Evidence of Mediterranean Monk Seal Predation on Loggerhead Sea Turtle in Lesvos Island, Greece

Loggerhead sea turtle (Caretta caretta) is globally classified as a Vulnerable (VU) species by the International Union for Conservation of Nature (IUCN) Red List, but its subpopulation in the Mediterranean sea is classified as Least Concern (LC) being the most common turtle species of the region [1,2]. Their nesting period takes place between May and October, while 75% of the nesting grounds are located in Greece and Turkey, with Laganas Bay in Zakynthos Island (Ionian Sea) being the largest one [3]. The northern Aegean Sea is primarily visited by loggerhead turtles for foraging and are often found to approach the coasts at a short distance [4,5], probably due to the wide continental shelves of this area [6].

Mediterranean monk seal (Monachus monachus) is considered as the most endangered mammal in Europe with a world population estimates between 400-600 individuals [7]. It is classified as a Critically Endangered (CR) species, comprising of 3-4 isolated sub-populations that are highly dispersed across the Mediterranean Sea [8]. The drop in monk seal population over the last decades makes its distribution becoming even more restricted, especially in the Aegean Sea [9]. Concerning their diet, they can ingest a high variety of food, preferably cephalopods and fish, while they show an opportunistic behaviour, praying upon the most abundant or/and vulnerable organisms at the time. Predation of Mediterranean monk seals on sea turtles is not a usual case and seems to take place at times when turtles are more vulnerable. Between 1984-2010, 29 cases of Mediterranean monk seal predation on loggerhead turtles have been documented in Zakynthos island during the Caretta caretta nesting season (when turtles are exhausted) showing a higher frequency in 2010 [10,11]. In addition, a video showing a monk seal attacking a loggerhead turtle was captured in late October 2017 by fishermen in Elounda bay, NE Crete [12], while remains of a green turtle (Chelonia mydas, Linnaeus 1758) were identified in the stomach content of a monk seal found stranded on the Turkish eastern Mediterranean coast near Antalya in May 2013 [13].

A Mediterranean monk seal was spotted (Lat: 26.490392 ºN, Lon: 38.970974 ºE, Depth: 26m) and video-recorded during a diving survey that took place on 30th March 2021 offshore the western cape of Tarti beach in Lesvos island (Figure 1). At the same time, the corpse of a loggerhead sea turtle was found at about 200m from the monk seal spot (Lat: 26.489042 ºN, Lon: 38.972536 ºE, Depth: 12m) and was retrieved for further investigations. The recorded video suggests that the spotted Mediterranean monk seal individual was an adult female. After examination of the turtle’s corpse, a main injury was evident at the plastron area towards the rear while there were signs of recent decomposition, especially in the eyes area (Figure 2). Examination of the turtle’s tail and body showed that it was an adult female, while further measurements took place. More specifically, the Curved Carapace Length (CCL) and Curved Carapace Width (CCW) were measured at 76.0 and 66.5cm respectively, while Total Tail Length (TTL) and Post-cloacal Tail Length (PTL) were found to be 11.3 and 7.0cm respectively.

Figure 1:A) Locations of the loggerhead sea turtle (Caretta caretta) and the Mediterranean monk seal (Monachus monachus) individuals, found offshore Tarti Beach, Lesvos Island, Greece on 30th March 2021. Photos taken from the locations of B) Female adult loggerhead turtle (Caretta caretta) and C) Female adult monk seal (Monachus monachus) observed on 30th March 2021.

Figure 2:Photographs of the Caretta caretta corpse. A) Full body of the turtle (dorsal side). B) Opening of the body cavity through the lower left side of its plastron, through the soft area between the plastron and the rear flipper. C) Insight of the opening. D) Inner side of the opening, showing the turtle’s entrails. E) Lateral and F) frontal images of the head of the turtle.

The injury spotted at the plastron area towards the rear (Figure 2B & 2C) comes in accordance with previous records of corpses of loggerhead sea turtles that have been attacked by Mediterranean monk seals [10,11]. The majority of the recorded predations of monk seals on loggerhead sea turtles are found to occur in Zakynthos island, during the nesting season [11]. This contribution presents the predation case of a loggerhead sea turtle which was most probably foraging, as the corpse was found in March, out of the turtle’s nesting period, while Tarti beach and the surrounding beaches are characterized by coarse sediments that are not suitable for nesting areas. Mediterranean monk seals show an opportunistic behaviour, and their diet varies, but feed normally on cephalopods and fish species that are most abundant at the time. Degradation of marine environment and reduction of the marine resources could possibly explain the reason for the predation on sea turtles, which is also supported by the relevant literature [11,13].

1. Casale P, Tucker AD (2017) Caretta caretta. The IUCN Red List of Threatened Species 2017. pp. 1-7.
2. Özdilek ŞY, Sönmez B, Sert M (2018) Stranded sea turtle records between 2010 and 2017 in northern Aegean and Sea of Marmara. Regional Studies in Marine Science 24: 17-22.
3. Broderick A, Camiñas JA, Cardona L, Casale P, Fuller WJ, et al. (2015) Demography of marine turtles nesting in the Mediterranean sea: A gap analysis and research priorities. 5^th Mediterranean Conference on Marine Turtles. Turkey, pp. 1-26.
4. Margaritoulis D, Rees AF (2011) Loggerhead turtles nesting at Rethymno, Greece, prefer the Aegean Sea as their main foraging area. Marine Turtle Newsletter 131: 12-14.
5. Patel SH (2013) Movements, behaviors and threats to Loggerhead Turtles (Caretta caretta) in the Mediterranean Sea. Ph.D thesis. Drexel University, USA, pp. 1-132.
6. Coll M, Piroddi C, Steenbeek J, Kashner K, Lasram FBR. et al. (2010) The biodiversity of the Mediterranean Sea: Estimates, Patterns and Threats. PLOS One 5(8).
7. Margaritoulis D, Touliatou S (2011) Mediterranean monk seals present an ongoing threat for loggerhead sea turtles in Zakynthos. Marine Turtle Newsletter 131: 18-23.
8. Karamanlidis A, Dendrinos P (2015) Monachus monachus, mediterranean monk seal. The IUCN red list of threatened species, pp. 1-25.
9. Voultsiadou E, Gerovasileiou V, Dailianis T (2012) Extinction trends of marine species and populations in the Aegean Sea and adjacent ecoregions. In: Marine Extinctions-patterns and processes, CIESM Workshop Monographs, CIESM Publisher 45: 59-74.
10. Margaritoulis D, Karavellas D, Irvine C (1996) Predation of adult loggerheads by Mediterranean monk seals. In: Keinath JA, Barnard DE, Musick JA, Bell BA (Eds.), Proceedings of the 15^th annual symposium on sea turtle biology and conservation. NOAA Tech Memo NMFS-SEFSC-387. pp. 193-196.
11. Margaritoulis D, Rees AF, Dean CJ, Riggall TE (2011) Reproductive data of loggerhead turtles in Laganas Bay, Zakynthos Island, Greece, 2003-2009. Marine Turtle Newsletter 131: 2-6.
12. Markakis S (2017) Monk seal attack on sea turtle, Elouda, Crete 22-10-2017.
13. Tonay AM, Danyer E, Dede A, Öztürk B, Öztürk AA (2016) The stomach content of a Mediterranean monk seal (Monachus monachus): Finding of green turtle (Chelonia mydas) remains. Zoology in the Middle East 62(3): 212-216.