Analysis of Plastic Ingestion by Juvenile Loggerhead Sea Turtles (Caretta caretta) Stranded from Tenerife, Canary Islands

Abstract

The exponential rise in plastic debris in oceans poses a severe threat to marine biodiversity, including loggerhead sea turtles (Caretta caretta) due to their widespread distribution and feeding habits. The present study aimed to assess plastic ingestion in juvenile loggerhead sea turtles stranded in Tenerife, Canary Islands. Among the 312 turtles admitted to the Wildlife Recovery Center “La Tahonilla” from July 2018 to November 2020, bycatch (20.8%) and entanglement (15.4%) were the primary admission causes, with significantly higher strandings in the island’s western region. Of these, 37 turtles (11.9%) had ingested plastic debris, totaling 546 pieces (average 14.59 ± 26.57 per turtle). Predominantly, filaments (44%), sheets (34%), and fragments (23%) were identified, with most being macroplastics (67%) in white or transparent colors. These findings, consistent with similar studies, underscore loggerhead sea turtles’ role as bioindicators of sea water pollution. They also highlight the urgent need for sustainable practices to mitigate plastic pollution in marine environments, preserve marine biodiversity, and achieve global sustainability goals.

1. Introduction

According to the Organization for Economic Co-operation and Development [1], an estimated 13.8 million tons of plastic enter the oceans annually, accounting for 80% of total marine debris [2]. Plastic presents a significant environmental threat due to its prolonged biodegradation, lasting potentially hundreds of years [3,4]. This persistence presents grave dangers to numerous organisms, as they may ingest or become entangled in plastic debris, especially those unable to distinguish it from food [5], such as various species of sea turtles [6,7,8]. The incidence of marine debris, especially plastics, and its high potential to cause damage to the marine environment has led to its recognition as a global problem, and it is currently included among the main threats affecting marine biodiversity [9].

Sea turtles are among the most impacted wildlife groups by marine debris [6]. Plastic ingestion has been documented across all seven sea turtle species [5,7,10,11,12]. Various characteristics make them susceptible to ingesting substantial quantities of marine litter or becoming entangled in items such as fishing gear [5,13]: they have an extensive distribution in European waters, including the Atlantic and Mediterranean [14,15,16,17,18,19]; inhabit diverse habitats and exhibit varied feeding behaviors throughout their life cycles [20,21]; often dwell within the upper layers of the water column, where plastic debris accumulates [22,23,24]; and are indiscriminate feeders [25]. This vulnerability positions them as valuable bio-indicators for assessing the impacts of marine debris in European waters [26].

Six species can be seen in Canary Islands waters, which are, in order of abundance of sightings: loggerhead turtle (Caretta caretta), green turtle (Chelonia mydas), leatherback turtle (Dermochelys coriacea), hawksbill turtle (Eretmochelys imbricata), olive ridley turtle (Lepidochelys olivacea), and Kemp’s ridley turtle (Lepidochelys kempii), with the last two being rare sightings [27,28]. The loggerhead turtle, the most commonly observed species in the Macaronesian region and in the Canary Islands [29], frequents this region in search of food during its oceanic juvenile stage [22,30]. This stage occurs from 7 to 11.5 years approximately for loggerhead turtles, when they reach an approximately size of 46–64 cm curved carapace length [22,31]. However, carapace length is not a precise measure of age, sexual maturity and feeding strategy [20].

Genetic studies on loggerhead turtle juveniles located in the Azores [32] and Madeira [33] suggest that these juveniles primarily originate from the eastern coast of the USA. In the Canary Islands, genetic analyses show the presence of juveniles from breeding populations in Florida, Mexico, and Cape Verde [34,35]. Historical nesting records exist for the island of Fuerteventura [36], although it does not currently nest naturally in the Canary Islands [29]. Currently, the loggerhead turtle is classified as “Endangered” on the Red List of Threatened Species by the International Union for Conservation of Nature (IUCN, www.iucnredlist.org/species/3897/119333622, accessed on 8 July 2024). Although global extinction of the species is improbable in the short to medium term, it is considered vulnerable due to the status of some of the subpopulations, which should be assessed independently [37]. In the Canary Islands, at least three subpopulations are found: Northwest Atlantic and Mediterranean subpopulations, listed as “Least Concern”, and the Northeast Atlantic subpopulation, listed as “Endangered”.

Recent studies have shown a correlation between plastic accumulation on Tenerife’s beaches and the surrounding currents, with debris from the North Atlantic Ocean gyre being carried towards the north and northeast of the island [38,39]. The eastern coast of Tenerife, influenced by strong currents, waves, and wind [38,39,40], has the highest levels of plastic pollution, with more than 1200 items (all of them between 1–5 mm) per square meter [39,41,42,43].

The first aim of this study was to determine the frequency of plastic ingestion by juvenile loggerhead sea turtles in Tenerife, Canary Islands by quantifying the presence of plastics in both the gastrointestinal tracts of deceased turtles and the feces of live specimens. The second aim was to characterize the plastics in terms of type, size, and color. Additionally, the study examined the relationship between the frequency of plastic ingestion and factors such as the body condition index (BCI) of the turtles, the stranding location, and the cause of stranding. This case study provides valuable insights into the impact of plastic pollution on the conservation and biology of sea turtles in the Canary Islands.

2. Materials and Methods

2.1. Area of Study and Sample Collection

All turtles sampled in this study belong to loggerhead turtles and were found on the coasts of the island of Tenerife, from July 2018 to November 2020, stranded on the beach, floating adrift, entangled or accidentally captured by fishing gear. They were taken to the Wildlife Recovery Center (WRC) “La Tahonilla” (Tenerife) and received an identification number and an individual record documenting the municipality of location, date and presumptive diagnosis upon admission. Biometric data, such as weight and carapace length measurements, were also recorded (see Appendix A,

Table A1. Information on the turtles and samples collected at the WRC “La Tahonilla” corresponding to the 37 turtles (Caretta caretta) affected by marine debris.

ID	CCL (cm)	Turtle Weight (kg)	Pieces	Pieces Weight (g)	Body Condition Index (bci)	Stranding Cause	Stranding Zone/Code
10446	26.0	2.00	25	0.318	1.132226	Indeterminate	Puerto Colón	4
10452	40.0	9.25	3	0.03	1.445313	Symptoms of illness	Puerto Colón	4
10506	26.0	2.00	4	0.007	1.126536	Entanglement	Punta de Teno	4
10519	30.0	4.38	1	0.014	1.622222	Collision	Puerto Colón	4
10808	67.1	51.00	140	6.50	1.688117	Indeterminate	Playa de Los Surfistas	2
10814	50.0	13.90	7	1.003	1.112000	Collision	Puerto de Los Cristianos	2
10904	40.0	7.70	51	1.853	1.203125	Symptoms of illness	Puerto Colón	4
11587	39.0	6.35	80	3.682	1.070483	Indeterminate	Puerto Colón	4
11604	32.0	3.65	1	4.812	1.113892	Entanglement	Puerto Colón	4
11886	37.0	6.10	1	0.04	1.206246	Indeterminate	Puerto Colón	4
11979	46.0	13.20	1	0.001	1.356127	Entanglement	Playas de Fañabé	4
11984	37.0	7.10	2	0.402	1.401694	Symptoms of illness	Costa Adeje	2
11986	40.0	8.80	3	0.198	1.375000	Symptoms of illness	Puerto Colón	4
11988	42.0	9.00	18	0.314	1.214772	Indeterminate	Puerto Colón	4
12002	52.5	18.00	40	5.59	1.243926	Symptoms of illness	Puerto Colón	4
12004	33.0	4.45	8	0.799	1.238556	Entanglement	Puerto Santiago	4
12017	64.0	34.75	7	0.029	1.325607	Bycatch	Playa de San Juan	4
12076	49.0	15.30	13	1.82	1.300479	Indeterminate	Puerto Colón	4
12083	28.0	2.45	1	0.001	1.116071	Bycatch	Puerto Colón	4
12089	60.0	23.65	2	1.51	1.094907	Indeterminate	Puerto Colón	4
12106	22.8	1.48	22	0.772	1.248697	Symptoms of illness	Puerto Colón	4
12121	30.2	3.60	1	0.004	1.307018	Bycatch	Puerto Colón	4
12124	31.0	3.05	2	0.056	1.023799	Entanglement	Puerto Colón	4
12125	30.0	3.50	4	1.150	1.296296	Indeterminate	Puerto Colón	4
12143	30.0	3.35	1	0.073	1.240741	Symptoms of illness	Puerto Colón	4
12157	63.5	30.00	2	0.209	1.171656	Bycatch	Los Gigantes	4
12292	51.5	14.15	3	0.191	1.035940	Entanglement	Punta del Hidalgo	1
12311	26.0	1.94	5	0.224	1.103778	Entanglement	El Poris	3
12415	26.0	2.06	10	0.104	1.172039	Entanglement	Punta del Hidalgo	1
12457	29.0	2.75	12	0.247	1.127558	Healthy caught	Puerto Colón	4
12541	41.0	6.50	23	0.67	0.943109	Entanglement	El Poris	3
12555	45.0	9.90	33	7.294	1.086420	Entanglement	Muelle de Santa Cruz	3
12941	17.5	0.80	4	0.287	1.492711	Bycatch	Puerto Colón	4
13056	42.0	8.90	6	0.175	1.201274	Entanglement	Los Gigantes	4
13216	54.0	18.00	2	0.637	1.143118	Indeterminate	Playa de Fañabe	4
13278	57.0	18.55	1	0.595	1.001658	Symptoms of illness	Malpaís	3
13280	33.0	3.80	6	1.794	1.057406	Bycatch	Los Gigantes	4

Stranding zone code: (1) North. (2) South. (3) East (4) West. (CCL) Curved length of the shell.

).

During their stay and recovery at the center, the turtles were kept in individual tanks, and they were fed three times a week with sardines, anchovies, and crustaceans. The daily food amount for each turtle was calculated in relation to the size of the animal and corresponded to approximately 5% of its weight. Tank water was changed every two to three days depending on turtles’ condition.

Sample collection was carried out following the INDICIT protocol [44]. INDICIT is a project funded by the European Union committed to support the implementation of the Marine Strategy Framework Directive (MSFD) [45] by developing a set of standardized tools for monitoring the impacts of litter on sea turtles. This protocol proposes a specific methodology for assessing ingested marine litter, including its composition and abundance, evaluating its impact on marine environments and establishing threshold values for its presence in turtles [44,45].

Following this protocol, sampling can be performed both in live and dead individuals. For live turtles, fecal samples were collected and the tank water was visually inspected for the presence of plastics. For deceased turtles, necropsies were performed, and the contents of the esophagus, stomach, and intestines—key areas for the presence and accumulation of marine debris—were analyzed. The minimum size of debris considered part of the “Garbage ingested by sea turtles” indicator was set at 1 mm [44,45].

2.2. Plastic Analysis and Identification

Similarly to the sampling process, the analysis and classification of marine debris followed the protocols outlined in the INDICIT project [44]. Turtle stomach contents were separated and prepared for measuring (maximum length, within 0.1 cm using a Tesa Digico 12 caliper) and weighing (dry mass, within 0.001 g using a Precisa 125A microbalance) (see Appendix A, Figure A1, Figure A2 and Figure A3). Plastics were described in as much detail as possible, also following the MSFD guide for monitoring marine debris in European seas [46]. They were classified into “item”, representing individual plastic units, and “pieces”, denoting fragments of an item.

Plastics were classified into nine categories

Table 1. Classification of marine debris items found in turtles, following the MSFD guidance [46].

Type	Code	Description
Industrial plastic	IND PLA	Industrial plastic granules, usually cylindrical but also sometimes oval spherical or cubical shapes, or suspected industrial item, used for the tiny spheres (glassy, milky, etc.)
Sheet	SHE	Remains of sheet (e.g., bags, cling-foil, agricultural sheets, rubbish bags, etc.)
Filament	THR	Threadlike materials (e.g., nylon wire, net-fragments, woven clothing, etc.)
Foam	FOA	All foamed plastics (polystyrene foam, foamed soft rubber, etc.)
Fragment	FRAG	Fragments, broken pieces of thicker type plastics, flexible but not like sheet.
Other	OTH	Other type of marine debris different from plastics

[44,46]: industrial plastic (IND PLA), sheet type (USE SHE), filament (USE THR), foam (USE FOA), fragments (USE FRAG), other plastics (USE OTH), non-plastic garbage (OTHER), natural food (FOO), and natural remains not identified as diet (NFO). Table 1 describes each category.

Finally, all plastics found were classified according to colour and size as microplastics (1–5 mm), mesoplastics (5–25 mm), and macroplastics (>25 mm) [44].

All plastic material is documented in Appendix A,

Table A2. Information on the turtles and samples collected at the WRC “La Tahonilla” corresponding to the 37 turtles (Caretta caretta) affected by marine debris.

ID	Type						Size
	She	Frag	Thr	Foa	Other	Ind	Macro	Meso	Micro
10446	8	11	6	0	0	0	3	14	8
10452	2	0	1	0	0	0	2	1	0
10506	0	0	4	0	0	0	1	3	0
10519	0	0	1	0	0	0	1	0	0
10808	26	11	103	0	0	0	119	21	0
10814	5	2	0	0	0	0	5	2	0
10904	14	6	30	0	0	0	37	13	0
11587	51	7	24	0	0	0	50	30	2
11604	0	0	0	0	1	0	1	0	0
11886	0	0	1	0	0	0	1	0	0
11979	0	0	1	0	0	0	1	0	0
11984	1	0	1	0	0	0	2	0	0
11986	2	1	0	0	0	0	1	2	0
11988	2	0	16	0	0	0	17	1	0
12002	31	6	3	0	0	0	28	12	0
12004	7	1	0	0	0	0	3	5	0
12017	1	0	6	0	0	0	6	1	0
12076	9	3	1	0	0	0	10	3	0
12083	0	0	1	0	0	0	0	1	0
12089	2	0	0	0	0	0	2	0	0
12106	3	0	19	0	0	0	22	0	0
12121	1	0	0	0	0	0	0	1	0
12124	1	0	1	0	0	0	2	0	0
12125	0	0	0	4	0	0	2	2	0
12143	1	0	0	0	0	0	1	0	0
12157	2	0	0	0	0	0	1	1	0
12292	0	3	0	0	0	0	0	3	0
12311	0	1	4	0	0	0	4	1	0
12415	0	5	5	0	0	0	3	4	3
12457	0	12	0	0	0	0	0	7	5
12541	1	4	18	0	0	0	19	4	0
12555	4	28	1	0	0	0	6	23	4
12941	4	0	0	0	0	0	4	0	0
13056	2	1	3	0	0	0	4	2	0
13216	0	2	0	0	0	0	0	2	0
13278	1	0	0	0	0	0	1	0	0
13280	6	0	0	0	0	0	4	0	2

.

2.3. Statistical Analysis

All analyses were performed using R Statistical Software v4.1.2 [47]. The percentage of turtles affected by ingestion of marine debris and plastic was estimated.

The average weight and total number of plastics found in each turtle were calculated and presented as mean ± standard error of the mean. The most abundant plastic category (lamella/SHE, fragment/FRAG, filiform/THR, foam/FOAM, and others/OTHER) and the predominant size (macro, meso, and micro) were calculated. Plastic items were also classified into the 12 observed color categories (black, blue, green, grey, red, pink, orange, purple, red, yellow, white and transparent). Items with more than one color were categorized as “multiple” [44,45].

Before conducting analyses, the Kolmogorov–Smirnov normality test assessed the normal distribution of all variables [48]. For those variables exhibiting non-normal distributions, logarithmic transformations were applied. Non-parametric tests were employed for variables that remained non-normally distributed after transformations [49].

To explore the relationship between plastic ingestion and turtle size, a Pearson correlation coefficient [50] was calculated, using the curved carapace length (CCL) as the independent variable and the total number of pieces per turtle as the dependent variable, with statistical significance set at 0.05.

The chi-square statistical test [51] was used to determine if there was any predominant type, size, or color of plastic, as well as any predominant stranding zone or cause of admission, with a statistical significance set at 0.05. Post hoc analysis, incorporating adjusted residuals and the Bonferroni correction, was conducted to identify specific groups exhibiting significant differences.

The body condition index (BCI) is a metric used to assess the overall health and physical state of a sea turtle based on its body size and weight. It provides insights into the animal’s nutritional status and helps determine whether a turtle is in good or poor physical condition. BCI can be established subjectively by assessing the physical appearance of the animal on a scale of 1–5, where 1 corresponds to an emaciated appearance characterized by sunken eyes and loss of musculature and 5 represents an obese turtle. A BCI value of 3 is considered normal [52]. Additionally, it can be calculated objectively according to the mass and length of the animal, using the formula:

$$\mathrm{B}\mathrm{C}\mathrm{I}=\left[{\displaystyle \frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\left(\mathrm{k}\mathrm{g}\right)}{{\mathrm{L}\mathrm{R}\mathrm{C}}^3\left({\mathrm{c}\mathrm{m}}^3\right)}}\right]\times 10000,$$

where LRC is the straight carapace length [52]. This formula was applied to compute the variable “BCI”.

To investigate distribution of the variable BCI between the two groups (turtles with and without plastics), the non-parametric Mann–Whitney U test [53] was employed. To specifically examine the correlation between BCI and the number of plastics ingested, the Pearson correlation test was used.

The Dunn–Bonferroni multiple comparisons test was used to identify significant differences between the plastics found in different zones sampled on the island, adjusting for the increased risk of Type I errors from multiple pairwise comparisons [54].

3. Results

During the study period, 312 sea turtles entered the WRC “La Tahonilla” (Tenerife) (Figure 1); 39 presented marine debris, of which 37 (11.9%) contained some plastic element. The two remaining turtles contained metal hooks with their respective nylon threads (Appendix A: Table A1 and Table A2). Of these 37 specimens (all loggerhead turtles), one was dead, three died at the center during the recovery process, and the rest were released. In total, 43,402 g of marine debris was collected, consisting of 546 pieces (

Table 2. Summary of results.

	Total	Average ± Standard Deviation	Minimum	Maximum
Turtle CCL (cm)	-	39.68 ± 12.68	17.50	67.10
Turtle weight (kg)	-	10.14 ± 10.61	0.80	51.00
Plastics (items)	546	14.59 ± 26.57	1.00	140.00
Plastic dry mass (g)	43.40	1.17 ± 1.90	0.01	7.29

).

The largest turtle (51 kg and 67.1 cm CCL; ID 10808) presented the highest number of plastics (140 pieces: 26 sheets (USE SHE), 11 filaments (USE THR), and 103 fragments (USE FRAG). The maximum weight of plastics was 7.29 g, found in a 9.9 kg, 45 cm CCL turtle (ID 12555). There are large differences in the ingestion of marine debris among the turtles sampled.

Considering the data obtained for the types of plastics found in marine debris ingested by loggerheads (Appendix A: Table A2), the most abundant plastic category was filiform plastics (USE THR), with 250 pieces (46%); followed by plastic in sheet form (USE SHE), with 187 pieces (34%); and fragments (USE FRAG), with 104 pieces (19%). The remaining types, foam plastics (USE FOAM) and other plastics (USE OTHER), were found in clearly smaller quantities (Figure 2). The chi-square test did not reveal any significant predominance within any category.

The predominant plastic size observed was macroplastic, constituting 66% of the total pieces, followed by mesoplastics at 29%, and microplastics at 4% (Figure 2). The chi-square test did not reveal any significant predominance within any size.

Among all plastic collected, white (31%) and transparent (20%) were the main colors ingested by sea turtles. Green (17%), blue (13%), and black (10%) were also quite abundant, and the remaining colors do not exceed 5% frequency. The chi-square test did not reveal any significant predominance within any color.

The linear regression correlating the ingestion of garbage with the turtle’s body size (logarithmic transformation of the variables “pieces” and “CCL” to comply with data normality) yields a Pearson correlation coefficient of p = 0.1243. Despite the p-value of 0.46, which is above the significance level of 0.05, indicating that there is not enough evidence to reject the null hypothesis of no correlation, the confidence interval includes both positive and negative values. The relationship is rather weak and is not considered statistically significant. Plastic was found in turtles of all size ranges (Figure 3).

The body condition indices of all turtles in this study varied between 0.175 and 2.963. The non-parametric Mann-Whitney U test resulted in a W coefficient of 5686.50 and a p-value greater than 0.05 (p = 0.24), indicating no significant difference between the groups (normal distribution of the data could not be achieved). The BCI of turtles with plastics is distributed in a tight range between 0.943 and 1.688 (Figure 4; Appendix A: Table A1). The average and standard deviation of the BCI for turtles with plastic were 1.218 ± 0.165. For the overall sample of turtles, the BCI was 1.183 ± 0.284.

The Pearson correlation test, conducted after logarithmic transformation of the “bci” and “pieces” variables, resulted in a coefficient value of −0.00289, indicating a weak negative very close to zero correlation between both variables. Moreover, the p-value exceeds 0.05 (p = 0.98), suggesting that there is no statistically significant correlation between the variables “pieces” and “bci”.

The strandings of the overall turtle population occur across the entire island, with a higher number observed in the southwest. In the specific case of turtles with plastics, strandings predominantly take place in the south and west, with almost no occurrences in the north. In both scenarios, strandings in the western part of the island are statistically and significantly higher than in the other regions, as indicated by the chi-square statistical test.

A compilation of studies on the presence of plastic debris (larger than 1 mm) on Tenerife’s beaches from 2016 and 2022 revealed a significant spatial variability in plastic concentrations. Samples were collected from beaches in the north (three beaches), south (four beaches), east (four beaches), and west (two beaches) zones. The findings indicate that Playa Grande in the east consistently shows high plastic concentrations across all studies. The Dunn-Bonferroni multiple comparisons test revealed significant differences between the eastern zone and the other zones.

When examining the cause of admission for all turtles, interaction with fishing gear, commonly known as “bycatch”, is the most prevalent cause, affecting 20.8% of the turtles, followed by entanglement, which affects 15.4% (excluding turtles with indeterminate causes of admission). In the specific analysis of turtles with plastic, it is observed that the majority, 29.7%, enter the center due to entanglement (Figure 5). The results of the chi-square test revealed statistically significant differences between the mentioned admission causes and the others for each group of turtles.

4. Discussion

The present study provides evidence and a comprehensive characterization of plastic debris found in the digestive systems of stranded loggerhead sea turtles in Tenerife, Canary Islands. The acquired data not only improve the current knowledge about plastic ingestion by loggerheads in the eastern North Atlantic but also highlights the importance of loggerhead turtles as key focal points in the research and conservation management of marine environmental challenges.

Other studies conducted in the Atlantic Ocean report incidences of 25% [55], 59% [12], and 83% [56]. In some cases, frequency of plastic ingestion may be underestimated, as only the stomach content was considered on dead turtles [55], and the intestines, where marine debris is often more abundant [56], were not examined. The significant variation in frequencies between studies could be attributed to various factors such as seasonality, life cycle stages (juveniles or adults) and status of the turtles sampled (alive or dead) and differences in sampling methodologies [7,10,56]. Nevertheless, results of this study should be taken into account when comparing with others, especially those using the INDICIT project protocol for sampling and analysis of live turtles.

The range of CCL of the turtles was very wide (17.5–67.1 cm), and plastic ingestion was not significantly related to age ranges, which was consistent with similar studies [57,58]. Some studies categorize turtles based of CCL to analyze plastic impact due to variations in feeding behavior [58,59]. The precise range and size limits of neritic–oceanic transitions remain undefined in the Northeast Atlantic [20,31], Northwest Atlantic [22,60], and Mediterranean [61,62]. This suggests that loggerhead turtles use adaptive ecological niches and demonstrates that carapace length is not conclusive for determining feeding strategy [20]. In this study, all turtles are treated as juveniles without habitat-based distinctions.

In our study, we treated all examined turtles as juveniles without distinguishing between their habitats, reflecting the undefined transitions mentioned earlier. Consequently, we deduce that plastic ingestion occurs across the different feeding strategies of juvenile turtles, given the broad size range analyzed. The presence of plastics across such a wide range of sizes indicates that the ingestion is not confined to a specific size class or feeding habitat. This underscores the pervasive nature of plastic pollution and its impact on sea turtles irrespective of their size or ecological niche. Therefore, our findings help us to understand the impact of plastics in the conservation and biology of sea turtle from Canary Islands and emphasize the need to reach comprehensive conservation strategies that address plastic pollution’s impacts to reach the sustainability and conservation of sea turtles.

The direct responsibility of waste ingestion for the death of marine turtles is rarely reported [63,64]. Although attributing a turtle’s death to plastic ingestion is complex, debris ingestion was not identified as a direct cause of death for most sampled individuals. Nearly all turtles in this study were successfully rehabilitated and released, with only two cases raising suspicions that ingested debris significantly contributed to their deaths:

(a)

In November 2020, a live loggerhead turtle (ID 13278) was admitted with signs of drowning, eye infection, and general weakness. Despite various treatments, it died after 11 days. Necropsy revealed a piece of soft white plastic (140 × 84 mm, 0.595 g) between the stomach and the intestine, obstructing the digestive tract along with severe inflammation and infection in the lungs, liver, and intestines. It was inferred that this obstruction played a significant role in its death, triggering infections in other organs.

(b)

In April 2019, a decomposed turtle was found in south Tenerife (ID 10808). Necropsy revealed 140 pieces: 103 filaments (thr), 11 fragments (frag), and 26 laminates (she). In total, 6.5 g of plastics, most of them macroplastics, including easily recognizable items such as two earbud sticks, a round washer over 3 cm in diameter, and food wrappers.

Studies on dead turtles often find significant plastic presence in the intestines, indicating their capacity to excrete most ingested debris [12,26,56]. The low mortality rate and effective waste excretion suggest turtles are robust bioindicators for assessing marine debris impact [5,8]. However, its known that it probably increases the risk of death due to sublethal effects that are more frequent and harder to detect [7,11].

The results showed differences between plastic types, although they were not statistically significant. The most frequent type of plastic was de filiforms—the same as in other studies [65,66]—followed by sheets and fragments. The high incidence of threads could be linked to interactions with fishing lines and materials related to fisheries [67]. This aligns with the primary stranding cause identified in this study, interaction with fishing gear, commonly referred to as “bycatch”, affecting 20.8% of turtles. This primary cause aligns with findings from other similar studies [12]. On the other hand, white and transparent plastic items are the most commonly ingested colors—the same as in other studies [26,56,58,63,65,66,68]—although they were not statistically significant. This finding corresponds with the predominant colors employed in the packaging industry in recent years [69]. Furthermore, these results align with the color findings of studies conducted on plastic debris on Tenerife beaches, where white and transparent were the most frequently observed colors [39,42]. The ingestion of light-colored debris may be attributed to its reduced visibility in the water, potentially leading to mistaken ingestion [70], or to its resemblance to jellyfish, a common prey item [5,11].

The decision to exclude particles smaller than 1 mm is based on MSFD Technical Sub-group guidelines, which set this as the minimum size for litter items [44,45]. However, this exclusion may lead to an underestimation of ingested debris, as smaller microplastics can still be harmful to sea turtles. This limitation should be noted, and future studies should consider including particles smaller than 1 mm for a more complete assessment of marine debris impacts on sea turtles.

The present study, the first in Tenerife analyzing and categorizing plastic ingestion by loggerhead turtles, increased the available data on the ingestion of plastics by this species. The findings not only highlight the threats faced by this species in the Northeast Atlantic subpopulation but also support the potential usage of live specimens of loggerhead turtles as indicator species for monitoring plastic debris in the marine environment.

This research underscores the urgent need for standardized procedures to accurately quantify ingested debris and calls for further extensive studies to evaluate the true impact of plastic ingestion on the health and survival of sea turtles. Moreover, the findings emphasize the critical importance of developing and implementing effective strategies to combat plastic pollution. By addressing these issues, we contribute to broader sustainability goals, including the preservation of marine biodiversity and the creation of science-based solutions for environmental protection.