Study Areas

Turtle nesting on Zakynthos Island (Fig. 1) occurs at 6 nesting beaches, totaling 5.5 km in length, fringing Laganas Bay at the southern coast of the island. Tagging in Zakynthos was primarily carried out at the 0.6-km-long Sekania beach (approximate middle of the beach: 37°43′30″N, 20°56′15″E, WGS 84), which hosts the highest density of nests (Margaritoulis 2005; Margaritoulis et al. 2011). In Kyparissia Bay (Fig. 1), loggerhead turtles nest along the 44-km beach from the river Alfios in the north to the river Arcadikos in the south (Margaritoulis and Rees 2001). Tagging effort at Kyparissia Bay was focused on the southernmost 9.5-km beach length (from about 37°22′30″N, 21°40′58″E to 37°17′30″N, 21°41′33″E, WGS 84), which comprises the core area of the nesting habitat because of the high concentration of nests (Margaritoulis and Rees 2001).

Methods

Tagging and re-identifying previously tagged turtles was completed every nesting season from 1982 until 2019 (except for the 1999 season on Zakynthos, when no observations were made), from about the beginning of June until beginning of August, with varying temporal and spatial intensity depending on human resources and logistics. Tagging teams consisting of 2–3 trained volunteers patrolled the assigned beach sectors at night by walking along the wet sand. When they spotted an upward track of an emerged turtle, they followed it until they neared the turtle from behind. Turtles were allowed to nest before they were checked for tags; if there were no tags present they were tagged with external tags. Two types of tags were used: 1) Monel metal tags (style 1005-49 and 1005-681; National Band and Tag Company, USA) applied on front flippers, and 2) plastic tags (cattle-ear Jumbo tags; Dalton Ltd., UK) applied mostly on hind flippers. Generally, turtles were double-tagged, and whenever a turtle was found with only one tag, a second tag was usually applied. Illegible tags, resulting from corrosion, fouling with marine organisms, or embodiment in tissue, were generally removed (Fig. 2) and replaced with new tags. Tags were placed in predefined locations through the membrane between scales at the trailing (posterior) edge of the flippers. We discontinued the usage of 1005-49 Monel tags in 1990 as these tags were rather large and we also stopped using plastic tags in 2015 because there was evidence of interference with fishing gears and increasing entanglement (see also Nichols et al. 1998). In addition to external tags, electronic passive integrated transponder (PIT) tags (11.5 × 2.1 mm; Trovan Ltd.) were also used starting in 2005 in Kyparissia Bay and in 2012 in Zakynthos. Turtles with existing tags or with scars at the defined tagging locations were characterized as “remigrants” while turtles without tags, or scars attributable to tagging, were registered as “neophytes.” Every newly tagged turtle was assigned a unique ID, usually the firstly applied tag code, and all subsequently applied tags were related to this ID. Turtles without tags but with scars attributed to lost tags were retagged and assigned a new ID. Tagging data were compiled annually in specially designed databases (Microsoft Excel) and were organized by the ID of each individual turtle. For the present study, we used this archive to investigate the reproductive longevity of loggerhead turtles in Greece.

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Results

During the 37 nesting seasons in Zakynthos (1982–1998, 2000–2019) and the 38 seasons in Kyparissia Bay (1982–2019), a total of 5707 “neophyte” turtles were tagged in both areas: 3406 in Zakynthos and 2301 in Kyparissia Bay. A number of these turtles subsequently appeared without tags but with scars at the defined tagging sites and so were retagged. Many of the above marked turtles, either neophytes or with scars, retained their tags and were identified in subsequent nesting seasons, with 28 of them revealing reproductive longevities of over 21 yrs. Specifically, 20 individuals on Zakynthos showed reproductive longevities ranging from 22 through 33 yrs (Table 1) and 8 turtles in Kyparissia Bay from 22 through 29 yrs (Table 2). These turtles were observed nesting within 3–12 seasons on Zakynthos (Table 1) and within 6–13 seasons in Kyparissia Bay (Table 2). It is noted that 9 of these turtles (32%) were recorded as already scarred on their first encounter (which contributed to their longevity record), and hence their actual reproductive time span is certainly longer. An example of repeated tagging manipulation of turtles with missing or illegible tags is shown in Table 3.

Table 1 Turtles with reproductive longevity of > 21 yrs, observed in Zakynthos following 37 seasons of tagging (1982–1998, 2000–2019). Asterisk denotes individuals with scars attributed to previous tagging. Turtle B2314 was observed making one nest in Kyparissia Bay during one season.

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Table 2 Turtles with reproductive longevity of > 21 yrs, observed in Kyparissia Bay following 38 seasons of tagging (1982–2019). Asterisk denotes individuals with scars attributed to previous tagging.

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Table 3 Observation dates and respective tag codes of Turtle B2494 with a 33-yr reproductive longevity in Zakynthos. Dagger (†) denotes illegible characters on tag.

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Discussion

As far as we can determine, the herein directly documented reproductive longevity of 33 yrs for Caretta caretta is the longest published so far at a global scale. In the southwestern Indian Ocean, loggerheads nest up to 18 yrs (Nel et al. 2013); in the United States, loggerhead turtles were recorded nesting over a time span of 23 yrs in Florida (Ehrhart et al. 2014) and 31 yrs in Georgia (Ondich and Andrews 2013); and in Brazil, loggerheads were recorded nesting over a time span of 32 yrs (Barreto et al. 2019). It is worth noting that reproductive longevity based on inference from skeletal growth mark analysis has been estimated up to 46 yrs for stranded loggerheads in the northwestern Atlantic (Avens et al. 2015).

It is apparent that the herein recorded longevities depend on several factors such as the duration of the tagging program over the years, the intensity of tagging effort, and the program's temporal and spatial coverage. Further, it is known that flipper tags, either metal or plastic, on female turtles exhibit significant losses over time (Limpus 1992; Bjorndal et al. 1996; Bellini et al. 2001; Ehrhart et al. 2014) as a result of dragging of flippers on sand during the nesting procedure, biting by males during courtship, faulty application, weakening of locking mechanism, coverage by fouling organisms, and seawater chemistry (see Limpus 1992, Bellini et al. 2001). On the other hand, PIT tags, applied in the last few years in our tagging program and which are documented to be retained by turtles for more years than are flipper tags (see also Braun McNeill et al. 2013; Omeyer et al. 2019), will improve long-term identification of nesting females and will increase the currently directly observed reproductive longevity of loggerhead turtles in Greece.