Fine-scale foraging ecology and habitat use of sympatric green and hawksbill turtles in the Western Indian ocean.

Using stable isotope analysis of carbon and nitrogen of turtle tissues and putative prey items, we investigated the diet of immature green turtles and hawksbill turtles foraging in the lagoon of Aldabra Atoll, a relatively undisturbed atoll in the southern Seychelles. Aldabra offers a unique environment for understanding sea turtle ecology. Green turtles mostly consumed seagrass and brown algae while hawksbill turtles mainly consumed mangroves and invertebrates. Green turtles showed a dietary shift with size (a proxy for age). There was minimal niche overlap between species and evidence of small-scale foraging site fidelity with turtle tissue reflecting site-specific prey. This highlights the ecological importance of seagrass and mangrove habitats and suggests that turtles play a role in controlling algal biomass at Aldabra. This study is the first to closely examine the foraging ecology of these sympatric turtle species in the Western Indian Ocean, a globally important region for both species.

Fine-scale intraspecific niche partitioning in a highly mobile, marine megafauna species: implications for ecology and conservation.

A species may partition its realized ecological niche along bionomic and scenopoetic axes due to intraspecific competition for limited resources. How partitioning manifests depends on resource needs and availability by and for the partitioning groups. Here we demonstrate the utility of analysing short- and long-term stable carbon and nitrogen isotope ratios from imperiled marine megafauna to characterize realized niche partitioning in these species. We captured 113 loggerhead sea turtles (Caretta caretta) at a high-use area in the eastern Big Bend, Florida, between 2016 and 2022, comprising 53 subadults, 10 adult males and 50 adult females. We calculated trophic niche metrics using established and novel methods, and constructed Bayesian ellipses and hulls, to characterize loggerhead isotopic niches. These analyses indicated that loggerheads partition their realized ecological niche by lifestage, potentially along both bionomic (e.g. trophic) and/or scenopoetic (e.g. habitat, latitude or longitude) axes, and display different characteristics of resource use within their niches. Analysis of stable isotopes from tissues with different turnover rates enabled this first characterization of intraspecific niche partitioning between and within neritic lifestages in loggerhead turtles, which has direct implications for ongoing research and conservation efforts for this and other imperiled marine species.

Characterizing stable isotope relationships between green turtle (Chelonia mydas) skin and unhatched eggs.

RATIONALE: Stable isotope analysis is used to understand the foraging habits and movements of a diverse set of organisms. Variability in stable isotope ratios among tissues derived from the same animal makes it difficult to compare data among study results in which different tissue types are evaluated. Isotopic relationships between two green turtle (Chelonia mydas) tissue types, skin and unhatched egg contents are unknown. Similarly, few data exist to evaluate the influence of time elapsed after oviposition (as a proxy for decomposition) on isotopic variability among unhatched eggs within the same nest. METHODS: Skin and unhatched egg contents were collected from 69 adult female green turtles and associated nests at the Archie Carr National Wildlife Refuge in Florida, USA. Values of δ13 C, δ15 N, and δ34 S were measured for both tissue types using a continuous flow isotope ratio mass spectrometer. Standardized major-axis (SMA) regression was used to generate conversion equations of carbon, nitrogen, and sulfur isotope ratios between the two tissue types. Model selection frameworks consisting of single-factor linear models were employed per isotope ratio to assess how egg time-in-nest affected intraclutch isotopic variability. RESULTS: Conversion equations for all three isotope ratios indicated significant relationships between skin and unhatched egg values, although model fits were lower than found in some studies examining similar patterns in other marine turtle species. The probability of increased intraclutch variability was significantly higher among eggs collected at longer intervals after deposition. CONCLUSIONS: This study reports the first-ever δ13 C and δ15 N conversion equations between skin and unhatched eggs for green turtles, and the first δ34 S conversion equation for any marine turtle species. SMA regression was used to directly convert tissue values bidirectionally, unlike equations generated using ordinary least-squares regression. Issues with increased intraclutch variability at later excavation dates highlight the importance of collecting unhatched eggs as soon as possible after hatchling emergence.

Foraging and recruitment hotspot dynamics for the largest Atlantic loggerhead turtle rookery.

Determining patterns of migratory connectivity for highly-mobile, wide-ranging species, such as sea turtles, is challenging. Here, we combined satellite telemetry and stable isotope analysis to estimate foraging locations for 749 individual loggerheads nesting along the east central Florida (USA) coast, the largest rookery for the Northwest Atlantic population. We aggregated individual results by year, identified seven foraging hotspots and tracked these summaries to describe the dynamics of inter-annual contributions of these geographic areas to this rookery over a nine-year period. Using reproductive information for a subset of turtles (n = 513), we estimated hatchling yields associated with each hotspots. We found considerable inter-annual variability in the relative contribution of foraging areas to the nesting adults. Also reproductive success differed among foraging hotspots; females using southern foraging areas laid nests that produced more offspring in all but one year of the study. These analyses identified two high priority areas for future research and conservation efforts: the continental shelf adjacent to east central Florida and the Great Bahama Bank, which support higher numbers of foraging females that provide higher rates of hatchling production. The implementation of the continuous-surface approach to determine geographic origins of unknown migrants is applicable to other migratory species.

Conservation hotspots for marine turtle nesting in the United States based on coastal development.

Coastal areas provide nesting habitat for marine turtles that is critical for the persistence of their populations. However, many coastal areas are highly affected by coastal development, which affects the reproductive success of marine turtles. Knowing the extent to which nesting areas are exposed to these threats is essential to guide management initiatives. This information is particularly important for coastal areas with both high nesting density and dense human development, a combination that is common in the United States. We assessed the extent to which nesting areas of the loggerhead (Caretta caretta), the green (Chelonia mydas), the Kemp's ridley (Lepidochelys kempii), and leatherback turtles (Dermochelys coriacea) in the continental United States are exposed to coastal development and identified conservation hotspots that currently have high reproductive importance and either face high exposure to coastal development (needing intervention), or have low exposure to coastal development, and are good candidates for continued and future protection. Night-time light, housing, and population density were used as proxies for coastal development and human disturbance. About 81.6% of nesting areas were exposed to housing and human population, and 97.8% were exposed to light pollution. Further, most (>65%) of the very high- and high-density nesting areas for each species/subpopulation, except for the Kemp's ridley, were exposed to coastal development. Forty-nine nesting sites were selected as conservation hotspots; of those high-density nesting sites, 49% were sites with no/low exposure to coastal development and the other 51% were exposed to high-density coastal development. Conservation strategies need to account for ~66.8% of all marine turtle nesting areas being on private land and for nesting sites being exposed to large numbers of seasonal residents.

Inferring foraging areas of nesting loggerhead turtles using satellite telemetry and stable isotopes.

In recent years, the use of intrinsic markers such as stable isotopes to link breeding and foraging grounds of migratory species has increased. Nevertheless, several assumptions still must be tested to interpret isotopic patterns found in the marine realm. We used a combination of satellite telemetry and stable isotope analysis to (i) identify key foraging grounds used by female loggerheads nesting in Florida and (ii) examine the relationship between stable isotope ratios and post-nesting migration destinations. We collected tissue samples for stable isotope analysis from 14 females equipped with satellite tags and an additional 57 untracked nesting females. Telemetry identified three post-nesting migratory pathways and associated non-breeding foraging grounds: (1) a seasonal continental shelf-constrained migratory pattern along the northeast U.S. coastline, (2) a non-breeding residency in southern foraging areas and (3) a residency in the waters adjacent to the breeding area. Isotopic variability in both δ(13)C and δ(15)N among individuals allowed identification of three distinct foraging aggregations. We used discriminant function analysis to examine how well δ(13)C and δ(15)N predict female post-nesting migration destination. The discriminant analysis classified correctly the foraging ground used for all but one individual and was used to predict putative feeding areas of untracked turtles. We provide the first documentation that the continental shelf of the Mid- and South Atlantic Bights are prime foraging areas for a large number (61%) of adult female loggerheads from the largest loggerhead nesting population in the western hemisphere and the second largest in the world. Our findings offer insights for future management efforts and suggest that this technique can be used to infer foraging strategies and residence areas in lieu of more expensive satellite telemetry, enabling sample sizes that are more representative at the population level.

Comparative morphology and sex identification of the reproductive system in formalin-preserved sea turtle specimens.

Sex identification in young sea turtles is challenging. Sea turtle neonates lack external dimorphic characteristics and heteromorphic sex chromosomes. We compared the morphology of the gonads and reproductive ducts of dead formalin-preserved hatchling and post-hatchling Caretta caretta, Dermochelys coriacea, and Chelonia mydas and identified sex-specific differences in these structures that are useful in assigning sex. We tested 11 gross gonadal and reproductive duct characteristics in 57 neonate sea turtles and verified the sex by histological examination. A suite of four characters was found to reliably indicate sex in the three species considered: paramesonephric duct size, mobility of the duct, presence of a complete lumen and gonad mobility. Additionally, gonad shape and edge form were dependable sex-specific characters in cheloniids but not in D. coriacea. Together, these morphological characteristics provide new and reliable methods to quickly distinguish sex in preserved neonate sea turtles without using more extensive histological methods.