerhead
turtles according to the coastline from which they were retrieved.
Location
East (n =29) North (n =19) West (n =7)
No. of pieces Mass (g) No. of pieces Mass (g) No. of pieces Mass (g)
Mean ±SE 8.1 ±2.6 0.8 ±0.4 6.2 ±4.8 0.6 ±0.3 11.4 ±4.8 0.7 ±0.3
Range 1.0–67.0 <0.01–9.66 1.0–25.0 <0.01–4.8 1.0–37.0 <0.01––1.9
E.M. Duncan et al.
Marine Pollution Bulletin 201 (2024) 116141
6
comparisons to some previous studies of debris selectivity in other
populations (Schuyler et al., 2012), green turtles (Chelonia mydas) from
the eastern Mediterranean exhibited similar selectivity as loggerhead
turtles in this study, with strong selections towards sheet-like and
thread-like plastic, being clear or black in colour with small length to
width ratios (Duncan et al., 2019). Although the selectivity for green
items demonstrated for green turtles by Duncan et al. (2019) was not
shown here. These patterns are contrary to what might have been ex-
pected as a result of species-specic foraging behaviour and dietary
preferences (Palmer et al., 2021). Despite loggerhead turtles ingesting a
Fig. 4. Marine turtle diet-related selectivity in macroplastic ingestion in the loggerhead turtles (Caretta caretta) (n =57) a) & d) type of plastic debris SHE =sheetlike
plastics, THR =threadlike plastics, FOAM =foamed plastics, FRAG =hard plastics, POTH =other ‘plastic like’ items, IND =industrial nurdles, b) & e) colour of
plastic debris. Cl =Clear, Blk =Black, Y =Yellow, Wh =White, Gn =Green, Bl =Blue, Br =Brown, Gy =Grey, O =Orange, P/P =Pink/Purple, R =Red, c) & f)
width/length ratio (WL ratio). If the ratio number produced was <0.2 this represented linear/rectangular shape whereas a ratio close to 1 indicated a more square or
circular piece of debris. Shape classes are as followed; SC1 =0.01–0.2, SC2 =0.21–0.4, SC3 =0.41–0.6, SC4 =0.61–0.8, SC5 =0.81–1.0. d), e) & f) Selectivity
Ratios. A value >1 this indicates a positive selectivity for that type/colour category than availability in the environment. Error bars indicate 95 % condence in-
tervals and ranked from strongest to weakest selection ratios. c) d) & f) Classication of ingested plastic from proportion (%) of plastic pieces. Original artwork by
Emma Wood. (For interpretation of the references to colour in this gure legend, the reader is referred to the web version of this article.)
E.M. Duncan et al.
Marine Pollution Bulletin 201 (2024) 116141
7
higher proportion of harder fragments and items such as bottle top lids,
that could be confused with hard bodied prey items, than green turtles in
this region (Duncan et al., 2019), they did not appear to actively select
these, possibly an artifact of the extremely high abundance of fragments
in the environment (Duncan et al., 2018).
Post-hatchling sized turtles showed high incidence of plastic inges-
tion, however, the small sample size of this size class prohibited any
further analysis but does add evidence for the concern for vulnerability
for this life stage (Pham et al., 2017a; Rice et al., 2021; White et al.,
2018). Previous studies have reported high occurrences and plastic
present in this marine turtle life stage and suggested this as a potential
evolutionary trap, with signs of morbidity and mortality presenting as
evidence of impacts such as gastrointestinal ulceration on necropsy
(Duncan et al., 2021). Life stage is likely be an important factor in
selectivity with post-hatchling turtles varying in not only their likeli-
hood of plastic ingestion but also being less selective and more oppor-
tunistic in their dietary choices (Schuyler et al., 2012; Schuyler et al.,
2014a, 2014b). However sample size and ability to collect environ-
mental baselines for this life stage often precludes them from detailed
analysis (Duncan et al., 2021; Pham et al., 2017b; Rice et al., 2021;
White et al., 2018). Further insights into the impacts of this rarely
encountered life stage will necessarily come from multiple research
groups combining sparse data.
Although bycaught and stranded turtles did not differ in patterns of
plastic ingestion within this study, origin of turtles sampled should be
maintained as a careful consideration within marine turtle plastic
ingestion studies. There is strong indication that different foraging be-
haviors could be represented in sample groups caught in different types
of gear (e.g. pelagic longline and trawl nets; Casale et al., 2016),
impacting the patterns of plastic ingestion. This is due to the close link
between gear types, habitat, diet and foraging behaviour (Palmer et al.,
2021). For example, turtles captured in pelagic longlines are likely to be
of a life stage tending to feed on epipelagic prey in comparison to other
life stages that target prey benthically being more prone to interaction
with set nets. Here the environmental baseline data was derived from
beach plastic surveys as they were the only logistically feasible way to
collect a baseline proxy. Limitations are recognised to using this method
as the resulting composition of plastic present on beaches can vary to
other environment compartments, but do provide a cost-effective,
widely accessible and a comprehensive method to estimate when at-
sea sampling is not possible (Di Beneditto and Awabdi, 2014; Duncan
et al., 2019; Schuyler et al., 2012). Sampling in known marine turtle
foraging areas remain a priority activity to help inform such studies.
Currently marine turtles are proposed as an indicator species for
monitoring marine litter in the Mediterranean region (Camedda et al.,
2022a; Darmon et al., 2022; Solomando et al., 2022). Monitoring this
region is important as the Mediterranean Sea shows non-homogeneous
values for plastic pollution in space and time, likely resultant from
geographic variability in levels of plastic input and the existence of non-
permanent eddies (Constantino et al., 2019; Mansui et al., 2015). The EU
Marine Strategy Framework Directive (MSFD) has including “Trends in
the amount and composition of litter ingested by marine animals”
among its indicators (Commission’s Decision 2010/477/EU) (Galgani
et al., 2014). Green turtles in the eastern Mediterranean have shown
higher FO% and ingestion abundance measures than loggerhead turtles
stranded or bycaught within the same area (Duncan et al., 2019). Similar
interspecic differences (FO%, number of pieces and mass of ingested
plastic) have been noted in the United Arab Emirates, with green turtles
ingesting higher quantities of marine debris than loggerhead turtles
(Yaghmour et al., 2021). Using only the loggerhead turtle as an indicator
species in the eastern Mediterranean may give a skewed view of the
current state of plastic pollution in this region (Darmon et al., 2022) as
eastern Mediterranean environments experience plastic values among
the highest levels reported globally (Duncan et al., 2018).
Loggerhead turtles have been suggested to be used to verify the
effectiveness of the Single-use Plastic Directive (EU 2019/904) and
patterns of ingestion presented here underscore this utility. Plastic bags
and food packaging are among some of the most observed litter items in
the Mediterranean Sea (Arcangeli et al., 2018; Constantino et al., 2019)
and these map to the materials ingested by loggerhead turtles within this
study and others (Camedda et al., 2022b; Darmon et al., 2022). Further
work is needed here to understand the processes leading to ingestion.
For example, questions arise as to whether marine turtles bite small thin
pieces of larger items when they interact with them, possibly explaining
the dominance of similar sheet-like pieces in the same samples. There-
fore, the utilisation of technology such as mounted camera tags on
turtles will improve knowledge of turtle/plastic interactions (Fukuoka
et al., 2016). Additionally, olfactory stimuli might also play an impor-
tant role on the ingestion of marine debris by turtles (Pfaller et al., 2020)
and is worthy of further investigation.
Overall loggerhead turtles in the eastern Mediterranean showed
consistent and considerable plastic ingestion over the course of an 11-
year period. Due to the widespread distribution of this species, with
harmonised monitoring implemented at a wide spatial scale they could
be considered a good bio-indicator species (Galgani et al., 2014; Matiddi
et al., 2017), albeit with larger sample sizes needed. It will, however, be
important to carefully consider interspecic differences and to make
sure a true representation of environmental status is reected and, we
suggest, green turtles are also considered. Further efforts should be
made towards increased standardisation of methodologies to continue
monitoring this threat alongside investigations into the potential lethal
and sublethal impacts of plastic on the health and survival of marine
turtles within this region and worldwide (Marn et al., 2020).
Fig. 5. Polymer identication of ingested plastic (n =130) from 33 loggerhead
turtles (Caretta caretta) in Cyprus. Proportion (%) of ingested pieces: PE,
Polyethylene; PP, Polyproplene; PA, Polyamide; PHMB, Polyhexamethylene;
PAU, Polyundecanoamide; PI, Polyisoprene; and PVC, Polyvinyl Chloride.
E.M. Duncan et al.
Marine Pollution Bulletin 201 (2024) 116141
8
CRediT authorship contribution statement
Emily M. Duncan: Writing – review & editing, Writing – original
draft, Visualization, Methodology, Investigation, Funding acquisition,
Formal analysis, Data curation, Conceptualization. Hasan Deniz
Akbora: Writing – review & editing, Data curation. Patrizia Baldi:
Writing – review & editing, Data curation. Damla Beton: Writing –
review & editing, Project administration, Funding acquisition, Data
curation. Annette C. Broderick: Writing – review & editing, Funding
acquisition, Data curation, Conceptualization. Burak Ali Cicek: Writing
– review & editing, Data curation. Charlotte Crowe-Harland: Writing –
review & editing, Data curation. Sophie Davey: Writing – review &
editing, Project administration, Data curation. Tess DeSerisy: Writing –
review & editing, Data curation. Wayne J. Fuller: Writing – review &
editing, Data curation. Julia C. Haywood: Writing – review & editing,
Data curation. Yu Jou Hsieh: Writing – review & editing, Data curation.
Ecem Kaya: Writing – review & editing, Data curation. Lucy C.M.
Omeyer: Writing – review & editing, Data curation. Meryem Ozkan:
Writing – review & editing, Data curation. Josie L. Palmer: Writing –
review & editing, Data curation. Emma Roast: Writing – review &
editing, Data curation. David Santillo: Writing – review & editing,
Methodology, Data curation. M. Jesse Schneider: Writing – review &
editing, Data curation. Robin T.E. Snape: Writing – review & editing,
Project administration, Funding acquisition, Data curation, Conceptu-
alization. Katrina C. Sutherland: Writing – review & editing, Data
curation. Brendan J. Godley: Writing – review & editing, Writing –
original draft, Resources, Project administration, Methodology, Funding
acquisition, Data curation, Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
doi still pending from the data repository (PANGAEA)
Acknowledgements
The authors would like to thank all the volunteers who assisted with
eldwork as part of the Marine Turtle Conservation Project (MTCP),
which is a collaboration between the Marine Turtle Research Group, The
Society for the Protection of Turtles in North Cyprus (SPOT) and the
North Cyprus Department of Environmental Protection. We thank the
latter department, as well as the North Cyprus Veterinary Department
and the North Cyprus Department for Animal Husbandry for their
continued permission and support. Field work in Cyprus was supported
by the Erwin Warth Foundation, Kuzey Kıbrıs Turkcell, Türkiye ˙
Is¸
Bankası, Kars¸ ıyaka Turtle Watch, MAVA Foundation, Roger de Freitas,
Peoples Trust for Endangered Species, Tony and Angela Wadsworth,
United States Agency for International Development, Presidency of
TRNC. EMD receives generous support from Roger de Freitas, the Sea
Life Trust and the University of Exeter. BJG, EMD and LCMO received
support from the National Research Foundation, Prime Minister’s Ofce
(Singapore) and the Natural Environment Research Council (United
Kingdom) under the NRF-NERC-SEAP-2020 grant call ‘Understanding
the Impact of Plastic Pollution on Marine Ecosystems in Southeast Asia
(South East Asia Plastics [SEAP]), under the project entitled Risks and
Solutions: Marine Plastics in Southeast Asia (RaSP-SEA; NRF Award No.
NRF-NERC-SEAP-2020-0004, NERC Award No. NE/V009354/1). BJG,
ACB and EMD were supported by European Commission project INDICIT
II (11.0661/2018/794561/SUB/ENV.C2). The authors would like to
thank Emma Wood for her wonderful illustration.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.marpolbul.2024.116141.
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