Multidecadal tag recoveries of loggerhead turtles nesting
in Greece reveal their foraging areas in the Mediterranean

La recuperación de marcas multidecadales de tortugas bobas que anidan
en Grecia revela sus zonas de alimentación en el Mediterráneo

Dimitris Margaritoulis

ARCHELON, the Sea Turtle Protection Society of Greece, Solonos 113, 10678 Athens, Greece

Corresponding author: margaritoulis@archelon.gr, https://orcid.org/0009-0001-0368-3260

ALan F. Rees

ARCHELON, the Sea Turtle Protection Society of Greece, Solonos 113, 10678 Athens, Greece

alanfrees@gmail.com, https://orcid.org/0000-0002-1353-8235

ABSTRACT

Tag recoveries from a more than 40-year tagging programme of loggerhead turtles nesting at the three major nesting areas of Greece (Zakynthos, Kyparissia Bay, Rethymno) revealed their post-nesting whereabouts in the Mediterranean. Most tag recoveries, signalling potential foraging areas, occurred in the Central Mediterranean, with major concentrations in the Adriatic Sea and the Tunisian Plateau (both characterized by shallow waters and rich benthic fauna), whereas fewer recoveries were recorded in the Aegean Sea. While turtles from Zakynthos and Kyparissia Bay shared the Adriatic Sea and the Tunisian Plateau, turtles from Rethymno (Crete) preferred the Aegean Sea and the Levantine Sea. Tag recoveries showed a much wider dispersal of post-nesting turtles than the dispersal recorded from satellite telemetry, highlighting the benefits of long-term tagging programmes. More than 60% of the recovered turtles were reported dead, indicating the substantial threats that reproductively active individuals, the most biologically valuable group in the loggerhead population, face in the Mediterranean Sea.

Keywords: Caretta caretta; mortality; Gulf of Gabès; Adriatic Sea; Aegean Sea; Tunisian Plateau; Levantine Sea; marine protected areas.

RESUMEN

La recuperación de marcas, tras un programa de marcado de más de 40 años de duración, de tortugas bobas que anidan en las tres principales zonas de anidación de Grecia (Zakynthos, bahía de Kyparissia y Rethymno), reveló su paradero en el Mediterráneo tras la anidación. La mayoría de las recuperaciones de marcas, que indican posibles zonas de alimentación, se produjeron en el Mediterráneo central, con mayores concentraciones en el mar Adriático y la meseta tunecina —ambas zonas conocidas por sus aguas poco profundas y su rica fauna bentónica— y, en menor medida, en el mar Egeo. Mientras que las tortugas de Zakynthos y la bahía de Kyparissia compartían el mar Adriático y la meseta tunecina, las tortugas de Rethymno (Creta) prefirieron el mar Egeo y el mar Levantino. La recuperación de marcas mostró una dispersión mucho mayor de las tortugas posanidación que la dispersión obtenida mediante telemetría satelital, lo que destaca los beneficios de los programas de marcado a largo plazo. Más del 60% de las tortugas recuperadas fueron reportadas muertas, lo que indica las importantes amenazas que enfrentan los individuos reproductivamente activos, el grupo de mayor valor biológico de la población de tortugas bobas, en el mar Mediterráneo.

Palabras clave: Caretta caretta; mortalidad; golfo de Gabés; mar Adriático; mar Egeo; meseta de Túnez; mar del Levante; áreas marinas protegidas.

Supplementary information ↓

Contents

1. INTRODUCTION

2. MATERIALS AND METHODS

3. RESULTS

3.1 Recovery rate

3.2 Spatial distribution

3.3 Temporal distribution

3.4 Recovery distance

3.5 Recovery interval

3.6 Body size

3.7 Fate of recovered turtles

4. DISCUSSION

4.1 Recovery rate

4.2 Spatial distribution

4.3 Temporal distribution

4.4 Recovery distance

4.5 Recovery interval

4.6 Body size

4.7 Fate of recovered turtles

4.8 Conservation implications

5. CONCLUSION

1. INTRODUCTION

Adult female loggerhead sea turtles (Caretta caretta) make long-distance reproductive migrations between nesting and foraging habitats (Schroeder et al. 2003Schroeder B.A., Foley A.M., Bagley D.A. 2003. Nesting patterns, reproductive migrations, and adult foraging areas of loggerhead turtles. In: Bolten A.B., Witherington B.E. (eds), Loggerhead Sea Turtles. Smithsonian Books, Washington DC, pp. 93-113.). Foraging habitats were traditionally identified through recoveries of turtles that were flipper-tagged at their nesting areas (e.g. Meylan 1982Meylan A. 1982. Sea turtle migration - Evidence from tag returns. In: Bjorndal K.A. (editor), Biology and Conservation of Sea Turtles. Smithsonian Institution Press, Washington DC, pp. 91-100.). In the last 30 years new technologies (i.e. satellite telemetry) have permitted the continuous tracking of migrating turtles, and hence more accurate information has been provided (Godley et al. 2008Godley B.J., Blumenthal J.M., Broderick A.C., et al. 2008. Satellite tracking of sea turtles: Where have we been and where do we go next? Endanger. Species Res. 4: 3-22. https://doi.org/10.3354/esr00060). However, the new technologies are expensive and considerably restrict the sample size (Luschi and Casale 2014Luschi P., Casale P. 2014. Movement patterns of marine turtles in the Mediterranean Sea: a review. Ital. J. Zoolog. 81: 478-495. https://doi.org/10.1080/11250003.2014.963714). Further, turtle-borne equipment (e.g. satellite tags) may change the fitness and behaviour of turtles (e.g. by increasing hydrodynamic drag and impeding mating; Frazier 2000Frazier J. 2000. Chelo-telemetry: more on great chelonian taboos. In: Abreu-Grobois F.A., Briseño-Dueñas R., Márquez R., Sarti L. (compilers), Proceedings of the Eighteenth International Sea Turtle Symposium. NOAA Tech Memo NMFS-SEFSC-436, National Marine Fisheries Service, Miami, pp. 97-99., Jones et al. 2013Jones T.T., Van Houtan K.S., Bostrom B.L., et al. 2013. Calculating the ecological impacts of animal borne instruments on aquatic organisms. Methods Ecol. Evol. 4: 1178-1186. https://doi.org/10.1111/2041-210X.12109) or increase threats (e.g. entanglement in nets; Godley et al. 2008Godley B.J., Blumenthal J.M., Broderick A.C., et al. 2008. Satellite tracking of sea turtles: Where have we been and where do we go next? Endanger. Species Res. 4: 3-22. https://doi.org/10.3354/esr00060).

Therefore, flipper-tagging continues to provide, together with reproductive behaviour (Miller 1997Miller J.D. 1997. Reproduction in sea turtles. In: Lutz P.L., Musick J.A. (eds), The Biology of Sea Turtles, Vol I. CRC Press, Boca Raton, Florida, pp. 51-81.), reproductive longevity (Margaritoulis et al. 2023Margaritoulis D., Dean C.J., Lourenço G., et al. 2023. CARETTA CARETTA (Loggerhead Sea Turtle). Reproductive longevity. Herpetol. Rev. 54(4): 639-640.) and growth rates (Bjorndal et al. 1983Bjorndal K.A., Meylan A.B., Turner B.J. 1983. Sea turtle nesting at Melbourne Beach, Florida, I. Size, growth and reproductive biology. Biol. Conserv. 26: 65-77. https://doi.org/10.1016/0006-3207(83)90049-6), a cost-effective means to obtain, over long periods of time, useful information on turtle migrations (Godley et al. 2003Godley B.J., Broderick A.C., Glen F., et al. 2003. Post-nesting movements and submergence patterns of loggerhead marine turtles in the Mediterranean assessed by satellite tracking. J. Exp. Mar. Biol. Ecol. 287: 119-134. https://doi.org/10.1016/S0022-0981(02)00547-6, Casale et al. 2007Casale P., Freggi D., Basso R., et al. 2007. A model of area fidelity, nomadism, and distribution patterns of loggerhead sea turtles (Caretta caretta) in the Mediterranean Sea. Mar. Biol. 152: 1039-1049. https://doi.org/10.1007/s00227-007-0752-7, Baldi et al. 2022Baldi G., Furii G., Del Vecchio M., et al. 2022. Behavioural plasticity in the use of a neritic foraging area by loggerhead sea turtles: insights from 37 years of capture–mark–recapture in the Adriatic Sea (Mediterranean Sea). ICES J. Mar. Sci. https://doi.org/10.1093/icesjms/fsac227) and other biological traits. Additionally, flipper-tagging and concomitant tag recoveries enhance collaboration among tag finders (usually fishers) and the tagging institutions, which does not occur in satellite telemetry studies (Lazar et al. 2004Lazar B., Margaritoulis D., Tvrtkovic N. 2004. Tag recoveries of the loggerhead sea turtle Caretta caretta in the eastern Adriatic Sea: implications for conservation. J. Mar. Biol. Ass. U.K. 84: 475-480. https://doi.org/10.1017/S0025315404009488h, Horrocks et al. 2011Horrocks J., Krueger B., Fastigi M., et al. 2011. International movements of adult female hawksbill turtles (Eretmochelys imbricata): First results from the Caribbean’s Marine Turtle Tagging Centre. Chelonian Conserv. Biol. 10(1): 18-25. https://doi.org/10.2744/CCB-08751). Nevertheless, the high tag loss involved in flipper-tagging (Limpus 1992Limpus C.J. 1992. Estimation of tag loss in marine turtle research. Wildl. Res. 19: 457-469. https://doi.org/10.1071/WR9920457) makes it necessary to tag a large number of nesting turtles in order to obtain a sufficient number of recovered tags in a distant area before characterizing it a foraging habitat (Casale et al. 2007Casale P., Freggi D., Basso R., et al. 2007. A model of area fidelity, nomadism, and distribution patterns of loggerhead sea turtles (Caretta caretta) in the Mediterranean Sea. Mar. Biol. 152: 1039-1049. https://doi.org/10.1007/s00227-007-0752-7).

In Greece, a long-term tagging programme has been carried out since 1982 at the two largest nesting aggregations of loggerhead turtles in the Mediterranean, Laganas Bay (Zakynthos) and Kyparissia Bay (Western Peloponnese) (Margaritoulis et al. 2023Margaritoulis D., Dean C.J., Lourenço G., et al. 2023. CARETTA CARETTA (Loggerhead Sea Turtle). Reproductive longevity. Herpetol. Rev. 54(4): 639-640.). An early publication covering the initial tag recoveries identified Tunisia as the most important location for post-nesting turtles (Margaritoulis 1988Margaritoulis D. 1988. Post-nesting movements of loggerhead sea turtles tagged in Greece. Rapp. Comm. int. Mer. Médit. 31(2): 284.). A subsequent preliminary analysis of 2868 tagged turtles over 18 years of tagging (1982-1999) highlighted a long-range (>150 km) tag recovery rate of 3.5% (100 turtles) and identified the Adriatic Sea and the Gulf of Gabès as major foraging areas of the loggerhead populations nesting in Zakynthos and Kyparissia Bay (Margaritoulis et al. 2003Margaritoulis D., Argano R., Baran I., et al. 2003. Loggerhead turtles in the Mediterranean Sea: Present knowledge and conservation perspectives. In: Bolten A.B., Witherington B.E. (eds), Loggerhead Sea Turtles. Smithsonian Books, Washington DC, pp. 175-198.). In 1990, flipper-tagging was initiated at the nesting area of Rethymno, Crete, and initial tag recoveries up to 2010 provisionally indicated the Aegean Sea as the preferred foraging habitat of this population (Margaritoulis and Rees 2011Margaritoulis D., Rees A.F. 2011. Loggerhead turtles nesting at Rethymno, Greece, prefer the Aegean Sea as their main foraging area. Mar. Turtle Newsl. 131: 12-14.).

Here, we present all tag recoveries received until the end of 2024 of turtles tagged at the three largest nesting areas of Greece (Zakynthos, Kyparissia Bay, and Rethymno), potentially indicating foraging and wintering areas in the Mediterranean. We further analyse the turtles’ destinations per nesting area and examine the relationship between turtles’ body size and travelled distance. This information can be useful for the conservation of these three breeding populations, which account for about 32% of the total nesting effort of loggerhead turtles in the Mediterranean (Casale et al. 2018Casale P., Broderick A.C., Camiñas J.A., et al. 2018. Mediterranean sea turtles: current knowledge and priorities for conservation and research. Endanger. Species Res. 36: 229-267. https://doi.org/10.3354/esr00901).

2. MATERIALS AND METHODS

Tagging, and identifying previously tagged turtles, has been conducted every nesting season in Zakynthos (Laganas Bay) and Kyparissia Bay since 1982 (with the exception of the 1999 season in Zakynthos, when no night observations were undertaken, and the 2020 season in Kyparissia Bay, when night observations were much reduced) and in Rethymno from 1990 to 2022 inclusive (with the exception of the 2013 and 2020 nesting seasons). The turtles were tagged, or checked for existing tags, after egg-laying. In general, the turtles were double-tagged, and whenever a turtle was found with only one tag, a second tag was usually applied. Turtles without tags but with scars at the tagging locations, attributed to lost tags, were re-tagged.

Two types of tags were used: metal tags (Monel, No 49 and No 681, National Band and Tag Company, USA) applied on front flippers (Fig. 1), and coloured plastic tags (Jumbo tags, Dalton Ltd, UK) on hind flippers. All tags had imprinted a serial code in Latin alphanumeric characters and the postal address of ARCHELON, with the exception of the plastic tags used in the period 1982–1984, which had the address of the Hellenic Society for the Protection of Nature (HSPN). Tags were placed in the membrane between the scales at the trailing edge of the flippers. We discontinued the usage of No 49 Monel tags in 1990 as these tags were rather large for loggerheads in Greece, and we stopped using plastic tags in 2015 as there was evidence of interference with fishing gears.

Figure 1. – Loggerhead turtle found stranded in North Evoikos Gulf (11 October 2020), Greece. The turtle was tagged after her nesting in Kyparissia Bay on 27 June 2018. Arrows show the two Monel tags on front flippers (Photo: Coast Guard of Halkis).

During tagging, four carapace measurements, covering length and width, were routinely taken. Of these, we use here only the measurement of straight carapace length notch-to-tip (Bolten (1999)Bolten A.B. 1999. Techniques for measuring sea turtles. In: Eckert K.L., Bjorndal K.A., Abreu-Grobois F.A., et al. (eds), Research and Management Techniques for the Conservation of Sea Turtles. IUCN/SSC Marine Turtle Specialist Group Publication No. 4, Washington DC, pp. 110-114.). Since all tagged turtles were adults, and growth rate in large-sized turtles is low (Bjorndal et al. 1983Bjorndal K.A., Meylan A.B., Turner B.J. 1983. Sea turtle nesting at Melbourne Beach, Florida, I. Size, growth and reproductive biology. Biol. Conserv. 26: 65-77. https://doi.org/10.1016/0006-3207(83)90049-6), we assumed that body size growth from the last observed nesting to tag recovery was negligible. Therefore, possible body measurements reported from the recovery site were not considered as this would influence the analysis with an additional source of uncertainty (see Baldi et al. (2022)Baldi G., Furii G., Del Vecchio M., et al. 2022. Behavioural plasticity in the use of a neritic foraging area by loggerhead sea turtles: insights from 37 years of capture–mark–recapture in the Adriatic Sea (Mediterranean Sea). ICES J. Mar. Sci. https://doi.org/10.1093/icesjms/fsac227).

Publicity of the tagging programme was sought in the Mediterranean through announcements to supranational organizations (the General Fisheries Commission for the Mediterranean, UNEP’s Regional Activity Centre for Specially Protected Areas, the International Union for Nature Conservation, the WWF Mediterranean Programme, the European Commission and the Council of Europe) in cooperation with the relevant national authorities. Within Greece, initially the Ministry of Environment and later ARCHELON informed fisheries departments and coast guard stations, and issued public appeals through the mass media to elicit reporting of tagged turtles.

We positioned all tag recoveries within the seven marine ecoregions recognized in the Mediterranean according to Spalding et al. (2007)Spalding M.D., Fox H.E., Allen G.R., et al. 2007. Marine ecoregions of the world: a bioregionalization of coastal and shelf areas. Biosci. 57: 573-583. https://doi.org/10.1641/B570707. The western part of Mediterranean Sea is split into two subdivisions (Western Mediterranean and Alboran Sea), the central part into three (the Adriatic Sea, the Ionian Sea, and Tunisian Plateau/Gulf of Sidra, hereafter the Tunisian Plateau) and the eastern part into two (the Aegean Sea and the Levantine Sea). In the Adriatic Sea we followed its subdivision into the northern, central and southern part after the General Fisheries Commission for the Mediterranean (GFCM 1999GFCM. 1999. Report of the twenty-fourth session. Alicante, Spain, 12-15 July 1999. GFCM Report, No. 24, Rome, FAO, 27 pp.).

Several tag recoveries at sea, usually a result of turtle capture in fishing gears, did not contain the exact location of capture. In these cases, we estimated a position based on other information describing the recovery event, and if such information was not available, we recorded as the location of recovery the base-port of the fishing vessel. In cases of fishers reporting tagged turtles, gifts in the form of a T-shirt or illustrated nature book were sent.

In our analysis, we consider only the long-range tag recoveries, i.e. the ones reported at distances over 150 km from the tagging site, because turtles in their inter-nesting period may conduct long forays from the nesting site, extending as far as 150 km (Schofield et al. 2010Schofield G., Hobson V.J., Lilley M.K.S., et al. 2010. Inter-annual variability in the home range of breeding turtles: Implications for current and future conservation management. Biol. Conserv. 143: 722-730. https://doi.org/10.1016/j.biocon.2009.12.011, Mingozzi et al. 2016Mingozzi T., Mencacci R., Cerritelli G., et al. 2016. Living between widely separated areas: Long-term monitoring of Mediterranean loggerhead turtles sheds light on cryptic aspects of females spatial ecology. J. Exp. Mar. Biol. Ecol. 485: 8-17. https://doi.org/10.1016/j.jembe.2016.08.007). Further, we assumed that all long-range recoveries occurred at the turtle’s foraging area because, according to Limpus et al. (1992)Limpus C., Miller J., Parmenter C., et al. 1992. Migration of green (Chelonia mydas) and loggerhead (Caretta caretta) turtles to and from eastern Australian rookeries. Wildl. Res. 19: 347-357. https://doi.org/10.1071/WR9920347, most tag recoveries occur during the turtle’s stay at the foraging grounds. To standardize methodologies across the region, we excluded tag recovery data from Amvrakikos Gulf in Greece, obtained from ARCHELON’s long-standing in-water capture-mark-recapture project, which has revealed links to Greek nesting areas (Rees et al. 2017Rees A.F., Carreras C., Broderick A.C., et al. 2017. Linking loggerhead locations: using multiple methods to determine the origin of sea turtles in feeding grounds. Mar. Biol. 164: 30. https://doi.org/10.1007/s00227-016-3055-z).

For the needs of the present study, we define recovery distance (in km) as the shortest possible route at sea between the nesting site and the recovery location, measured through Google Earth (Google Inc., USA) and recovery interval (in days) as the days elapsed between the last observed appearance of the turtle on the nesting beach and its recovery date.

Throughout the paper, data averages are presented as means ±standard deviation. Statistical comparisons among groups of variables were assessed using a one-way ANOVA test when the data were shown to be approximately normally distributed and with equal variance using the Shapiro-Wilk and Levene test, respectively. Post hoc determination of significant differences among groups was carried out using Tukey’s HSD test. For other cases, the Kruskal-Wallis test was used. The relationship between two variables was assessed using the Pearson product-momentum test. All statistical tests were carried out in R, version 4.5.1 (R Core Team 2025R Core Team. 2025. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria. https://www.R-project.org/).

3. RESULTS

3.1 Recovery rate

In the period 1982–2024, a total of 6909 different turtles were tagged at the nesting areas of Zakynthos (3747 turtles, 54.2%), Kyparissia Bay (2717 turtles, 39.3%) and Rethymno (445 turtles, 6.4%). By the end of 2024, 254 turtles (3.7% of the tagged turtles) had been reported at distances greater than 150 km from their respective nesting area. Of the 254 recovered turtles, 160 originated from Zakynthos (4.3% of those tagged in Zakynthos), 75 from Kyparissia Bay (2.8%), and 19 from Rethymno (4.3%).

3.2 Spatial distribution

Spatial distribution of tag recoveries shows a wide dispersal in the Mediterranean, from the Balearic Islands in the west to the coast of Israel in the east (Fig. 2), with four ecoregions concentrating 93.7% of total recoveries: the Adriatic Sea (39.4%), the Tunisian Plateau (23.2%), the Aegean Sea (18.9%) and the Ionian Sea (12.2%) (Table 1). No recoveries were reported from the Alboran Sea.

Figure 2. – Approximate locations of 254 tag recoveries (coloured dots) positioned in the seven Mediterranean ecoregions. Subdivisions in the Adriatic Sea (northern, central, southern) are separated by straight lines. Dot colour represents the source nesting area, shown as solid stars (red, Zakynthos; green, Kyparissia Bay; black, Rethymno). Country codes (clockwise from west): ES, Spain; FR, France; IT, Italy; SI, Slovenia; HR, Croatia; ME, Montenegro; AL, Albania; GR, Greece; TR, Turkey; CY, Cyprus; SY, Syria; LB, Lebanon; IL, Israel; EG, Egypt; LY, Libya; MT, Malta; TN, Tunisia; DZ, Algeria; MO, Morocco. Inset maps show concentration areas of tag recoveries within Greece.

Table 1. – Spatial analysis of long-range recoveries of loggerhead turtles tagged in Greece, by Mediterranean ecoregion and source nesting area.

Mediterranean ecoregion	From Zakynthos		From Kyparissia		From Rethymno		All recoveries
	No	%	No	%	No	%	No	%
Adriatic Sea	62	38.8	37	49.3	1	5.3	100	39.4
Aegean Sea	27	16.9	10	13.3	11	57.9	48	18.9
Ionian Sea	19	11.9	12	16.0	0	0	31	12.2
Levantine Sea	5	3.1	2	2.7	2	10.5	9	3.5
Tunisian Plateau	42	26.3	12	16.0	5	26.3	59	23.2
W. Mediterranean	5	3.1	2	2.7	0	0	7	2.8
Total	160	100	75	100	19	100	254	100

Analysing the spatial distribution of recoveries per tagging area, we noted that Zakynthos and Kyparissia turtles were more common in the Adriatic Sea (38.8% and 49.3%, respectively) and the Tunisian Plateau (26.3% and 16.0%, respectively), while Rethymno turtles were mostly reported from the Aegean Sea and the Tunisian Plateau (57.9% and 26.3%, respectively), with only a small representation in the Adriatic Sea (5.3%) (Table 1; Fig. 2).

The 254 recovered turtles were reported from 15 Mediterranean countries, including Greece. Four countries accounted for 83.6% of all recaptures (Italy, 26.2%; Greece, 23.8%; Tunisia, 17.6%; Croatia, 16.0%) (Fig. 2). In the analysis per nesting area, turtles recovered from Zakynthos followed a similar pattern (Greece, 23.1%; Italy, 21.9%; Tunisia, 19.4%; Croatia, 17.5%), while those recovered from Kyparissia Bay showed a different pattern, with a relative increase in observations from Italy and reduced reports from Tunisia (Italy, 40.3%; Greece, 22.1%; Croatia, 16.9%; Tunisia, 13.0%). Rethymno turtles were recovered mainly in Greece (36.8%), Tunisia (21.1%), and Turkey (21.1%) (Fig. 2).

In the Adriatic Sea, of the 100 recovered turtles, 62 were from Zakynthos and 37 from Kyparissia Bay, with only one turtle from Rethymno (Table 1). The northern subdivision of the Adriatic Sea accounted for 56.0% of all recoveries, while the central and southern subdivisions accounted for lower concentrations (23.0% and 21.0%, respectively) (Table 2). The overall pattern across the Adriatic was valid for turtles from both Zakynthos and Kyparissia Bay, with Rethymno turtles being an exceptional case (Table 2; Fig. 2).

Table 2. – Spatial analysis of long-range recoveries of loggerhead turtles tagged in Greece in the Adriatic Sea, by source nesting area.

Subdivisions of the Adriatic Sea	From Zakynthos		From Kyparissia		From Rethymno		Total recoveries
	No	%	No	%	No	%	No	%
Northern	34	54.8	21	56.8	1	100	56	56.0
Central	15	24.2	8	21.6	0	0	23	23.0
Southern	13	21.0	8	21.6	0	0	21	21.0
Total	62	100	37	100	1	100	100	100

The 61 tagged turtles recovered in Greece were dispersed in the Aegean Sea (62.3%) and in the Ionian Sea (37.7%) (Fig. 2). Recoveries in the Aegean Sea provided three concentrations in the Thracian Sea, Lesvos Island and the Saronikos Gulf (Fig. 2 (insets B, C, D respectively)). In the Ionian Sea, recoveries showed aggregations in the wider area of the Amvrakikos Gulf (43.5% of turtles recovered in the Greek Ionian) and in the Gulf of Corinth (30.4%) (Fig. 2 (insets A, E respectively)).

3.3 Temporal distribution

In the period 1982–2024, the number of tag recoveries per year averaged 5.9±2.8 turtles (range: 1-12 turtles/year, n=252 turtles over 43 years) and showed a slight decreasing trend (two recoveries did not have an exact date) (Fig. 3). Monthly distribution of tag recoveries ranged from 11 (4.4%) in May to 37 (14.7%) in August (Fig. 4). However, a striking difference was observed by comparing the seasonality of recoveries on the Tunisian Plateau and in the Adriatic Sea. In winter (December–February) 33.9% were recovered on the Tunisian Plateau and 20.4% in the Adriatic, while in summer (June–August) the figures were 18.6% and 34.7%, respectively. In the Aegean Sea, 16.7% were recovered in winter and 29.2% in summer.

Figure 3. – Annual long-range recoveries of loggerhead turtles tagged in Greece over the 43-year period 1982–2024 (n=252 recoveries of known year).

Figure 4. – Monthly distribution of long-range recoveries of loggerhead turtles tagged in Greece in the period 1982–2024 (n=252 recoveries of known month).

3.4 Recovery distance

In respect to recovery distance, 136 turtles (53.5% of the total recovered) travelled a minimum distance of more than 800 km, and of these 77 turtles (30.3%) travelled more than 1000 km. The mean recovery distance was 798.7±330.8 km (range: 151–1900 km, n=254 turtles) for all recoveries, 807.4±321.8 km (range: 151–1900 km, n=160) for Zakynthos turtles, 801.8±315.7 (range: 151–1307 km, n=75) for Kyparissia Bay turtles, and 712.9±452.3 km (range: 205–1520 km, n=19) for Rethymno turtles. The differences among nesting areas were not significant (Kruskal-Wallis, χ²=1.03, df=2, p=0.597). The two longest distances (1765 and 1900 km) were recorded for turtles from Zakynthos that travelled to the Balearic Sea (Fig. 2).

3.5 Recovery interval

The recovery interval was known for 251 turtles (two turtles, A1381 and L8441, had no date of recovery, and another turtle had no tag code reported and hence no tagging date). The mean recovery interval for all turtles was 636.3±733.0 days (range: 11-5493 days, n=251), while it was 588.5±689.3 days (range: 16-4925 days, n=157) for Zakynthos turtles, 755.3±831.3 days (range: 14-5493 days, n=75) for Kyparissia Bay turtles, and 561.5±647.2 days (range: 11-2924 days, n=19) for Rethymno turtles. The differences among the nesting areas were not significant (Kruskal-Wallis, χ²=4.27, df=2, p=0.118). The largest recovery interval was 5493 days (15 years) for a turtle (tag code A1245) that was last observed nesting in Kyparissia Bay on 23 July 1986 and by-caught/released alive outside Amvrakikos Gulf on 6 August 2001. It is interesting to note that the turtle’s metal tag (Monel, No 49), applied on 4 July 1984, survived 17.1 years on the turtle.

Recovery distance was loosely correlated with recovery interval, and the correlation strengthened with shortening recovery time. Forty-two percent of variation in recovery distance was explained by recovery interval during the first two months (61 days) (Fig. 5), but only 10.3% of variation was explained at 6 months. Some turtles were recovered after a short time (11–16 days) in distances, indicating high speeds of travel (26.4–38.4 km/day).

3.6 Body size

The mean body size (straight carapace length) of the recovered turtles for which this measurement was available was 78.3±4.7 cm (range: 64.8-92.0 cm, n=233) (Table 3). We found no correlation between body size and recovery distance (Pearson’s product-momentum, t=0.44, df=227, p=0.658). However, turtles recovered in the Adriatic Sea were largest, with an average body size of 80.1±4.4 cm (range: 70.0-92.0 cm, n=91), while those recovered in the Levantine Sea were smallest, with an average body size of 74.7±5.7 cm (range: 64.8-82.5 cm, n=9) (Table 3). Excluding locations from which we only had small sample sizes (Levantine Sea and W. Mediterranean) we found that body size differed significantly among destinations (ANOVA, F(3,213)=9.138, p<0.001). Tukey’s HSD test revealed that the only significant differences were that Adriatic turtles were significantly larger than those in the Aegean Sea (p<0.05), the Ionian Sea (p<0.001) and the Tunisian Plateau (p<0.001).

Figure 5. – Relation between recovery distance and recovery interval for turtles recovered in less than two months (61 days) from their last observed nesting (n=14).

Table 3. – Body size (straight carapace length notch-to-tip, SCLn-t in cm) of recovered loggerhead turtles tagged in Greece, by Mediterranean ecoregion.

Mediterranean ecoregion	Mean	SD	MIN	MAX	n
Adriatic Sea	80.1	4.4	70.0	92.0	91
Aegean Sea	77.8	4.7	68.0	85.0	43
Ionian Sea	77.6	4.8	71.0	88.7	27
Levantine Sea	74.7	5.7	64.8	82.5	9
Tunisian Plateau	76.8	3.8	70.0	85.0	56
W. Mediterranean	77.2	5.0	72.0	87.5	7
Overall	78.3	4.7	64.8	92.0	233

3.7 Fate of recovered turtles

Of the 254 recovered turtles, 145 were reported dead, 90 alive and 19 of unknown fate. Hence, of the recovered turtles of reported fate (235 turtles), 61.7% were dead and 38.3% alive. High mortalities (>60%) were noted in all ecoregions, with the exception of the Tunisian Plateau, which showed the lowest mortality (40.8%) (Table 4). In the Adriatic Sea, the northern subdivisions had a mortality of 69.2%, the central subdivision of 65.2%, and the southern subdivision of 44.4%. Several live turtles found stranded or by-caught in fisheries were transported to rescue facilities, from where they were subsequently released. A couple of turtles captured alive were reportedly killed by fishers, and one was butchered and sold to clients.

Table 4. – Number and percentage of tagged turtles recovered dead, by Mediterranean ecoregion.

Mediterranean ecoregion	Known fate recoveries	Dead	%
Adriatic Sea	93	59	63.4
Aegean Sea	48	35	72.9
Ionian Sea	30	21	70.0
Levantine Sea	9	6	66.7
Tunisian Plateau	49	20	40.8
W. Mediterranean	6	4	66.7
Total	235	145	61.7

4. DISCUSSION

We have presented here the long-range tag recoveries over the 43-year period 1982–2024 from the ARCHELON tagging programme carried out at three nesting areas of Greece. This is the longest such programme in the Mediterranean, and its results may provide empirical confirmation pertaining to the regional distribution of foraging and/or wintering areas of loggerhead turtles breeding in Greece and their ecological implications. Considering that these three nesting areas host about 32% of all loggerhead nests in the Mediterranean (Casale et al. 2018Casale P., Broderick A.C., Camiñas J.A., et al. 2018. Mediterranean sea turtles: current knowledge and priorities for conservation and research. Endanger. Species Res. 36: 229-267. https://doi.org/10.3354/esr00901), the confirmed foraging/wintering areas deserve increased attention in order to mitigate impacts threatening the turtles that inhabit these waters.

4.1 Recovery rate

Tag recovery rates away from nesting beaches are generally low. This is mainly due to the high tag loss (Limpus 1992Limpus C.J. 1992. Estimation of tag loss in marine turtle research. Wildl. Res. 19: 457-469. https://doi.org/10.1071/WR9920457) and to the inherent difficulties in reporting tagged turtles, especially those by-caught in fisheries (Bjorndal et al. 2003Bjorndal K.A., Bolten A.B., Chaloupka M.Y. 2003. Survival probability estimates for immature green turtles Chelonia mydas in the Bahamas. Mar. Ecol. Prog. Ser. 252: 273-281. https://doi.org/10.3354/meps252273). We experienced both problems in our tagging programme. More than 36% of tagged turtles re-appear for nesting in subsequent seasons without their tags (ARCHELON, unpublished data). In addition to tag loss, tag corrosion and epibionts hindered legibility of the tag code and/or the return address. Further, in the case of captured tagged turtles, fishers must make a special effort to take note of the tag code and report it to the return address inscribed on the tag. In the early years, before the advent of electronic communications, several tag returns came in through the ordinary post, especially from Tunisian fishers writing in French. However, posting a tag recovery put an additional burden on fishers, who had to write a note, put in an envelope and post it to the tagging institution, possibly in another country. Hence, collecting a sufficient number of tag recoveries required a large number of turtles to be flipper-tagged over an extended period of time.

In the face of the above problems, institutions and organizations conducting coastal surveys or working with fishers proved a great source of information, as their staff located the tagging institution and reported the tag code. This data collection of tagged turtles, either stranded or by-caught, was active (for a given period) in Italy, Croatia, Slovenia and Tunisia. For instance, the Adriatic Marine Turtle Programme (AMTP), active in Croatia and Slovenia in the period 1993-2002, communicated a higher number of tag recoveries to us than those reported previously (estimated seven-fold; Lazar et al. 2004Lazar B., Margaritoulis D., Tvrtkovic N. 2004. Tag recoveries of the loggerhead sea turtle Caretta caretta in the eastern Adriatic Sea: implications for conservation. J. Mar. Biol. Ass. U.K. 84: 475-480. https://doi.org/10.1017/S0025315404009488h). The Institute of Marine Sciences and Technologies (INMST) in Tunisia provided information on by-caught tagged turtles in Tunisian waters, and the Sea Turtle Rescue Centre in Lampedusa provided tag codes of turtles by-caught by Italian fishing vessels on the Tunisian Plateau. We recognize that these concerted efforts may have introduced some bias in the spatial distribution of recoveries.

In the Mediterranean, there are no similar tagging programmes with which to compare our tag recovery rates. However, comparing with tagging programmes in other parts of the world, our overall return rate (3.7%) appears higher. In Australia, a 21-year programme with 10149 tagged loggerheads had only 118 tag returns (rate: 1.2%) (Limpus et al. 1992Limpus C., Miller J., Parmenter C., et al. 1992. Migration of green (Chelonia mydas) and loggerhead (Caretta caretta) turtles to and from eastern Australian rookeries. Wildl. Res. 19: 347-357. https://doi.org/10.1071/WR9920347). In South Africa, Hughes (1974)Hughes G.R. 1974. The Sea Turtles of South-East Africa. South African Association for Marine Biological Research, Oceanographic Research Institute, Investigational Report No 36, 96 pp. had 34 recoveries from 1800 tagged loggerheads (rate: 1.9%). In Florida, USA, of 2910 loggerheads tagged in the period 1972–1978 only 34 were recovered (rate: 1.2%) (Meylan et al. 1983Meylan A.B., Bjorndal K.A., Turner B.J. 1983. Sea turtles nesting at Melbourne Beach, Florida: II. Post-nesting movements of Caretta caretta. Biol. Conserv. 26: 79-90. https://doi.org/10.1016/0006-3207(83)90050-2). In Japan, 55 loggerheads were recovered from 2219 tagged ones, providing a recovery rate of 2.5% (Kamezaki et al. 1997Kamezaki N., Miyawaki I., Suganuma H., et al. 1997. Post-nesting migration of Japanese loggerhead turtle, Caretta caretta. Wildl. Conserv. Jpn. 3: 29-39 (in Japanese with English abstract).). We attribute the higher recovery rate in the Mediterranean to the densely populated coastal zone and to the higher environmental awareness, which may have contributed to increased reporting.

The low recovery rate of turtles tagged in Kyparissia Bay (2.8%) in comparison with the other two areas (4.3% each) could suggest that Kyparissia turtles may use marine areas with less chance of being captured (e.g. remaining further offshore). This can be answered by satellite tracking of a sufficient number of post-nesting turtles in Kyparissia, as done in Zakynthos where satellite tracking corroborated previous flipper-tag recoveries (Zbinden et al. 2008Zbinden J.A., Aebischer A., Margaritoulis D., et al. 2008. Important areas at sea for adult loggerhead sea turtles in the Mediterranean Sea: satellite tracking corroborates findings from potentially biased sources. Mar. Biol. 153: 899-906. https://doi.org/10.1007/s00227-007-0862-2, Schofield et al. 2013Schofield G., Dimadi A., Fossette S., et al. 2013. Satellite tracking large numbers of individuals to infer population level dispersal and core areas for the protection of an endangered species. Divers. Distrib. 19: 834-844. https://doi.org/10.1111/ddi.12077).

4.2 Spatial distribution

Our results generally confirm the spatial distribution of post-nesting turtles from Greece revealed previously through flipper-tag recoveries (Margaritoulis et al. 2003Margaritoulis D., Argano R., Baran I., et al. 2003. Loggerhead turtles in the Mediterranean Sea: Present knowledge and conservation perspectives. In: Bolten A.B., Witherington B.E. (eds), Loggerhead Sea Turtles. Smithsonian Books, Washington DC, pp. 175-198., Margaritoulis and Rees 2011Margaritoulis D., Rees A.F. 2011. Loggerhead turtles nesting at Rethymno, Greece, prefer the Aegean Sea as their main foraging area. Mar. Turtle Newsl. 131: 12-14.), satellite telemetry (Zbinden et al. 2008Zbinden J.A., Aebischer A., Margaritoulis D., et al. 2008. Important areas at sea for adult loggerhead sea turtles in the Mediterranean Sea: satellite tracking corroborates findings from potentially biased sources. Mar. Biol. 153: 899-906. https://doi.org/10.1007/s00227-007-0862-2, Schofield et al. 2013Schofield G., Dimadi A., Fossette S., et al. 2013. Satellite tracking large numbers of individuals to infer population level dispersal and core areas for the protection of an endangered species. Divers. Distrib. 19: 834-844. https://doi.org/10.1111/ddi.12077, Patel et al. 2015Patel S.H., Morreale S.J., Panagopoulou A., et al. 2015a. Changepoint analysis: a new approach for revealing animal movements and behaviors from satellite telemetry data. Ecosphere 6(12): 219. https://doi.org/10.1890/ES15-00358.1) or analysis of stable isotopes (Haywood et al. 2020Haywood J.C., Fuller W.J., Godley B.J., et al. 2020. Spatial ecology of loggerhead turtles: Insights from stable isotope markers and satellite telemetry. Divers. Distrib. 26: 368-381. https://doi.org/10.1111/ddi.13023). The similarity of our tag-return distribution with the end-points of telemetry studies for Zakynthos turtles is striking; see Figure 1 in Zbinden et al. (2008Zbinden J.A., Aebischer A., Margaritoulis D., et al. 2008. Important areas at sea for adult loggerhead sea turtles in the Mediterranean Sea: satellite tracking corroborates findings from potentially biased sources. Mar. Biol. 153: 899-906. https://doi.org/10.1007/s00227-007-0862-2) and Figure 1 in Schofield et al. (2013)Schofield G., Dimadi A., Fossette S., et al. 2013. Satellite tracking large numbers of individuals to infer population level dispersal and core areas for the protection of an endangered species. Divers. Distrib. 19: 834-844. https://doi.org/10.1111/ddi.12077. Resemblance of spatial distribution of migration end-points between satellite tags and flipper-tag returns has also been reported for loggerheads in Australia (Perez et al. 2022Perez M.A., Limpus C.J., Hofmeister K., et al. 2022. Satellite tagging and flipper tag recoveries reveal migration patterns and foraging distribution of loggerhead sea turtles (Caretta caretta) from eastern Australia. Mar. Biol. 169: 80. https://doi.org/10.1007/s00227-022-04061-8), as well as for green turtles in Tortuguero (Troëng et al. 2005Troëng S., Evans D.R., Harrison E., et al. 2005. Migration of green turtles Chelonia mydas from Tortuguero, Costa Rica. Mar. Biol. 148: 435-447. https://doi.org/10.1007/s00227-005-0076-4). This is apparently a result of the direct, and generally non-stop, movements towards foraging areas that post-nesting turtles usually undertake (Godley et al. 2008Godley B.J., Blumenthal J.M., Broderick A.C., et al. 2008. Satellite tracking of sea turtles: Where have we been and where do we go next? Endanger. Species Res. 4: 3-22. https://doi.org/10.3354/esr00060, Dujon et al. 2017Dujon A.M., Schofield G., Lester R.E., et al. 2017. Fastloc-GPS reveals the timing of departure and arrival during sea turtle migration and speed of travel patterns. Mar. Biol. 164: 187. https://doi.org/10.1007/s00227-017-3216-8, Perez et al. 2022Perez M.A., Limpus C.J., Hofmeister K., et al. 2022. Satellite tagging and flipper tag recoveries reveal migration patterns and foraging distribution of loggerhead sea turtles (Caretta caretta) from eastern Australia. Mar. Biol. 169: 80. https://doi.org/10.1007/s00227-022-04061-8).

We can therefore assume that the general distribution of tag recoveries depicts the foraging habitats where most post-nesting loggerheads from Greece have settled in the Mediterranean. The Adriatic Sea (especially its northern part) and the Tunisian Plateau (Gulf of Gabès), two large, shallow (<200 m) neritic areas, host rich benthic communities that play a significant role in the loggerheads’ feeding (Argano et al. 1992Argano R., Basso R., Cocco M., et al. 1992. New data on loggerhead (Caretta caretta) movements within Mediterranean. Bollett. Museo Ist. Biol. Univ. Genova 56/57: 137-164., Lazar et al. 2004Lazar B., Margaritoulis D., Tvrtkovic N. 2004. Tag recoveries of the loggerhead sea turtle Caretta caretta in the eastern Adriatic Sea: implications for conservation. J. Mar. Biol. Ass. U.K. 84: 475-480. https://doi.org/10.1017/S0025315404009488h, Karaa et al. 2016Karaa S., Bradai N.M., Mahmoud S., et al. 2016. Migration of the Mediterranean sea turtles into the Tunisian waters, importance of the tag recoveries as conservation management method. Cah. Biol. Mar. 57: 103-111.). These areas are used for foraging by large proportions of turtles from Zakynthos and Kyparissia Bay, the two largest breeding aggregations of loggerheads in the Mediterranean. The frequent occurrence of tagged turtles from Greece in the Adriatic Sea and in the Gulf of Gabès (Tunisian Plateau) has been also reported in local studies concerning tagged turtles found in these marine areas (Lazar et al. 2004Lazar B., Margaritoulis D., Tvrtkovic N. 2004. Tag recoveries of the loggerhead sea turtle Caretta caretta in the eastern Adriatic Sea: implications for conservation. J. Mar. Biol. Ass. U.K. 84: 475-480. https://doi.org/10.1017/S0025315404009488h, Karaa et al. 2016Karaa S., Bradai N.M., Mahmoud S., et al. 2016. Migration of the Mediterranean sea turtles into the Tunisian waters, importance of the tag recoveries as conservation management method. Cah. Biol. Mar. 57: 103-111., Baldi et al. 2022Baldi G., Furii G., Del Vecchio M., et al. 2022. Behavioural plasticity in the use of a neritic foraging area by loggerhead sea turtles: insights from 37 years of capture–mark–recapture in the Adriatic Sea (Mediterranean Sea). ICES J. Mar. Sci. https://doi.org/10.1093/icesjms/fsac227).

We received few tag returns from the Western Mediterranean (seven records) and from the Levantine Sea (nine records). Despite possible biases in reporting, it seems that the majority of loggerheads nesting in Greece do not venture outside the boundaries of the Central Mediterranean Sea. However, recent satellite tracks have shown turtles from Greece to conduct long wandering migrations in the Western Mediterranean (Rees and Margaritoulis 2019Rees A.F., Margaritoulis D. 2019. The longest migratory distance recorded for a loggerhead nesting in Greece. Mar. Turtle Newsl. 158: 10-11., Haywood et al. 2021Haywood J., Margaritoulis D., Theodorou P., et al. 2021. Identifying critical marine habitats of the largest nesting population of loggerhead sea turtles in the Mediterranean: insights from stable isotope markers and satellite telemetry. Testudo 9(3): 80-88.).

We should be cautious, however, how we interpret the relatively few tag returns from Libya and Egypt, in contrast with Tunisia (see Fig. 2). A challenging point is that the tags bear inscriptions in the Latin alphabet, which can be read by Tunisian fishers, who are familiar with the it, but not from the great majority of Egyptian and Libyan fishers. This drawback may have biased our results, because important foraging areas, in the form of extensive continental shelves with increased by-catch of large-sized loggerheads, are found in the Levantine Sea, e.g. the bays of Mersin and Iskenderun in Turkey and the Nile delta in Egypt (Margaritoulis et al. 2003Margaritoulis D., Argano R., Baran I., et al. 2003. Loggerhead turtles in the Mediterranean Sea: Present knowledge and conservation perspectives. In: Bolten A.B., Witherington B.E. (eds), Loggerhead Sea Turtles. Smithsonian Books, Washington DC, pp. 175-198.).

In addition, our study has shown that the dispersal of females from Zakynthos and Rethymno is geographically wider than the dispersal revealed by satellite tracks for turtles from the same rookeries (Zbinden et al. 2008Zbinden J.A., Aebischer A., Margaritoulis D., et al. 2008. Important areas at sea for adult loggerhead sea turtles in the Mediterranean Sea: satellite tracking corroborates findings from potentially biased sources. Mar. Biol. 153: 899-906. https://doi.org/10.1007/s00227-007-0862-2, Schofield et al. 2013Schofield G., Dimadi A., Fossette S., et al. 2013. Satellite tracking large numbers of individuals to infer population level dispersal and core areas for the protection of an endangered species. Divers. Distrib. 19: 834-844. https://doi.org/10.1111/ddi.12077, Patel et al. 2015aPatel S.H., Morreale S.J., Panagopoulou A., et al. 2015a. Changepoint analysis: a new approach for revealing animal movements and behaviors from satellite telemetry data. Ecosphere 6(12): 219. https://doi.org/10.1890/ES15-00358.1). A similar conclusion has been reported for post-nesting loggerheads in Australia, where the foraging distribution identified from satellite telemetry accounted for 50% of the distribution identified from a large flipper-tag recovery database (Perez et al. 2022Perez M.A., Limpus C.J., Hofmeister K., et al. 2022. Satellite tagging and flipper tag recoveries reveal migration patterns and foraging distribution of loggerhead sea turtles (Caretta caretta) from eastern Australia. Mar. Biol. 169: 80. https://doi.org/10.1007/s00227-022-04061-8). This demonstrates the benefits obtained from tagging programmes sustained over decades in comparison with telemetry studies, which are usually of short duration and cannot represent the evolution of changing preferences.

Identifying the foraging areas per nesting population is of special importance. In our study the two largest nesting populations, those of Zakynthos and Kyparissia Bay, seem to share the same foraging habitats (the Adriatic Sea and the Tunisian Plateau). However, Rethymno turtles seem to use primarily the Aegean Sea (57.9% of the total turtles from Rethymno) and, to a lesser extent, the Tunisian Plateau (26.3%), with two individuals reaching the easternmost coast of the Levantine Sea and only one recovered in the Adriatic. Similarly, satellite tracked turtles from Rethymno headed towards the Aegean Sea and the Tunisian Plateau and none of them towards the Adriatic Sea (Margaritoulis and Rees 2011Margaritoulis D., Rees A.F. 2011. Loggerhead turtles nesting at Rethymno, Greece, prefer the Aegean Sea as their main foraging area. Mar. Turtle Newsl. 131: 12-14., Patel et al. 2015aPatel S.H., Morreale S.J., Panagopoulou A., et al. 2015a. Changepoint analysis: a new approach for revealing animal movements and behaviors from satellite telemetry data. Ecosphere 6(12): 219. https://doi.org/10.1890/ES15-00358.1). This indicates that Rethymno turtles have likely differentially selected foraging areas. In another example, loggerheads from Calabria (southern Italy) forage mostly on the Tunisian Plateau (Mingozzi et al. 2016Mingozzi T., Mencacci R., Cerritelli G., et al. 2016. Living between widely separated areas: Long-term monitoring of Mediterranean loggerhead turtles sheds light on cryptic aspects of females spatial ecology. J. Exp. Mar. Biol. Ecol. 485: 8-17. https://doi.org/10.1016/j.jembe.2016.08.007), not venturing northwards. Outside the Mediterranean, loggerheads from mainland Australia have different foraging areas than those nesting on the islands of the Great Barrier Reef (Perez et al. 2022Perez M.A., Limpus C.J., Hofmeister K., et al. 2022. Satellite tagging and flipper tag recoveries reveal migration patterns and foraging distribution of loggerhead sea turtles (Caretta caretta) from eastern Australia. Mar. Biol. 169: 80. https://doi.org/10.1007/s00227-022-04061-8), and those from nesting areas of the NW Atlantic use different foraging habitats (Pfaller et al. 2020Pfaller J.B., Pajuelo M., Vander Zanden H.B., et al. 2020. Identifying patterns in foraging-area origins in breeding aggregations of migratory species: Loggerhead turtles in the Northwest Atlantic. PLoS ONE 15(4): e0231325. https://doi.org/10.1371/journal.pone.0231325). This shows that each nesting population has evolved a specific pattern concerning its migrations between foraging and breeding habitats, exhibiting a considerable fidelity to both habitats (Limpus et al. 1992Limpus C., Miller J., Parmenter C., et al. 1992. Migration of green (Chelonia mydas) and loggerhead (Caretta caretta) turtles to and from eastern Australian rookeries. Wildl. Res. 19: 347-357. https://doi.org/10.1071/WR9920347).

Defining the foraging area per nesting population is critical because natural (environmental) and anthropogenic conditions at the foraging habitat may affect annual nest numbers and hatchling output and hence influence long-term population trends (Pfaller et al. 2020Pfaller J.B., Pajuelo M., Vander Zanden H.B., et al. 2020. Identifying patterns in foraging-area origins in breeding aggregations of migratory species: Loggerhead turtles in the Northwest Atlantic. PLoS ONE 15(4): e0231325. https://doi.org/10.1371/journal.pone.0231325).

4.3 Temporal distribution

We attribute the decreasing trend of tag recoveries over the years, despite the overall increase of tagged turtles at large, to the relatively high tag loss (e.g. Limpus 1992Limpus C.J. 1992. Estimation of tag loss in marine turtle research. Wildl. Res. 19: 457-469. https://doi.org/10.1071/WR9920457) and/or the potentially increasing adult mortality away from the nesting areas. Another reason is the loss of enthusiasm or fatigue on the part of potential finders (e.g. fishers) to report a tagged turtle, especially if this is not accompanied by a direct benefit.

The many tag recoveries on the Tunisian Plateau during winter, in contrast to the Adriatic recoveries, which occur mostly in summer, if not biased from potentially different fishing methods (see Karaa et al. (2016)Karaa S., Bradai N.M., Mahmoud S., et al. 2016. Migration of the Mediterranean sea turtles into the Tunisian waters, importance of the tag recoveries as conservation management method. Cah. Biol. Mar. 57: 103-111.), may be attributed to the different temperature regimes between the two areas. Winter temperatures in the Adriatic Sea may fall below 13°C, the minimum wintering temperature for Mediterranean loggerheads (Hochscheid et al. 2007Hochscheid S., Bentivegna F., Bradai M.N., et al. 2007. Overwintering behaviour in sea turtles: dormancy is optional. Mar. Ecol. Prog. Ser. 340: 287-298. https://doi.org/10.3354/meps340287), which may drive them to move to southern latitudes where higher temperatures prevail (Pastor et al. 2018Pastor F., Valiente J.A., Palau J.L. 2018. Sea surface temperature in the Mediterranean: trends and spatial patterns (1982–2016). Pure Appl. Geophys. 175: 4017-4029. https://api.semanticscholar.org/CorpusID:133718397). Telemetry studies have recorded southward movements of foraging loggerheads from Zakynthos in winter (Zbinden et al. 2008Zbinden J.A., Aebischer A., Margaritoulis D., et al. 2008. Important areas at sea for adult loggerhead sea turtles in the Mediterranean Sea: satellite tracking corroborates findings from potentially biased sources. Mar. Biol. 153: 899-906. https://doi.org/10.1007/s00227-007-0862-2, Schofield et al. 2013Schofield G., Dimadi A., Fossette S., et al. 2013. Satellite tracking large numbers of individuals to infer population level dispersal and core areas for the protection of an endangered species. Divers. Distrib. 19: 834-844. https://doi.org/10.1111/ddi.12077).

4.4 Recovery distance

This study has shown that loggerheads nesting in Greece are capable of large-scale migrations in the Mediterranean. The longest distances (minimum) travelled in the present study between tagging and recovery site (1756 and 1900 km) are comparable to distances travelled by tagged post-nesting loggerheads in Western Atlantic (e.g. 1352 km in Georgia, USA (Bell and Richardson 1978Bell R., Richardson J.I. 1978. An analysis of tag recoveries from loggerhead sea turtles (Caretta caretta) nesting in Little Cumberland Island, Georgia. Fla. Mar. Res. Publ. 33: 20-24.), 1500 km in Florida (Meylan et al. 1983Meylan A.B., Bjorndal K.A., Turner B.J. 1983. Sea turtles nesting at Melbourne Beach, Florida: II. Post-nesting movements of Caretta caretta. Biol. Conserv. 26: 79-90. https://doi.org/10.1016/0006-3207(83)90050-2)). However, longer distances were covered by migrating flipper-tagged post-nesting loggerheads in other ocean basins: e.g. 2620 km in the SW Pacific (Limpus et al. 1992Limpus C., Miller J., Parmenter C., et al. 1992. Migration of green (Chelonia mydas) and loggerhead (Caretta caretta) turtles to and from eastern Australian rookeries. Wildl. Res. 19: 347-357. https://doi.org/10.1071/WR9920347), 2880 km in the Indian Ocean (Hughes 1974Hughes G.R. 1974. The Sea Turtles of South-East Africa. South African Association for Marine Biological Research, Oceanographic Research Institute, Investigational Report No 36, 96 pp.) and 3000 km in the NW Pacific (Kamezaki et al. 1997Kamezaki N., Miyawaki I., Suganuma H., et al. 1997. Post-nesting migration of Japanese loggerhead turtle, Caretta caretta. Wildl. Conserv. Jpn. 3: 29-39 (in Japanese with English abstract).).

Recovery distances in our study did not differ among nesting areas, as also reported for loggerheads in Australia (Perez et al. 2022Perez M.A., Limpus C.J., Hofmeister K., et al. 2022. Satellite tagging and flipper tag recoveries reveal migration patterns and foraging distribution of loggerhead sea turtles (Caretta caretta) from eastern Australia. Mar. Biol. 169: 80. https://doi.org/10.1007/s00227-022-04061-8). In addition, the recovery distance was not correlated to body size, as also reported by Schofield et al. (2013)Schofield G., Dimadi A., Fossette S., et al. 2013. Satellite tracking large numbers of individuals to infer population level dispersal and core areas for the protection of an endangered species. Divers. Distrib. 19: 834-844. https://doi.org/10.1111/ddi.12077 for post-nesting loggerheads from Zakynthos. These findings, combined with the large variability of recovery distance (151–1900 km), indicate that post-nesting loggerheads exhibit a variety of migration strategies, including a percentage of turtles that migrate to short distances from the nesting area (Schofield et al. 2013Schofield G., Dimadi A., Fossette S., et al. 2013. Satellite tracking large numbers of individuals to infer population level dispersal and core areas for the protection of an endangered species. Divers. Distrib. 19: 834-844. https://doi.org/10.1111/ddi.12077, Patel et al. 2015aPatel S.H., Morreale S.J., Panagopoulou A., et al. 2015a. Changepoint analysis: a new approach for revealing animal movements and behaviors from satellite telemetry data. Ecosphere 6(12): 219. https://doi.org/10.1890/ES15-00358.1). It would be interesting to know whether turtles making long journeys spend more energy and hence invest less energy in reproduction, i.e. have longer remigration intervals or lay smaller clutches.

4.5 Recovery interval

In our study, short recovery intervals (less than two months) were proportional to recovery distances indicating a purposeful movement of post-nesting turtles, with some of them attaining travel speeds comparable to those acquired through satellite tracking, e.g. 31.3–40.0 km/day (Zbinden et al. 2008Zbinden J.A., Aebischer A., Margaritoulis D., et al. 2008. Important areas at sea for adult loggerhead sea turtles in the Mediterranean Sea: satellite tracking corroborates findings from potentially biased sources. Mar. Biol. 153: 899-906. https://doi.org/10.1007/s00227-007-0862-2). Turtles with short intervals were probably recovered before reaching their final destinations. On the other hand, long recovery intervals may represent residency in the specific foraging area or site fidelity to the same marine area between breeding migrations (Lazar et al. 2004Lazar B., Margaritoulis D., Tvrtkovic N. 2004. Tag recoveries of the loggerhead sea turtle Caretta caretta in the eastern Adriatic Sea: implications for conservation. J. Mar. Biol. Ass. U.K. 84: 475-480. https://doi.org/10.1017/S0025315404009488h). Despite high tag loss, flipper tags remaining for a long-time on turtles may assist interpretation of turtle movements and habitat use changes over large time scales (Limpus and Limpus 2003Limpus C.J., Limpus D.J. 2003. Loggerhead turtles in the equatorial and southern Pacific Ocean: A species in decline. In: Bolten A.B., Witherington B.E. (eds), Loggerhead Sea Turtles. Smithsonian Books, Washington DC, pp. 199-209.). Conversely, satellite telemetry is intrinsically limited to turtle tracking over short periods, often less than a year.

4.6 Body size

It has been hypothesized that large turtles may travel farther (Godley et al. 2003Godley B.J., Broderick A.C., Glen F., et al. 2003. Post-nesting movements and submergence patterns of loggerhead marine turtles in the Mediterranean assessed by satellite tracking. J. Exp. Mar. Biol. Ecol. 287: 119-134. https://doi.org/10.1016/S0022-0981(02)00547-6). However, in our study we did not observe any relevance of body size to distance travelled. Instead, we noticed significant differences in body size based on foraging location. Significant body size differences between turtles foraging in the Adriatic Sea and in the Gulf of Gabès (Tunisian Plateau) have been reported previously and were attributed to differences in abundance of benthic prey, which comprises the main diet of loggerhead turtles (Zbinden et al. 2011Zbinden J.A., Bearhop S., Bradshaw P., et al. 2011. Migratory dichotomy and associated phenotypic variations in marine turtles revealed by satellite tracking and stable isotope analysis. Mar. Ecol. Prog. Ser. 421: 291-302. https://doi.org/10.3354/meps08871, Patel et al. 2015b). Our findings support this result and extend it to reveal that turtles in the Adriatic were significantly bigger than those from other main recovery sites. The Adriatic Sea, being the marine area with the highest primary production in the Eastern Mediterranean (Antoine et al. 1995Antoine D., Morél A., André J-M. 1995. Algal pigment distribution and primary production in the eastern Mediterranean as derived from coastal zone colour scanner observations. J. Geophys. Res. 100: 16193-16209. https://doi.org/10.1029/95JC00466), appears to be a resource-rich region where turtles may grow to achieve larger body size and likely have higher reproductive output than turtles from other regions in the Mediterranean.

4.7 Fate of recovered turtles

About 61.7% of our tagged turtles were reported dead in all ecoregions. This percentage is considered high compared with the 45.8% of tagged turtles recovered dead in Australia (Limpus et al. 1992Limpus C., Miller J., Parmenter C., et al. 1992. Migration of green (Chelonia mydas) and loggerhead (Caretta caretta) turtles to and from eastern Australian rookeries. Wildl. Res. 19: 347-357. https://doi.org/10.1071/WR9920347). The large differences observed between Mediterranean ecoregions, from 40.8% on the Tunisian Plateau to almost 73% in the Aegean Sea, may have been caused by the different fishing gears and methods used in each ecoregion (Casale 2011Casale P. 2011. Sea turtle by-catch in the Mediterranean. Fish. Fish. 12: 299-316. https://doi.org/10.1111/j.1467-2979.2010.00394.x). Nevertheless, the combined high percentage of deaths (55.6%) in the two ecoregions accounting for most recoveries (the Adriatic Sea and the Tunisian Plateau; 62.6%), and likely attributed to the considerable turtle by-catch in these two regionally important fishing grounds (Casale et al. 2004Casale P., Laurent L., De Metrio G. 2004. Incidental capture of marine turtles by the Italian trawl fishery in the north Adriatic Sea. Biol. Conserv. 119: 287-295. https://doi.org/10.1016/j.biocon.2003.11.013, Jribi et al. 2007Jribi I., Bradai M.N., Bouain A. 2007. Impact of trawl fishery on marine turtles in the Gulf of Gabés, Tunisia. Herpetol. J. 17: 110-114.), certainly impacts the population of turtles breeding in Greece.

4.8 Conservation implications

In the Mediterranean, loggerhead turtles cross the boundaries of many countries because of their long migrations. In our study, the recovered turtles were reported in the economic zones of 15 countries, including Greece. These turtles are apparently exposed to numerous threats of varying intensity depending on national fisheries regulations, including illegal fisheries, and the associated levels of enforcement. Turtle captures in these 15 countries account for 86% of all turtle by-catch in the Mediterranean (Casale 2011Casale P. 2011. Sea turtle by-catch in the Mediterranean. Fish. Fish. 12: 299-316. https://doi.org/10.1111/j.1467-2979.2010.00394.x). This emphasizes the need to reinforce regional fisheries regulations and establish (or expanding existing) marine protected areas in the known foraging or wintering habitats.

In Greece, most of the tag recovery concentration areas overlap with marine protected sites of the NATURA 2000 Network of the European Union. Indeed, most recoveries in the Gulf of Corinth, in Amvrakikos Gulf, in the Thracian Sea and at Lesvos Island are within a protected site (see Fig. 2). However, Saronikos Gulf, with several tag recoveries, is not included in this network as it contains the large Piraeus port and many tourist facilities, which challenge the establishment of a protected site. We note that these marine sites, with the exception of Amvrakikos Gulf (declared a National Park), still lack adequate protective legislation and active management. It is therefore questionable whether the adult female turtles that migrate to these nominally protected areas after their reproduction will remain unharmed by unregulated activities.

It is globally recognized that the adult female turtles are the biologically most valuable individuals within a sea turtle population (Crouse et al. 1987Crouse D.T., Crowder L.B., Caswell H. 1987. A stage-based population model for loggerhead sea turtles and implications for conservation. Ecology 68: 1412-1423. https://doi.org/10.2307/1939225), and hence their foraging/wintering areas should receive protection equivalent to that afforded to their breeding areas.

5. CONCLUSION

Our study, resulting from a multidecadal flipper-tagging programme, highlighted the regional distribution of loggerhead turtles after their nesting in Greece, indicating apparent foraging/wintering areas in the Mediterranean. The empirical data, although potentially biased, are useful in determining the ecological range of the loggerhead turtle populations breeding in Greece. Transboundary migrations and increased deaths advocate the establishment of new (or the expansion of existing) marine protected areas, preferably at regional or multi-national scales. Further, monitoring environmental and anthropogenic changes in the foraging/wintering areas may elucidate the drivers behind trends in the abundance and productivity of nesting populations.

Supplementary information ↑

Funding sources

Tagging work was generally funded by ARCHELON in the course of its annual programmes on the nesting beaches of Greece. In certain periods of time, tagging work benefited from various LIFE-Nature projects.

Supplementary material

Not applicable.

Data availability

Not applicable.

Acknowledgements

We thank all those who provided information on recovered tagged turtles, especially fishers, divers and coast guard and fisheries officers. Many thanks to international institutions and organizations (e.g. GFCM, RAC/SPA) who informed national authorities in the Mediterranean about our tagging programme. We thank the many hundreds of trained ARCHELON volunteers who worked with dedication to tag turtles at night on the nesting beaches of Greece. We thank the Archie Carr Center for Sea Turtle Research (ACCSTR) in Florida, USA, for providing, through its Tag Inventory, our contact details to finders of tags, especially those with unreadable return addresses. The tagging programme of ARCHELON is implemented under research permits, issued initially by the Ministry of Agriculture, and later by the Ministry of the Environment and the National Marine Park of Zakynthos. Lenio Margaritouli drafted the map and the graphics of Figure 2. We thank Sara Abalo and Pilar Santidrian-Tomillo for their reviewing work and useful comments, which improved an early draft.

Authorship contribution statement

Dimitris Margaritoulis: conceptualization; data curation; writing-original draft. ALan F. Rees: statistical analysis; writing-reviewing and editing.

Competing interests

The authors of this article declare that they have no financial, professional or personal conflicts of interest that could have inappropriately influenced this work.

Statement on the use of Artificial Intelligence

Not applicable.

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