Between December 2015 and December 2017 a total of 63 sea turtles were recorded as being stranded along the Algerian coast. The loggerhead sea turtle Caretta caretta was the most commonly stranded species (n = 44) (69.8%), followed by the leatherback Dermochelys coriacea (n = 18) (28.6%) and the green turtle Chelonia mydas (n = 1). There was a slight dominance of the adult size class for stranded loggerhead turtles, while, for the leatherback, late juveniles and adults prevailed. Most loggerhead turtles stranded during the summer months (July and August), whereas most leatherbacks stranded during winter. The breakdown of the strandings by region shows a slight dominance along the western and central shores for C. caretta and a clear dominance in the west for D. coriacea.

The primary cause of death was determined in 50.8% of the stranded turtles. Regarding the evidence of interactions with humans the major cause of stranding in loggerhead turtles was incidental catch by artisanal fisheries, followed by boats’ collisions. The main causes of leatherback strandings were boats’ collisions. Algerian data show that human activities affect loggerhead turtles and also prove a significant presence of the leatherback turtle on this coast.

Algerian coast, Caretta caretta, Chelonia mydas, Dermochelys coriacea, Mediterranean Sea

Systematic gathering of data on stranded sea turtles represents an opportunity for scientists to understand mortality factors, such as by-catch, and can provide information about distribution patterns (Casale et al. 2010). Sea turtles are slow growing and have a long lifespan. This is due to their complex life history patterns which encompass a diversity of ecosystems (Bolten 2003). Seven species of sea turtles representing two families, Cheloniidae and Dermochelyidae, are the only living members of what has been a large and diverse marine radiation of cryptodiran turtles.

Three species of sea turtles are frequently found in the Mediterranean: leatherback turtles (Dermochelys coriacea) are widely distributed in the Mediterranean, along with loggerhead turtles (Caretta caretta), and green turtles (Chelonia mydas) (Casale et al. 2018). Only the last two species have been detected breeding in the basin. Loggerhead turtles and leatherback turtles are classified under the ‘vulnerable’ species category (Wallace et al. 2013; Casale and Tucker 2017), whereas the green turtle is listed as endangered (Seminoff 2004). The survival of sea turtles is threatened due to their slow reproductive dynamics as a result of delayed sexual maturity and high natural mortality rate of early juveniles. However, the biggest challenges that sea turtles face currently are due to anthropogenic threats in terrestrial habitats, such as coastal development, erosion and beach armoring, driving on the beach, and in marine habitats, including the interaction with fisheries, human exploitation, marine debris, pollution, and boat collisions (Casale et al. 2018). Along the Algerian coast, the presence of these marine reptiles has been mentioned in old literature. The loggerhead turtle was considered to be a common visitor to Algerian coasts (Doumergue 1901). Poiret (1789) considered the leatherback as a very common marine life species which resided at the barbary coasts. The presence of the green turtle in these coasts was, however, not described in the literature. The Algerian Basin is now considered as a foraging ground for both the Mediterranean and Atlantic stock of the Caretta caretta species (Clusa et al. 2014), and also for the leatherback (Casale et al. 2003). Hence, further research was required to better understand biology and conservation of these species frequently present on Algerian coasts, and to develop a strategy to mitigate the effects of anthropogenic threats in these waters.

The present study reports for the first time records on sea turtle strandings on the Algerian coasts over a two-year period (2016–2017) in order to gain knowledge about threats to these species. Especially since the Algerian population has grown strongly in recent decades, reaching 40.6 million inhabitants in 2015, concentrated in a fringe of approximately 150 km, along the Mediterranean coast. More than 130 million people frequented the beaches during summer while fishing in this country has developed strongly and been modernized over the last decade (Wiefels 2014).

According to Eriksson (1965), the western Mediterranean is considered a vast ocean-type basin, in which the Algerian-provincial basin is the largest. The Algerian coast, stretching across more than 1600 km, from the east to the west makes up approximately 3.4% of the total Mediterranean coast. This continental margin borders the southern part of the large Algiers-Provencal basin, an important morphological part of the Western Mediterranean Sea (Leclaire 1972). The present study was conducted along the Algerian coast from the far East of Altaref (36.940920N, 8.642120W) to the far West of Tlemcen (35.085006N, 2.211779E) (Fig. 1).

Figure 1. 

Map showing the subdivision of the three zones considered in this study: Western, Central and Eastern zone, and the distribution of stranding sea turtles Caretta caretta (brown circle), Dermochelys coriacea (black circle) and Chelonia mydas (green circle) along Algerian coast (2016–2017).

In the present study, stranded sea turtles were defined as those that had been washed ashore, dead or alive, or were found floating dead or alive (generally in a weakened condition) in coastal waters.

The sex was determined in nineteen loggerhead turtles using the tail length for adult individuals and examining the gonads at necropsy when it was possible. However, the sex for the leatherback turtles could not be determined due to difficulties in recovering the carcasses and limitations in the necropsies performed in situ, at the site of stranding. Necropsies were performed on seventeen and four carcasses of stranded C. caretta and D. coriacea, respectively. The cause of stranding was determined through this necropsy and direct visual observation. We also collected samples (stomach contents, humerus, tissues muscle, kidney, liver) which were frozen for further research.

The coastline was subdivided in 14 stranding areas, according to the administrative division and grouped in three sectors: western region (Tlemcen, Aintemouchent, Oran, Mostaganem), center region (Chlef, Tipaza, Algiers, Boumerdese, Tizi Ouzou) and the eastern region (Bijaia, Jijel, Skikda, Annaba, Altaref) (Fig. 1).

The year was divided into four seasons: winter (January to March), spring (April to June), summer (July to September), and autumn (October to December).

Records of marine turtles found stranded in this study were collected from December 2015 to December 2017, both months inclusive. Data were collected through two methodologies: firstly, as systematic surveys on beaches and other areas of difficult access (rocky areas). We performed these surveys periodically almost every day with the help of 20 volunteers. Most of them were part of fishing communities or people who lived near the coast. The coordination between the teams took place through telephone calls.

The second methodology consisted of an opportunist effort of surveys by receiving information through different sources and going to the stranding location to gather data. We received alerts regularly from diverse sources such as the coast guard, the national gendarmerie, people living in coastal villages, fishermen, civil protection, diving clubs, environmental associations, the National Center for Fisheries and Aquaculture Research (CNRDPA), University Oran 1, University Annaba Badji Mokhtar, social networks, and others. Both methodologies were adopted on the western and central coasts. Only the opportunist survey was adopted on the eastern coast.

This difference in the survey effort was mainly due to two factors: first, the University to which the team belonged is located to the west, second the east coast is characterized by isolated beaches. Therefore, strandings in the eastern part probably were underestimated.

The data on dead turtles were collected according to the established stranding protocols (Wyneken 2001), namely: species identification, curved carapace length notch to tip (CCL), and general biometric measurements (standard length, curved carapace width, head width) using a soft millimetric tape (Wyneken 2001). No complete records of CCL were obtained in 4 C. caretta turtles because the measurements were taken by unqualified persons. The CCL values were grouped by size-classes using the Sturges’ rule to visualize their distribution (Sturges 1926). For loggerheads, a carapace length (CCL) of 70 cm was used to distinguish juveniles from adults, since this was the threshold for nesting females and adult males with secondary sexual characteristics in the Mediterranean considered in the literature (Margaritoulis et al. 2003; Casale et al. 2005; Casale et al. 2009). For leatherback turtles, we used a size class CCL < 125 to provide information about the juveniles, because this is the minimum carapace size for nesting leatherbacks at the nearest nesting sites (Stewart et al. 2007).

In the present study, we considered all types of evidence from human interactions that could be linked to the causes for stranding. The causes of stranding were classified into four categories as below: 1) Incidental catch: including bycatch in fishing nets, fishing line or hook detected in the mouth; 2) Collision with boats; directly crashed turtle carapace, injuries in carapace or severed body parts; 3) Marine debris: intestinal impactions; 4) Undetermined cause of stranding; when no apparent cause of death was observed due to the advanced state of decomposition of the stranded carcass.

A total of 63 stranded sea turtles were found during the study period: 44 (69.8%) C. caretta, 18 (28.6%) D. coriacea and one C. mydas (1.6%) (Table 1). The CCL of the C. caretta ranged between 43 and 93.5 cm, with a mean of 71.18 ± 12.73 SD cm. The CCL of D. coriacea ranged between 86 and 137 cm, with a mean of 120.25 ± 14.59 SD cm. The length of the C. mydas was CCL = 42 cm. The majority of the stranded loggerheads ranged between 66 and 81 cm (Fig. 2). For D. coriacea turtle strandings most turtle measurements fell between 111 and 140 cm (Fig. 3). We found no significant differences in size between dead and live turtles in both species C. caretta (T test, t = 1.26 p > 0.05) and D. coriacea (t = 0.017, p > 0.05). Live stranded loggerheads accounted for only 6.8% of all the recorded stranding events. Live leatherbacks were 5.6% of all the stranded leatherbacks. Males and females of C. caretta did not differ in size (t = 1.10, p > 0.05). Concerning sex, we identified 3 male loggerheads (6.8%), 16 female loggerheads (36.4%), and 25 (56.8%) from which we could not determine the sex. We found more strandings in year 2016, with n = 42 (66.66%) as compared to 2017, where n = 21 (33.33%) for both species.

Figure 2. 

The size frequency distribution of marine turtle Caretta caretta strandings along the Algerian coast (2016–2017).

Figure 3. 

The size frequency distribution of marine turtle Dermochelys coriacea strandings along the Algerian coast (2016–2017).

There was no significant difference in CCL among seasons for both loggerhead turtles (ANOVA, F = 0.084, p > 0.05) and leatherbacks (F = 2.063, p > 0.05). The CCL of sea turtles stranded according to the sex, status (alive/dead), regions and season is shown in Table 2. No significant difference was found in CCL according to regions for both, loggerhead turtles (ANOVA, F = 2.74, p > 0.05) and leatherbacks (F = 1.50, p > 0.05). From 44 stranded loggerhead, n = 16 (36.36%) were found in the west, the same value in the center. For the D. coriacea a high percentage of stranding was found in the western area, n = 8 (44.4%). The distribution of the stranding data for loggerheads and leatherbacks on the basis of regions and seasons is shown in Figures 4, 5, respectively.

Figure 4. 

The distribution of the strandings loggerheads on the basis of regions and seasons along the Algerian coast (2016–2017).

The percentage of loggerheads turtle strandings was relatively homogenous between adults 57.5% and juveniles 42.5%. The leatherback turtles stranded show an equality of late juveniles and adults along the Algerian coast (Table 3). From the 44 stranded loggerhead turtles, we could not identify the cause of stranding of 19 (45.5%) of the turtles. Human interactions linked to the causes for stranding were incidental catch with artisanal fishing (set and drift net) (n = 15, 34.1%), followed by boat collisions (n = 9, 20.5%), and marine debris ingestion (n = 1, 2.3%).

Among the 18 leatherback stranded, no sign of anthropogenic interaction was reported in n = 12 (66.7%) while the carcasses were in very good condition. Three had injuries in carapace or severed body parts (16.7%), presumably due to impacts with boats, and two had intestinal impactions (11.1%), with plastic bags. Only one was released alive after it had been bycaught by an on set net (5.6%). Getting entangled in a fishing net was the cause of stranding of the green turtle.

Figure 5. 

The distribution of the strandings leatherbacks on the basis of regions and seasons along the Algerian coast (2016–2017).

The majority of turtles stranded during the studied period were loggerheads. The frequency of stranding illustrates the regular presence of C. caretta and D. coriacea in the Algerian coastal waters, and accidentally, the green turtle. The latter species was considered ‘rare’ in this area and has Atlantic origins, entering the Mediterranean through the Gibraltar Strait (Carreras et al. 2014).

From our survey, it appears that loggerheads on the Algerian coast coming from the Atlantic (as shown by previous genetic studies) and moving east with the main currents will face anthropogenic threats.

The present study cannot provide detailed information about the primary cause of mortality, apart from those caused by fisheries or boat collisions. This is due to a low number of necropsies and the lack of information about the interaction with fishing gear.

When discussing the status of the leatherback turtles on this coast, this study shows a significant presence of this species. Additionally, this species also suffers from human interaction, given the large number of strandings over two years of study.

This article is offered in memory of my professor Boutiba Zitouni, who died 3 years ago, Professor at University of Oran 1, one of the famous Mediterranean marine specialists. The authors would like to thank Jesús Tomás for his comments which improved the quality of the paper. We also thank all the staff members of our LRSE laboratory, I also thank Amina Tifoura, PhD student at the University of Algiers. I am deeply indebted to the Chief Commander of the Civil Protection Unit of Cherchel Bahiri for his involvement in the conservation of sea turtles. A big thanks to all those who have contributed to this work in various ways.