Western Mediterranean immature loggerhead turtles: habitat use in spring and summer assessed through satellite tracking and aerial surveys

An aerial survey was conducted in early spring 2002 over the continental shelf of the Balearic Archipelago to study the distribution of neritic loggerhead turtles. Furthermore, five juvenile loggerhead turtles [straight carapace length (SCL) range 37.1–48.7 cm], were instrumented with transmitters and monitored during 2003 by satellite tracking to study habitat use over a broader geographical range. The distribution of the turtles over the continental shelf matched habitat availability, as defined by depth. However, those tracked by satellite spent most of the time in the oceanic waters of the Algerian basin and generally avoided continental shelf areas. In these turtles, average speed of travel and mean cosine of turning angle did not significantly differ between habitats, indicating that avoidance of shelf areas is not due to active habitat selection. On average, tracked turtles spent 35.1±19.7% of the time at the surface, although surface time was much greater in the turtle with the shortest carapace length, suggesting that this individual had limited swimming capacity. We conclude that the transition between passive drifting and active habitat selection occurs at an SCL of about 40 cm. The turtles followed tracks that matched prevailing currents, but on some occasions they also swam upstream. Hence, the distribution of late juvenile loggerhead turtles in the southern and central western Mediterranean may reflect a combination of passive drifting and active habitat selection.Communicated by O. Kinne, Oldendorf/Luhe     

Satellite tracking reveals a dichotomy in migration strategies among juvenile loggerhead turtles in the Northwest Atlantic

Few data are available on the movements and behavior of immature Atlantic loggerhead sea turtles (Caretta caretta) from their seasonal neritic foraging grounds within the western north Atlantic. These waters provide developmental habitat for loggerheads originating from several western Atlantic nesting stocks. We examined the long-term movements of 23 immature loggerheads (16 wild-caught and seven headstart turtles) characterizing their seasonal distribution, habitat use, site fidelity, and the oceanographic conditions encountered during their migrations. We identified two movement strategies: (1) a seasonal shelf-constrained north–south migratory pattern; and (2) a year-round oceanic dispersal strategy where turtles travel in the Gulf Stream to the North Atlantic and their northern dispersal is limited by the 10–15°C isotherm. When sea surface temperatures dropped below 20°C, neritic turtles began a migration south of Cape Hatteras, North Carolina (USA) where they established fidelity to the waters between North Carolina’s Outer Banks and the western edge of the Gulf Stream along outer continental shelf. Two turtles traveled as far south as Florida. Several turtles returned to their seasonal foraging grounds during subsequent summers. Northern movements were associated with both increased sea surface temperature (>21°C) and increased primary productivity. Our results indicate strong seasonal and interannual philopatry to the waters of Virginia (summer foraging habitat) and North Carolina (winter habitat). We suggest that the waters of Virginia and North Carolina provide important seasonal habitat and serve as a seasonal migratory pathway for immature loggerhead sea turtles. North Carolina’s Cape Hatteras acts as a seasonal “migratory bottleneck” for this species; special management consideration should be given to this region. Six turtles spent time farther from the continental shelf. Three entered the Gulf Stream near Cape Hatteras, traveling in the current to the northwest Atlantic. Two of these turtles remained within an oceanic habitat from 1 to 3 years and were associated with mesoscale features and frontal systems. The ability of large benthic subadults to resume an oceanic lifestyle for extended periods indicates plasticity in habitat use and migratory strategies. Therefore, traditional life history models for loggerhead sea turtles should be reevaluated.     

Habitat use by immature loggerhead sea turtles in the Algerian Basin (western Mediterranean): swimming behaviour,seasonality and dispersal pattern

To study habitat use by loggerhead sea turtles in the Algerian Basin (western Mediterranean), ten juveniles (straight carapace length range: 39.0–63.3 cm) were tracked by satellite from March 2004 to September 2005. Swimming behaviour (characterized by speed of travel, time spent at the surface, and the cosine of turning angles) varied individually, but these differences were unrelated to body size. Despite individual differences in swimming behaviour, the ten immature loggerhead sea turtles spent most of their time in the oceanic waters of the Algerian Basin, although simulations indicated that the average tracking time (235.7 ± 98.7 SD days) was sufficiently long for them to leave the Algerian Basin and disperse through most of the Mediterranean. Furthermore, none of the ten turtles swam in any preferred direction, and their bearings were all randomly distributed. Finally, all them consistently avoided the continental shelf and did not migrate seasonally, as the average latitude, the average longitude, and the average distance of the population to the release point did not change seasonally. Seasonality also had only a weak influence in swimming behaviour, as the time spent at the surface during light hours was the only parameter that changed seasonally. We conclude that immature loggerhead sea turtles in the south of the western Mediterranean exhibit a strong fidelity to the Algerian Basin, where distribution is ruled mainly by the bathymetry, without any influence of seasonality. That fidelity to the Algerian Basin matches predictions based on genetic structuring and might result from a combination of factors: surface circulation patterns and habitat selection by the loggerhead sea turtles.     

A model of area fidelity,nomadism, and distribution patterns of loggerhead sea turtles (<Emphasis Type="Italic">Caretta caretta</Emphasis>) in the Mediterranean Sea

Sea turtle tagging carried out in Italy in the period 1981–2006 resulted in 125 re-encounters of loggerhead turtles (Caretta caretta) after a mean of 2.5 years, from different marine areas in the Mediterranean. At first finding, turtles ranged 25–83 cm of curved carapace length. Data were analyzed according to size, area, habitat type, season, in order to provide indication of movement patterns. When integrated with other information, results indicate that: (1) a part of turtles in the oceanic stage show a nomad behavior with movements among different oceanic areas; (2) another part show fidelity to an oceanic area; (3) turtles in the neritic stage show fidelity to neritic areas, and once settled to one area, change to other neritic areas is unlikely; (4) nomad oceanic turtles are significantly larger than sedentary ones, and also larger than turtles found in neritic areas; it is hypothesized that these could be Atlantic turtles that eventually leave the Mediterranean; (5) ecological transition from oceanic to neritic habitats occurs at a wide range of sizes, and some turtles may have a very brief oceanic stage; (6) turtles in the oceanic stage are more likely to recruit to neritic areas close to their oceanic areas than to distant ones; (7) part of turtles from some Mediterranean nesting beaches might frequent a relatively limited area range, including both oceanic and neritic areas; (8) in most of the Mediterranean, latitudinal seasonal migrations are unlikely. A general model of movement patterns of loggerhead turtles in the Mediterranean is proposed.     

Genetic structuring of immature loggerhead sea turtles (Caretta caretta) in the Mediterranean Sea reflects water circulation patterns

The analysis of mitochondrial DNA in loggerhead sea turtles (Caretta caretta) from eight foraging grounds in the Mediterranean and the adjoining Atlantic revealed deep genetic structuring within the western Mediterranean. As a consequence, the foraging grounds off the North-African coast and the Gimnesies Islands are shown to be inhabited mainly by turtles of the Atlantic stocks, whereas the foraging grounds off the European shore of the western Mediterranean are shown to be inhabited mainly by turtles from the eastern Mediterranean rookeries. This structuring is explained by the pattern of sea surface currents and water masses and suggests that immature loggerhead sea turtles entering the western Mediterranean from the Atlantic and the eastern Mediterranean remain linked to particular water masses, with a limited exchange of turtles between water masses. As the north of the western Mediterranean comprises mostly individuals from the highly endangered eastern Mediterranean rookeries, conservation plans should make it a priority to reduce the mortality caused by incidental by-catch in these areas.     

The relationship between loggerhead turtle (<Emphasis Type="Italic">Caretta caretta</Emphasis>) movement patterns and Mediterranean currents

Previous studies have shown that loggerhead sea turtles (Caretta caretta), monitored by satellite telemetry, complete long-distance migration between the western and eastern Mediterranean basins following a seasonal pattern. This study investigated if these migration routes may be influenced by surface currents by superimposing the tracks of three loggerhead turtles (curved carapace length >55 cm), migrating from the western to the eastern Mediterranean basin, on Lagrangian data of current developed into pseudo-eulerian speed fields. The average travel speed of the turtles was 1.6 km h⁻¹ and did not depend on the current speed or direction. We observed a connection between surface currents and the turtles’ migration routes, although not a conclusive one. These observations show that neritic stage loggerhead turtles conduct migration in two distinct alternate phases: the first characterized by high and constant speed of travel both when swimming with or against currents and the second typified by low travel speeds and a good concurrence between the trailed routes and the course of the currents. These two phases corresponded to two types of movements, one where the turtle migrates actively to reach a specific destination (either neritic foraging, wintering or nesting ground) and the other, where the turtle drifts with the mesoscale current and forages pelagically. It seemed thus, that the influence of currents on a turtle’s movements depends on the turtle’s momentary behaviour and location of residence.     

Different growth rates between loggerhead sea turtles (<Emphasis Type="Italic">Caretta caretta</Emphasis>) of Mediterranean and Atlantic origin in the Mediterranean Sea

We estimated for the first time the growth rates of loggerhead sea turtles of Mediterranean and of Atlantic origin found in the Mediterranean Sea, combining both skeletochronological and genetic analyses. Our growth models suggested that the growth rate of loggerhead sea turtles of Mediterranean origin was faster than that of their conspecifics with an Atlantic origin exploiting the feeding grounds in the Mediterranean Sea. The age at maturity for Mediterranean origin loggerhead sea turtles, estimated using our best fitting model, was 24 years, which suggests that loggerhead sea turtles nesting in the Mediterranean are not only smaller than those nesting in the western North Atlantic but also younger.     

Spatio-temporal distribution and migration of adult male loggerhead sea turtles (Caretta caretta) in the Mediterranean Sea: further evidence of the importance of neritic habitats off North Africa

Ten adult male loggerhead sea turtles, captured by trawlers or dip nets, were satellite-tracked from a neritic foraging ground in the Mediterranean in order to investigate adult spatio-temporal distribution and breeding migration. Five individuals migrated to potential breeding sites in Libya and one to Greece. The results complement previous studies and show that: (1) the Tunisian shelf may be more important for turtles from Libyan rookeries than previously thought; (2) male tracks corroborate a conservation hotspot previously identified for juveniles; (3) the north African coast represents a preferred migratory corridor, unless open sea routes are more direct; (4) adult males may exhibit high fidelity to relatively small areas, without evident seasonal differences; (5) adults home ranges were smaller and more neritic than juveniles frequenting the same area; (6) males may frequent multiple courtship areas; (7) the average remigration interval of males frequenting this region is longer than 1 year.     

Seasonal distribution patterns of juvenile loggerhead sea turtles (<Emphasis Type="Italic">Caretta caretta</Emphasis>) following capture from a shipping channel in the Northwest Atlantic Ocean

Thirty-four juvenile loggerhead sea turtles captured by trawling from the Charleston, South Carolina (USA), shipping channel (32°42′N; −79°47′W) between May 2004 and August 2007 were tagged with satellite transmitters to assess the extent to which they remained near the capture location given their collection along a seasonal migratory corridor. Seventy-five percent of juveniles were classified as seasonal residents. Migrants predominantly swam north in the spring and nomads wandered south in the summer, but predictive indicators for non-resident status were not identified. All but one juvenile generally remained south of 34°N, within 40 km of shore, and in waters <30 m deep throughout the year. Nine of 14 loggerhead sea turtles monitored during the winter remained exclusively over the continental shelf, three briefly occurred in oceanic habitats, and two foraged extensively in oceanic habitats. Residents distributed >15 km from shore between spring and autumn were three times as likely to occur in oceanic habitats in winter. Modest seasonal movements contrasted with adults tagged at similar latitudes and with juveniles tagged further north and suggest distinct foraging groups within a regional foraging ground.     

Comparative skeletochronological analysis of Kemp’s ridley (<Emphasis Type="Italic">Lepidochelys kempii</Emphasis>) and loggerhead (<Emphasis Type="Italic">Caretta caretta</Emphasis>) humeri and scleral ossicles

Skeletochronological analysis of Kemp’s ridley (Lepidochelys kempii) and loggerhead (Caretta caretta) sea turtle humeri and scleral ossicles was conducted to (1) describe the characteristics of scleral ossicles in these species, (2) determine whether the scleral ossicles contain annually deposited skeletal growth marks and (3) evaluate the potential for skeletochronological analysis of ossicles to obtain age data for size classes and species of sea turtles whose humeri exhibit prohibitive amounts of growth mark resorption. Humeri, entire eyes, and/or individual scleral ossicles were collected from stranded, dead sea turtles that were found along the coasts of Florida, North Carolina, Virginia, and Texas, USA. Samples were taken from a total of 77 neritic, juvenile Kemp’s ridleys ranging from 21.1 to 56.8 cm straightline carapace length (SCL), as well as two Kemp’s ridley hatchlings. For loggerheads, samples were obtained from 65 neritic juvenile and adult turtles ranging from 44.7 to 103.6 cm SCL and ten hatchlings. Examination of the ossicles revealed the presence of marks similar in appearance to those found in humeri. The number of marks in the ossicles and humeri of individual juvenile Kemp’s ridleys for which both structures were collected (n = 55) was equivalent, strongly indicating that the marks are annual. However, in large juvenile and adult loggerhead turtles (n = 65), some significant resorption of early growth marks was observed, suggesting that although ossicles might be useful for skeletochronological analysis of small juveniles, they may not provide a reasonable alternative to humeri for obtaining age estimates for older loggerhead sea turtles.     

Development and pre-validation of a testosterone enzyme immunoassay (EIA) for predicting the sex ratio of immature loggerhead sea turtles (Caretta caretta) recovered along the western coast of the central Adriatic Sea

Plasma testosterone levels were evaluated, by validating and using a commercially available testosterone enzyme immunoassay (EIA) as an indicator of the sex of immature loggerhead turtles (Caretta caretta) recovered along the western coast of the central Adriatic Sea between November 2011 and February 2012. Testosterone levels were measured in blood samples collected from 28 immature turtles kept in short-term maintenance at the Fondazione Cetacea Rescue Centre (Italy). Overall, plasma testosterone ranged between 229.3 and 2628.6 pg ml^?1, suggesting that the EIA procedure is effective for determining androgen titers in immature loggerhead sea turtles. Analysis of the obtained data indicates an unbiased sex ratio supporting previous studies of juvenile loggerhead turtle sex ratios in the Mediterranean Sea. The present work can be considered a starting point for augmenting knowledge on the dynamics of juvenile loggerhead aggregations increasingly found in the northern-central Adriatic Sea and for promoting local management for conservation actions.     

Intra-Mediterranean migrations of loggerhead sea turtles (Caretta caretta) monitored by satellite telemetry

The movements of four Mediterranean loggerhead sea turtles (Caretta caretta; three females, one male) were tracked via satellite telemetry for between 108 and 457 days. Total length of the routes traveled by the turtles varied between 2554 and 7098 km, and the average travel rate was 1.2 km h^-1. Long-distance movement between the western and eastern Mediterranean basins followed a seasonal pattern and seemed to be triggered by temperature and food availability. In the autumn/winter months turtles moved from west to east in search of warmer waters, and returned to the western basin in spring, where food resources are generally more plentiful. Three (two females, one male) of the four turtles migrated eastward through the Straits of Messina, which is characterized by high fishing pressure and intense boat traffic. Information about turtle migration patterns and routes will serve to plan effective conservation strategies.     

Oceanic residents, neritic migrants: a possible mechanism underlying foraging dichotomy in adult female loggerhead turtles (Caretta caretta)

To reveal the mechanism underlying intrapopulation variation in the use of feeding habitats (oceanic vs. neritic) by adult female loggerhead turtles (Caretta caretta), we compared telomere length in the epidermis (a proxy for age) between oceanic- and neritic-foraging recruits (first-time nesters). Based on egg-yolk stable isotope ratios, recruits at Yakushima Island, Japan, were clearly divided into small oceanic planktivores and large neritic benthivores. There were no significant differences in telomere length between oceanic and neritic foragers, suggesting that they start reproduction at similar ages. Turtles that experienced faster growing conditions during their oceanic early lives may achieve sexual maturity there, while others may move from oceanic areas into neritic habitats, switching diets from nutrient-poor macroplankton to nutrient-rich benthic fauna in order to compensate for their earlier slow growth rate and continue their sexual development, reaching maturity in neritic waters.     

Important areas at sea for adult loggerhead sea turtles in the Mediterranean Sea: satellite tracking corroborates findings from potentially biased sources

Sea turtle populations worldwide suffer from reduced survival of immatures and adults due to fishery bycatch. Unfortunately, information about the whereabouts of turtles outside the breeding habitat is scarce in most areas, hampering the development of spatially explicit conservation plans. In the Mediterranean, recoveries of adult females flipper-tagged on nesting beaches suggest that the Adriatic Sea and Gulf of Gabès are important foraging areas for adults, but such information could be heavily biased (observing and reporting bias). In order to obtain unbiased data, we satellite-tracked seven loggerhead sea turtles after they completed nesting in the largest known Mediterranean rookery (Bay of Laganas, Zakynthos, Greece). Three females settled in the north Adriatic Sea, one in the south Adriatic Sea and two in the Gulf of Gabès area at the completion of their post-nesting migrations (one individual did not occupy a distinct foraging area). The concordance of tracking results with information from recoveries of flipper-tagged turtles suggests that the north Adriatic Sea and the Gulf of Gabès represent key areas for female adult Mediterranean loggerhead sea turtles.     

Migration,distribution, and diving behavior of adult male loggerhead sea turtles (<Emphasis Type="Italic">Caretta caretta</Emphasis>) following dispersal from a major breeding aggregation in the Western North Atlantic

Sixteen satellite-tagged adult male loggerhead sea turtles (Caretta caretta) dispersed widely from an aggregation near Port Canaveral, Florida, USA (28°23′N, −80°32′W) after breeding. Northbound males migrated further (990 ± 303 km) than southbound males (577 ± 168 km) and transited more rapidly (median initial dive duration = 6 (IQR = 4–16) versus 19 (IQR = 10–31) min, respectively).. Migration occurred along a depth corridor (20–40 m) except where constricted by a narrow continental shelf width. Males foraged in areas 27 ± 41 km² day⁻¹ at locations <1–80 km from shore for 100.1 ± 60.6 days, with variability in foraging patterns not explained by turtle size or geography. Post-breeding dispersal patterns were similar to patterns reported for adult female loggerhead sea turtles in this region and adult male loggerhead sea turtles elsewhere in the northern hemisphere; however, foraging ground distributions were most similar to adult female loggerhead sea turtles in this region.     

Remigration and growth of loggerhead turtles (<Emphasis Type="Italic">Caretta caretta</Emphasis>) nesting on Senri Beach in Minabe,Japan: life-history polymorphism in a sea turtle population

There are size-related differences in the use of feeding habitats (planktonic or benthic; oceanic or neritic) by adult female loggerhead sea turtles (Caretta caretta) within Japanese populations. We thus hypothesized that the differences may be reflected in their remigration and growth patterns, and investigated the relationships between body size and remigration intervals, growth rates, and remigration percentages, for female loggerheads nesting on a Japanese beach between 1991 and 2001. Although remigration intervals, growth rates, and remigration percentages were not significantly different among females, there were trends for longer remigration intervals and lower remigration percentages in smaller females. All females grew little. Considering these results along with previous findings, we speculated on the life-history strategy of female Japanese loggerheads.Communicated by T. Ikeda, Hakodate     

Fine-scale distribution of juvenile Atlantic and Mediterranean loggerhead turtles (Caretta caretta) in the Mediterranean Sea

Loggerhead turtles nesting in the Mediterranean Sea exhibit remarkable genetic structuring. This paper tests the hypothesis that young loggerhead turtles from different rookeries do not distribute homogeneously among the major Mediterranean foraging grounds, due to a complex pattern of surface currents. We extracted long fragments of mitochondrial DNA from 275 stranded or bycaught juvenile turtles from six foraging grounds (Catalano-Balearic Sea, Algerian basin, Tyrrhenian Sea, Adriatic Sea, northern Ionian Sea and southern Levantine Sea). We used a Bayesian mixed-stock analysis to estimate the contributions from rookeries in the Mediterranean, the North-west Atlantic and Cape Verde to the studied foraging grounds. Differences were found in the relative contribution of juvenile turtles of Atlantic and Mediterranean origin to each foraging ground. A decreasing proportion of Atlantic juveniles was detected along the main surface current entering the Mediterranean, with a high prevalence of turtles from eastern Florida in the Algerian basin and lower numbers elsewhere. In regard to the turtles of Mediterranean origin, juveniles from Libya prevailed in central and western Mediterranean foraging grounds other than the Algerian basin. Conversely, the Adriatic Sea was characterised by a large presence of individuals from western Greece, while the southern Levantine Sea was inhabited by a heterogeneous mix of turtles from the eastern Mediterranean rookeries (Turkey, Lebanon and Israel). Overall, the distribution of juveniles may be related to surface circulation patterns in the Mediterranean and suggests that fisheries might have differential effects on each population depending on the overlap degree between foraging and fishing grounds.     

The stomach contents of post-hatchling green and loggerhead sea turtles in the southwest Pacific: an insight into habitat association

Dietary information obtained from stomach contents can provide a wealth of information on an animal’s ecology. Where animals are cryptic, such as the post-hatchling life history stage of a sea turtle, the ecological insight that dietary analyses can provide, may be otherwise unobtainable. Investigations into post-hatchling turtle stomach contents have found planktonic organisms, dominated by pelagic molluscs and crustaceans, hydrozoans, Sargassum and fish eggs. The nature of these dietary organisms provides evidence for the widely accepted hypothesis that, with the exception of the flatback turtle (Natator depressus), the post-hatchling stage of a sea turtle’s life history is pelagic and oceanic. As the majority of studies that have investigated the stomach contents of post-hatchling sea turtles have been conducted on loggerhead turtles (Caretta caretta) in the northern Atlantic and Pacific Oceans, insight derived from dietary investigations into post-hatchling ecology is biased. This study investigates the diet of post-hatchling green turtles (Chelonia mydas) and loggerhead turtles in the southwest Pacific Ocean. Stomach contents were obtained from 55 green and loggerhead post-hatchling turtles that had stranded or been consumed by Coryphaena hippurus. Our findings demonstrate that loggerhead and green post-hatchlings in the southwest Pacific share similar feeding ecology and feed on a variety of neustonic items that are indicative of an oceanic and pelagic existence. The dietary items consumed by both species investigated belong to similar taxonomic groups as those found in previous studies with species level distinctions occurring owing to the different geographical location.     

Post-nesting migrations of loggerhead sea turtles in the Gulf of Mexico: dispersal in highly dynamic conditions

This study is the first report of post-nesting migrations of loggerhead sea turtles (Caretta caretta) nesting in Sarasota County (Florida, USA), their most important rookery in the Gulf of Mexico (GOM). In total, 28 females (curved carapace length CCL between 82.2 and 112.0 cm) were satellite-tracked between May 2005 and December 2007. Post-nesting migrations were completed in 3–68 days (mean ± SD = 23 ± 16 days). Five different migration patterns were observed: six turtles remained in the vicinity of their nesting site while the other individuals moved either to the south-western part of the Florida Shelf (n = 9 turtles), the Northeast GOM (n = 2 turtles), the South GOM (Yucatán Shelf and Campeche Bay, Mexico, and Cuba; n = 5 turtles) or the Bahamas (n = 6 turtles). In average, turtles moved along rather straight routes over the continental shelf but showed more indirect paths in oceanic waters. Path analyses coupled with remote sensing oceanographic data suggest that most of long-distance migrants reached their intended foraging destinations but did not compensate for the deflecting action of ocean currents. While six out of seven small individuals (CCL < 90 cm) remained on the Florida Shelf, larger individuals showed various migration strategies, staying on the Florida Shelf or moving to long-distance foraging grounds. This study highlights the primary importance the Western Florida Shelf in the management of the Florida Nesting Subpopulation, as well as the need of multi-national effort to promote the conservation of the loggerhead turtle in the Western Atlantic. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.