First Atlantic satellite tracks of 'lost years' green turtles support the importance of the Sargasso Sea as a sea turtle nursery

Daily, seasonal, and long-distance movements inferred from Fastloc-GPS telemetry of immature green turtles (Chelonia mydas) at a high-latitude, mid-ocean developmental site

To characterize the movements and habitat use of juvenile green turtles (Chelonia mydas) in benthic developmental habitat, we deployed Fastloc-GPS-enabled satellite transmitters on 16 individuals captured as part of a multi-decade study of green turtles on the Bermuda Platform. We characterized residence areas, distinct use areas within them, and seasonal movements based on an average of 562 Fastloc-GPS positions and 284 tracking days per turtle. We estimated residence area sizes using traditional home range methods, e.g., 90% utilization distribution (UD) (mean 2.29 ±2.71 km2) and 50% UD (mean 0.54 ±0.69 km2). Total residence area size increased significantly over the 8-year study, from <1 km2 before 2013 to ≥3 km2 in 2018 (R2 = 0.51, F1,14 = 14.55, p = 0.0019), corresponding to a period of decline in seagrass habitat and suggesting increased foraging effort. We identified three types of distinct use areas within residence areas where tracked turtles typically exhibited behavioral fidelity: foraging, resting, and cool weather refugia. These distinct use areas were smaller than high-use areas from previous studies; e.g., seagrass meadow foraging areas averaged 0.05 km2. Most turtles made daily transits between foraging and resting sites; for some individuals, these involved crossing frequently used vessel navigation channels. Seasonal variation in behavior suggested that the overwintering strategy for green turtles on the Bermuda Platform involves "optional dormancy," during which turtles spent less time on seagrass meadows and made brief excursions to distinct deeper habitats. Four individuals made directed (mean path straightness = 0.93 ±0.02 SD) developmental migrations away from Bermuda toward known adult foraging range. Results of our study further knowledge of the green turtle life cycle at a high-latitude site; they demonstrate that green turtles show fidelity to distinct use areas within developmental habitats over many years and exhibit seasonal movements.

Physiological differences between wild and captive animals: a century-old dilemma

Laboratory-based research dominates the fields of comparative physiology and biomechanics. The power of lab work has long been recognized by experimental biologists. For example, in 1932, Georgy Gause published an influential paper in Journal of Experimental Biology describing a series of clever lab experiments that provided the first empirical test of competitive exclusion theory, laying the foundation for a field that remains active today. At the time, Gause wrestled with the dilemma of conducting experiments in the lab or the field, ultimately deciding that progress could be best achieved by taking advantage of the high level of control offered by lab experiments. However, physiological experiments often yield different, and even contradictory, results when conducted in lab versus field settings. This is especially concerning in the Anthropocene, as standard laboratory techniques are increasingly relied upon to predict how wild animals will respond to environmental disturbances to inform decisions in conservation and management. In this Commentary, we discuss several hypothesized mechanisms that could explain disparities between experimental biology in the lab and in the field. We propose strategies for understanding why these differences occur and how we can use these results to improve our understanding of the physiology of wild animals. Nearly a century beyond Gause's work, we still know remarkably little about what makes captive animals different from wild ones. Discovering these mechanisms should be an important goal for experimental biologists in the future.

Holopelagic Sargassum aggregations provide warmer microhabitats for associated fauna

Drifting aggregations of Sargassum algae provide critical habitat for endemic, endangered, and commercially important species. They may also provide favorable microclimates for associated fauna. To quantify thermal characteristics of holopelagic Sargassum aggregations, we evaluated thermal profiles of 50 aggregations in situ in the Sargasso Sea. Sea surface temperature (SST) in the center of aggregations was significantly higher than in nearby open water, and SST differential was independent of aggregation volume, area, and thickness. SST differential between aggregation edge and open water was smaller than those between aggregation center and aggregation edge and between aggregation center and open water. Water temperature was significantly higher inside and below aggregations compared to open water but did not vary inside aggregations with depth. Holopelagic Sargassum aggregations provide warmer microhabitats for associated fauna, which may benefit marine ectotherms, though temperature differentials were narrow (up to 0.7 °C) over the range of aggregation sizes we encountered (area 0.01-15 m2). We propose a hypothetical curve describing variation in SST differential with Sargassum aggregation size as a prediction for future studies to evaluate across temporal and geographic ranges. Our study provides a foundation for investigating the importance of thermal microhabitats in holopelagic Sargassum ecosystems.

Satellite Tracking of Head-Started Juvenile Green Turtles (Chelonia mydas) Reveals Release Effects and an Ontogenetic Shift

Juveniles of marine species, such as sea turtles, are often understudied in movement ecology. To determine dispersal patterns and release effects, we released 40 satellite-tagged juvenile head-started green turtles (Chelonia mydas, 1–4 years) from two separate locations (January and July 2023) off the coast of the Cayman Islands. A statistical model and vector plots were used to determine drivers of turtle directional swimming persistence and the role of ocean current direction. More than half (N = 22) effectively dispersed in 6–22 days from the islands to surrounding areas. The January turtles radiated out (185–1138 km) in distinct directions in contrast to the northward dispersal of the July turtles (27–396 km). Statistical results and vector plots supported that daily swimming persistence increased towards the end of tracks and near coastal regions, with turtles largely swimming in opposition to ocean currents. These results demonstrate that captive-reared juvenile greens have the ability to successfully navigate towards key coastal developmental habitats. Differences in dispersal (January vs. July) further support the importance of release timing and location. Our results inform conservation of the recovering Caymanian green turtles and we advise on how our methods can be improved and modified for future sea turtle and juvenile movement ecology studies.

Marina Observation of Sea Turtles: Establishing a Database of Intracoastal Waterway Green Sea Turtles in Northeast Florida

As conservation efforts regarding green sea turtles, Chelonia mydas, continue, it is imperative to document behaviors and foraging habits/habitats of understudied populations. We have conducted an 18-month study dedicated to photographing the local population feeding alongside floating docks within the Guana Tolomato Matanzas estuary to determine the capability of matching head scale patterns efficiently through a pattern matching program: HotSpotter. To date, 195 unique sea turtles have been identified between two different marinas located in St. Augustine, FL. Of these, 98 were spotted more than once, with 39 of them being "tracked" for longer than a year. Temperature trends were also monitored in conjunction, showing that more individuals appeared during the warmer months of the year. The evidence, overall, indicates that these locations host a resident population of green sea turtles, leading to the need for a discussion on potential threats originating from the usage of these marinas by humans.

Incorporating distance metrics and temporal trends to refine mixed stock analysis

The distribution of marine organisms is shaped by geographic distance and oceanographic features like currents. Among migratory species, individuals from multiple populations may share feeding habitats seasonally or across life stages. Here, we introduce a modification for many-to-many mixed stock models to include distance between breeding and foraging sites as an ecological covariate and evaluate how the composition of green turtle, Chelonia mydas, juvenile mixed stock aggregations changed in response to population growth over time. Our modified many-to-many model is more informative and generally tightens credible intervals over models that do not incorporate distance. Moreover, we identified a decrease in genetic diversity in a Florida nesting site and two juvenile aggregations. Mixed stock aggregations in central Florida have changed from multiple sources to fewer dominant source populations over the past ~ 20 years. We demonstrate that shifts in contributions from source populations to mixed stock aggregations are likely associated with nesting population growth. Furthermore, our results highlight the importance of long-term monitoring and the need for periodical reassessment of reproductive populations and juvenile aggregations. Understanding how mixed stock aggregations change over time and how different life stages are connected is fundamental for the development of successful conservation plans for imperiled species.

Genetic variation among sea turtle life stages and species suggests connectivity among ocean basins

Regional genetic differentiation of mitochondrial lineages occurs in migratory species with natal philopatry such as sea turtles. However, early juvenile dispersal represents a key opportunity for gene flow and colonization of new regions through founder events, making it an important yet under-studied life stage. To assess connectivity among sea turtle life stages and ocean basins, we sequenced mitochondrial DNA (mtDNA) fragments from 35 juveniles sampled in the Gulf of Mexico from the rarely observed dispersal stage across three species: green turtles (Chelonia mydas; n = 30), hawksbills (Eretmochelys imbricata; n = 3), and loggerheads (Caretta caretta; n = 2). We estimated green turtle rookery contributions using a many-to-many Bayesian mixed stock analysis that incorporated dispersal probabilities based on rookery size and transport via ocean currents. We assembled a gene tree including 709 distinct mtDNA control region haplotypes from the literature for all seven extant sea turtle species to assess gaps in life-stage data across ocean basins, as well as contextualize the lineages we sampled from dispersing juveniles. Our results indicate a high likelihood that green turtles sampled in the Gulf of Mexico originated from rookeries along the coast of Mexico, with smaller contributions from Costa Rica and Suriname. The gene tree analysis yielded species-level relationships consistent with those presented previously, while intra-species relationships between lineages and ocean basins differed, particularly within loggerhead and green turtle clades. Our results highlight the lack of genetic data from juvenile sea turtles, especially the early dispersal stage, and the potential for these data to answer broader questions of connectivity and diversification across species and lineages.

Tracking movements and growth of post-hatchling to adult hawksbill sea turtles using skeleto+iso

In the eastern Pacific Ocean, hawksbill sea turtles (Eretmochelys imbricata) are adapted to use coastal habitats and ecosystems uncharacteristic of most other sea turtles. Once considered extirpated from this region, hawksbills had sought refuge in estuaries, nesting on muddy banks among the tangles of mangrove roots. This population is at high risk of bycatch during fishing efforts in the estuaries (blast fishing) and adjacent coastal rocky reefs (gillnets), and is further impacted by habitat degradation from coastal development and climate change. The conservation and population recovery of hawksbills in this region is highly dependent on management actions (e.g., nest relocation, habitat protection, bycatch mitigation), and a better understanding of how hawksbills use and move between distinct habitats will help prioritize conservation efforts. To identify multi-year habitat use and movement patterns, we used stable carbon (δ13C) and nitrogen (δ15N) isotope analysis of skin and bone growth layers to recreate movements between two isotopically distinct habitats, a nearshore rocky reef and a mangrove estuary, the latter distinguishable by low δ13C and δ15N values characteristic of a mangrove-based foodweb. We applied skeletochronology with sequential δ13C and δ15N analysis of annual growth layers, “skeleto+iso,” to a dataset of 70 hawksbill humeri collected from coastal El Salvador. The results revealed at least two unique habitat-use patterns. All turtles, regardless of stranding location, spent time outside of the mangrove estuaries during their early juvenile years (&lt; 35 cm curved carapace length, CCL, age 0–5), showing that an oceanic juvenile stage is likely for this population. Juveniles ca. &gt; 35 cm then began to recruit to nearshore areas, but showed divergent habitat-use as some of turtles occupied the coastal rocky reefs, while others settled into the mangrove estuaries. For turtles recruiting to the estuaries, settlement age and size ranged from 3 to 13 years and 35–65 cm CCL. For the adult turtles, age-at-sexual-maturity ranged from 16 to 26 years, and the maximum reproductive longevity observed was 33 years. The skeleto+iso also showed that adult hawksbills have long-term habitat fidelity, and the results demonstrate the importance of both mangrove estuary and nearshore rocky reefs to the conservation of hawksbills in the eastern Pacific.

Movement Patterns of Juvenile Loggerhead Turtles (Caretta caretta L. 1758) and Green Turtles (Chelonia mydas L. 1758) Hatched in Captivity and Released in the Korean Waters

With most sea turtle populations declining, activities to conserve their habitat and nesting grounds and restore their populations are being implemented worldwide. To preserve the Northwestern Pacific populations, the National Marine Biodiversity Institute of Korea has been releasing artificially propagated sea turtles, but whether these individuals join the wild population remains unknown. The present study aimed to determine the movement patterns of artificially propagated juvenile loggerhead (Caretta caretta) and green (Chelonia mydas) turtles fitted with satellite transmitters on their carapaces and released in the waters of Jeju or Yeosu, Republic of Korea, between August 2018 and April 2022. Loggerheads traveled northward to the East Sea, whereas green turtles moved west or southwest. Two 36-month-old and two 48-month-old loggerheads moved toward their potential nursery grounds and toward their feeding grounds, respectively. Three green turtles with a curved carapace length (CCL) of <40 cm moved toward their nursery or feeding grounds, while three individuals (CCL > 45 cm) moved toward their inshore foraging areas. The travel paths were closely related to the direction of local sea currents. Our results implied that releasing artificially propagated sea turtles, considering their age and CCL, can positively contribute to the conservation of Northwestern Pacific populations.

Age-specific growth and maturity estimates for the flatback sea turtle (Natator depressus) by skeletochronology

To address a major knowledge gap for flatback sea turtles (Natator depressus), a species endemic to Australia and considered ‘Data Deficient’ for IUCN Red List assessment, we present the first-ever skeletochronology-derived age and growth rate estimates for this species. Using a rare collection of bone samples gathered from across northern Australia, we applied skeletochronology and characterized the length-at-age relationship, established baseline growth rates from the hatchling to adult life stages, and produced empirical estimates of age-at- and size-at-sexual-maturation (ASM, SSM). We analyzed humeri from 74 flatback sea turtles ranging in body size from 6.0–96.0 cm curved carapace length (CCL), and recovered from Western Australia (n = 48), Eastern Australia (n = 13), central Australia (n = 8; Northern Territory n = 3, the Gulf of Carpentaria n = 5), and unknown locations (n = 5). We identified the onset of sexual maturity for 29 turtles, based on rapprochement growth patterns in the bones. Estimates for ASM ranged from 12.0 to 23.0 years (mean: 16.3 ± 0.53 SE), SSM ranged from 76.1 to 94.0 cm CCL (mean: 84.9 ± 0.90 SE), and maximum observed reproductive longevity was 31 years for a 45-year old male flatback. Growth was modeled as a smoothing spline fit to the size-at-age relationship and at the mean SSM (84.9 cm CCL) corresponded with a spline-predicted maturity age of 18 years (95% CI: 16 to 24), while mean nesting sizes reported in the literature (86.4 to 94 cm CCL) corresponded to estimated ages of 24+ years. A bootstrapped von Bertalanffy growth model was also applied and showed consistencies with the spline curve, yielding an estimated upper size limit, L_inf, at 89.2 ± 0.04 cm (95% CI: 85.5 to 95.9 cm) with the intrinsic growth rate parameter, k, at 0.185 ± 0.0004 (0.16 to 0.22); at the same mean SSM (84.9 cm CCL) the estimated ASM was 16.3 ± 0.05 years (95% CI: 12.8 to 27.7 years). Lastly, four of the samples analyzed were collected from deceased adult females that had previous sizes known from on-going mark/recapture studies at nesting sites in Western Australia. The paired CCL data (measured at nesting and back-calculated) did not significantly differ (p = 0.875). This first skeletochronology study for flatback sea turtles generates valuable empirical estimates for ongoing conservation and management efforts.

Litter ingestion and entanglement in green turtles: An analysis of two decades of stranding events in the NE Atlantic

Survivorship of early life stages is key for the well-being of sea turtle populations, yet studies on animals that distribute around oceanic areas are very challenging. So far, the information on green turtles (Chelonia mydas) that use the open NE Atlantic as feeding grounds is scarce. Strandings occurring in oceanic archipelagos can provide relevant information about the biology, ecology and current anthropogenic pressures for megafauna inhabiting the open ocean. In this study, we analysed stranding events of green turtles found in the Azores archipelago to investigate interactions with marine litter. In addition, we quantified and characterized litter items stranded on beaches to provide a direct comparison between the ingested items with the debris found in the environment. A total of 21 juvenile green turtles were found stranded in the region between 2000 and 2020 (size range: 12-49 cm, CCL). Overall, 14% of the animals were entangled in marine litter and 86% of the turtles necropsied had ingested plastic. The mean abundance of items ingested was 27.86 ± 23.40 and 98% were white/transparent. Hard plastic fragments between 1 and 25 mm were the most common shape recovered in the turtles, similarly to what was found on the coastline. All of the litter items analysed with pyrolysis GC-MS revealed to be polyethylene (PE). This study provides the first baseline assessment of interactions of plastic litter with juvenile green turtles found at the east edge of the North Atlantic Subtropical Gyre. The combination of these results supports the hypothesis that migratory megafauna that use remote oceanic islands as a feeding ground are exposed to anthropogenic litter contamination dominated by plastics, even when these regions are located far away from big industrial centers or populated cities.

First Atlantic satellite tracks of 'lost years' green turtles support the importance of the Sargasso Sea as a sea turtle nursery

In-water behaviour and long-term movements of oceanic-stage juvenile sea turtles are not well described or quantified. This is owing to technological or logistical limitations of tracking small, fast-growing animals across long distances and time periods within marine habitats. Here, we present, to our knowledge, the first long-term offshore tracks of oceanic green turtles (Chelonia mydas) in western North Atlantic waters. Using a tag attachment technique developed specifically for young (less than 1 year old) green turtles, we satellite-tracked 21 oceanic-stage green turtles (less than 19 cm straight carapace length) up to 152 days using small, solar-powered transmitters. We verify that oceanic-stage green turtles: (i) travel to and remain within oceanic waters; (ii) often depart the Gulf Stream and North Atlantic Subtropical Gyre currents, orienting towards waters associated with the Sargasso Sea; (iii) remain at the sea surface, using thermally beneficial habitats that promote growth and survival of young turtles; and (iv) green turtles orient differently compared to same stage loggerhead turtles (Caretta caretta). Combined with satellite tracks of oceanic-stage loggerhead turtles, our work identifies the Sargasso Sea as an important nursery habitat for North Atlantic sea turtles, supporting a growing body of research that suggests oceanic-stage sea turtles are behaviourally more complex than previously assumed.