The spatial ecology of juvenile loggerhead turtles (Caretta caretta) in the Indian Ocean sheds light on the “lost years” mystery

While our understanding of the early oceanic developmental stage of sea turtles has improved markedly over recent decades, the spatial context for this life history stage remains unknown for Indian Ocean loggerhead turtle populations. To address this gap in our knowledge, 18 juvenile loggerheads were satellite tracked from Reunion Island (21.2°S, 55.3°E) between 2007 and 2011. Nine turtles swam north toward Oman (20.5°N, 58.8°E), where one of the world’s largest rookeries of loggerheads is located. Three individuals traveled south toward South Africa and Madagascar, countries that also host loggerhead nesting grounds. Fourteen of the transmitters relayed diving profiles. A dichotomy between diurnal and nocturnal diving behavior was observed with a larger number of shorter dives occurring during the day. Diving behavior also differed according to movement behavior as individuals spent more time in subsurface waters (between 10 and 20 m) during transit phases. The study provides an understanding of the oceanic movement behavior of juvenile loggerheads in the Indian Ocean that suggests the existence of an atypical trans-equatorial developmental cycle for the species at the ocean basin scale in the Indian Ocean. These results address a significant gap in the understanding of loggerhead oceanic movements and may help with the conservation of the species.     

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.     

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.     

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.     

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.     

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.     

Habitat use by loggerhead sea turtles <Emphasis Type="Italic">Caretta caretta</Emphasis> off the coast of eastern Spain results in a high vulnerability to neritic fishing gear

Previous studies of loggerhead sea turtles have concluded that drifting longlines were the main threat for immature specimens in the western Mediterranean, because immature loggerhead sea turtles mainly inhabit oceanic waters. However, recent aerial surveys have revealed large numbers of immature loggerhead sea turtles over the continental shelf of eastern mainland Spain, where turtles are exposed to neritic fishing gears but not to drifting longlines. We satellite-tracked seven loggerhead sea turtles (minimum straight carapace length (SCLmin) range: 36.5–55.0 cm) to assess whether the turtles in this region are vagrants from the adjoining oceanic regions or whether these loggerheads mostly inhabit the continental shelf. Satellite-tracking revealed that six of the tagged turtles avoided the oceanic realm and made extended use of the continental shelf, whereas only one individual could be considered a true vagrant as it avoided the continental shelf and primarily used the oceanic habitat. These results are in sharp contrast with those previously reported for immature loggerhead sea turtles of similar size from the south-western Mediterranean and fit well a relaxed ontogenic model that was recently proposed for loggerhead sea turtles in the central Mediterranean. Furthermore, these results demonstrate the vulnerability of loggerhead sea turtles of eastern mainland Spain to neritic fishing gears, as three of the seven turtles died and one was bycaught incidentally while being tracked over the continental shelf.     

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.     

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.     

Tracking post-nesting movements of loggerhead turtles (Caretta caretta) with sonic and radio telemetry on the southwest coast of Florida, USA

Nine post-nesting loggerhead turtles (Caretta caretta) were tracked using sonic and radio telemetry. Tracking began immediately after the turtles left the beach and continued until contact was either lost or terminated. As sonic tags transmit continuously underwater, they were much more effective than the radio tags in determining the paths of the turtles. Radio tags transmit only at the surface and were useful in ascertaining submergence durations. For nine of the ten turtles tracked with sonic signals, the gross movement was away from the beach in a westerly direction. The tracking periods ranged from 3.35 to 8.25 h, while the straight-line movements ranged from 3.05 to 12.88 km, respectively. Sixty-seven percent of the submergence durations recorded were <3 min. This respiratory behavior suggests continuous swimming, and the paths of the turtles suggested directed movement offshore immediately after nesting. The gradual littoral slope and lack of nearshore structure in this part of the Gulf of Mexico could be contributing factors to the patterns of dispersal observed, as benthic structures provide resting and foraging habitat for loggerheads.     

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.     

Oceanographic influences on the dive behavior of juvenile loggerhead turtles (Caretta caretta) in the North Pacific Ocean

Satellite telemetry data from 17 juvenile loggerhead turtles (43.5–66.5 cm straight carapace length) were used in conjunction with oceanographic data to analyze the influence of regional and seasonal oceanography on dive behavior in the North Pacific Ocean. Combined dive behavior for all individuals showed that turtles spent more than 80% of their time at depths <5 m, and more than 90% of their time at depths <15 m. Multivariate classifications of dive data revealed four major dive types, three representing deeper, longer dives, and one representing shallower dives shorter in duration. Turtles exhibited variability in these dive types across oceanographic regions, with deeper, longer dives in the Hawaii longline swordfish fishing grounds during the first quarter of the year, as well as in the Kuroshio Extension Bifurcation Region and the region near the Baja California Peninsula, Mexico. Turtles in the Kuroshio Extension Bifurcation Region also exhibited dive variability associated with mesoscale eddy features, with turtles making deeper, longer dives while associated with the strongest total kinetic energy. Turtles in the central North Pacific exhibited seasonality in dive behavior that appeared to reflect synchronous latitudinal movements with the North Pacific Subtropical Front and the associated seasonal, large-scale oceanography. Turtles made deeper, longer dives during the first quarter of the year within this region, the reported time and area where the highest loggerhead bycatch occurs by the longline fishery. These results represent the first comprehensive study of dive data for this species in this region. The increased understanding of juvenile loggerhead dive behavior and the influences of oceanography on dive variability should provide further insight into why interactions with longline fisheries occur and suggest methods for reducing the bycatch of this threatened species.     

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.     

Detection of coastal mud odors by loggerhead sea turtles: a possible mechanism for sensing nearby land

For sea turtles, an ability to detect land masses from a considerable distance away, and to distinguish coastal areas from the open sea, might be adaptive. The loggerhead turtle, Caretta caretta, can detect airborne odorants associated with food. To investigate whether sea turtles can also detect odors associated with land, we studied the responses of juvenile loggerheads to odors from coastal mud. Turtles were tested in a water-filled arena in which odorants could be introduced to the air above the water surface. Turtles exposed to air that had passed over a cup containing mud spent more time with their noses out of the water than did control turtles exposed to air that had passed over a cup containing distilled water. The results demonstrate for the first time that loggerheads can detect airborne odorants associated with land, an ability that might play a role in foraging, navigation, or both.     

Site fidelity and homing behavior in juvenile loggerhead sea turtles (<Emphasis Type="Italic">Caretta caretta</Emphasis>)

To investigate site fidelity and homing behavior in juvenile loggerheads ( Caretta caretta, L.), a mark-recapture study spanning four years (1998–2001) was conducted in Core Sound, N.C., USA. Each year of the study, approximately half of the turtles captured were tagged and released near the capture sites ( n=207), while the remaining turtles were displaced 15–20 km and released ( n=198). Loggerheads in both groups were recaptured in equal proportions near the original capture sites and many individuals were also recaptured in subsequent years. These data imply that juvenile loggerheads often returned to their capture sites following displacement, because if turtles dispersed randomly or remained near their release sites, then fewer displaced turtles should have been caught again. Moreover, because turtles migrate out of North Carolina sounds each winter, turtles recaptured at the same locations in different years evidently returned to specific sites following long migrations. To further investigate homing behavior, a small number of displaced turtles ( n=28) were tracked using radio telemetry following their release. Although transmitters detached from most turtles within a few days, analyses of initial headings showed strong orientation in the direction of the capture site. In addition, four turtles successfully tracked for longer periods of time all returned rapidly to the vicinity of the capture location and remained in the area. Taken together, the results of this study indicate that juvenile loggerheads exhibit fidelity to specific areas during summer months and possess the navigational abilities to home to these areas following forced displacements and long-distance migrations.     

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.     

Trans-Pacific dispersal of loggerhead turtle hatchlings inferred from numerical simulation modeling

We used Lagrangian numerical simulations to examine the trans-Pacific dispersal processes of loggerhead turtle hatchlings. Ten thousand simulated particles were released from each of the three nesting regions in Japan and tracked for 5 years. Results showed many particles moving eastward, drifting in the Kuroshio Current followed by the Kuroshio Extension Current. However, no particles reached Baja California, a known feeding area, through passive processes, indicating that trans-Pacific transportation requires active swimming by turtles. The duration of the trans-Pacific dispersal was estimated to be at least 1.6–3.4 years, with some turtles drifting in the Kuroshio Countercurrent and remaining in the western Pacific even after 5 years. This indicates that as revealed by previous genetic studies, not all loggerheads always disperse along a trans-Pacific route. The findings showed that survival and expected growth rates varied widely according to ambient temperatures during drifting, which in turn depended on nesting location.     

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.     

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.     

Foraging areas differentially affect reproductive output and interpretation of trends in abundance of loggerhead turtles

Diet items and habitat constitute some of the environmental resources that may be used differently by individuals within a population. Long-term fidelity by individuals to particular resources exemplifies individual specialization, a phenomenon that is becoming increasingly recognized across a wide range of species. Less is understood about the consequences of such specialization. Here, we investigate the effects of differential foraging ground use on reproductive output in 183 loggerhead sea turtles (Caretta caretta) nesting at Wassaw Island, Georgia (31.89°N, 80.97°W), between 2004 and 2011 with resulting possible fitness effects. Stable isotope analysis was used to assign the adult female loggerheads to one of three foraging areas in the Northwest Atlantic Ocean. Our data indicate that foraging area preference influences the size, fecundity, and breeding periodicity of adult female loggerhead turtles. We also found that the proportion of turtles originating from each foraging area varied significantly among the years examined. The change in the number of nesting females across the years of the study was not a result of uniform change from all foraging areas. We develop a novel approach to assess differential contributions of various foraging aggregations to changes in abundance of a sea turtle nesting aggregation using stable isotopes. Our approach can provide an improved understanding of the influences on the causes of increasing or decreasing population trends and allow more effective monitoring for these threatened species and other highly migratory species.