ISTS Symposium43 Program/Agenda

Until recently, scientists had no idea how loggerhead sea turtles (Caretta caretta) migrate from their nesting beaches in Japan to nursery grounds in Baja California, Mexico. The Thermal Corridor Hypothesis (TCH, Briscoe et al. 2021) combined 16 years of satellite-tracked movement data to propose an intermittent thermal corridor that allows juvenile loggerheads to transition from the Central North Pacific (CNP) to the west coast of North America. The TCH proposes that this migratory corridor opens during anomalously warm conditions and closes during cool conditions, causing turtles to stay in the CNP. Here we report on the deployment of two experimental cohorts, one in 2023, during warm El Niño conditions, and one on 2024, during the onset of cooler La Niña conditions. In 2023, all loggerheads experienced warmer than average SST, due to a marine heatwave and an El Niño. The entire cohort moved north until September 2023, and then they moved south, with 7 of 23 turtles moving towards North America — 3 of which entered coastal waters of Southern California and Baja Mexico. These responses confirmed the TCH that under warm conditions, loggerheads can pass through the thermal corridor. In 2024, the experimental cohort was deployed with a projected onset of La Niña into the fall. Initially, nearly 2/3rds of the turtles headed east. It is possible that strong Ekman transport and/or warm SST anomalies, in response to especially strong westerlies, may have driven these eastward movements, leading to the loggerheads entering the California Current further north than is typical. Late fall movements tended to shift southward again, but the cold California current may prevent reaching the coastline. Distributional shifts due to changing ocean conditions will allow us to dynamically manage and protect this species. If a corridor were to open more frequently or in a changing ocean location, it could alter abundances and risks for these endangered turtles.

Sea turtles are highly migratory, occupying breeding and foraging areas often hundreds to thousands of kilometers apart. Their breeding phenology is influenced by body condition and environmental cues at foraging and breeding grounds. While breeding typically follows a defined seasonal pattern, some locations experience year-round nesting or dual breeding seasons. These aspects make sea turtle conservation challenging if threats and management vary across their range or if research and conservation efforts are limited to a too-narrow time frame. Understanding sea turtle spatiotemporal distribution across the geographic areas they use is thus essential for conservation and ecological insight.

Poilão Island, part of the Bijagós Archipelago in Guinea-Bissau, West Africa, hosts one of the world’s largest green turtle (Chelonia mydas) nesting populations, with over 15,000 nesting females. The primary nesting season spans June to November, peaking in August-September during the rainy season, but nesting occurs year-round.

Tracking data from 2018 to 2021 for both adult females (n = 44) and males (n = 9) that breed during the main season revealed the primary foraging grounds for this population. Most post-breeding females migrate 1,000 km north to Mauritania's Banc d’Arguin, with about 25% traveling 300-400 km north to Joal Fadiouth, the Saloum Delta, or Bijol Islands in Senegal and The Gambia. The remaining 25% stay within 40-90 km of the nesting site, at the Bijagós Archipelago. Males generally use similar foraging areas but tend to stay closer to the breeding site.

To expand our understanding of this population’s range, we deployed satellite tags in 2023 on breeding females (n = 9) nesting during the dry season at Poilão (January - March). All but one turtle migrated about 2,000 km south to forage offshore near Accra, Ghana, revealing a new migratory pattern for Poilão’s population linked to nesting phenology.

Our findings suggest that two subpopulations of green turtles nest at Poilão Island, during contrasting seasons, and reinforce the importance of sampling outside the main breeding season for a better understanding of spatiotemporal distribution of sea turtles. This seasonal dichotomy may hold particular significance in the context of climate change. Cooler sand temperatures during the dry season at Poilão could enhance egg incubation and hatchling survival in the near future, while in recent years, intense rainfall and tidal surges during the rainy season have led to high clutch mortality. Should turtles nesting in the dry season gain a fitness advantage under these changing conditions, this could drive phenological adaptation at Poilão over evolutionary timescales, with possible shifts in main foraging locations.

The Sea Turtle Conservancy has been using satellite telemetry to study the post-nesting migration of female sea turtles since 2000. Over 25 years, we have deployed 240 transmitters on loggerhead (Caretta caretta), green (Chelonia mydas), hawksbill (Eretmochelys imbricata) and leatherback (Dermochelys coriacea) turtles at eight nesting beaches (Archie Carr National Wildlife Refuge (ACNWR) and Anna Maria Island, Florida USA; Nevis, St. Kitts & Nevis; Playa del Carmen, Mexico; Tortuguero and Jalova, Costa Rica; Chiriquí and Soropta Beaches, Panama). This data set allows us to answer additional questions related to sea turtle behavior and movement, specifically the inter-nesting period, both before migration to a foraging area (n = 159) and tracked returning to the nesting beach for a second nesting season (n = 6). Of the 240 turtles satellite tracked, 160 provided at least 10 days of inter-nesting data. This includes 42 loggerheads, 28 hawksbills, 49 greens and 41 leatherbacks. We used daily locations to determine a kernel density estimate (KDE) for individual core areas (50% contour), plus combined core and resident areas (95% contour) for all turtles from the same nesting beach (grouped by species for ACNWR and Tortuguero). When turtles were tracked over multiple nesting seasons, we calculated separate KDE for each nesting season. The centroid was calculated for individual core areas. For each centroid, the depth (m) and distance to the nearest shoreline (km) were calculated. Inter-nesting period ranged from 10 to 131 days (mean 39 + 23 days). The shape of combined inter-nesting areas reflected the influence of the bathymetry adjacent to the nesting beach, as well as the roaming nature of leatherbacks. For loggerheads, greens and hawksbills, core inter-nesting areas were constrained to the continental shelf (< 200 m). Combined core inter-nesting area size ranged from 20.8 km² for greens nesting in Playa del Carmen, Mexico to 3,759.5 km² for leatherbacks nesting in Chiriquí Beach, Panama. Both core and residential inter-nesting areas overlapped between hawksbill and green turtles from Tortuguero, with hawksbills having larger core and residential areas and centroids farther from shore than greens. A similar trend was seen for turtles from the ACNWR, with loggerheads having larger core and residential areas with centroids being farther from shore compared to green turtles. This large dataset allows for a comparison of inter-nesting movements and habitat use among different species of sea turtles, as well as among different nesting beaches. Interestingly, we observed many similarities in the inter-nesting movement patterns and habitat use across sea turtle species suggesting that local conditions, such as bathymetry and ocean currents, play a key role. Turtle movements during the nesting season should be an important consideration for resource managers when looking to establish critical habitat areas, considering slow boat zones in areas with high overlap between boats and sea turtles, and where to focus in-water anti-poaching efforts.

Pacific leatherback turtles (Dermochelys coriacea), are federally listed as endangered under the United States Endangered Species Act (ESA) and are recognized as being under threat of extirpation within the Pacific Ocean. Leatherbacks that nest at beaches in the tropical western Pacific migrate across the Pacific to forage on seasonally abundant sea nettles, Chrysaora fuscescens, in two known areas off the U.S. West Coast: central California and Oregon-Washington (OR-WA). Both areas are designated as Leatherback Critical Habitat (77 FR 4169, 27 February 2012). Since 2000, integrated aerial survey, satellite telemetry, and in-water sampling have been successfully conducted off central California to characterize leatherback distribution, movements, abundance, habitat use, foraging behavior, and health. More limited information on leatherback occurrence is available off OR-WA, and no estimate of leatherback abundance is available for this region. Here we present an overview of the methodologies and results from over twenty-five years of leatherback monitoring research efforts off the US West Coast, including 1) replicated aerial surveys to document distribution and estimate abundance via line transect methodology; 2) satellite and acoustic telemetry to identify movements following at sea capture of leatherbacks using a specially designed leatherback capture vessel, with plane support to guide the boat to surfacing leatherbacks; and 3) suction-cup attached VHF/camera tags with time-depth recorders for fine-scale foraging and behavior studies.

The loggerhead sea turtle (Caretta caretta) is the most abundant species of sea turtle in the Mediterranean Sea, with major nesting grounds located in the eastern basin. In recent years, this species has expanded the nesting range into the Western Mediterranean Sea. The Cilento National Park, South-west Italy, has become a hotspot for these emerging nesting areas, offering an opportunity to study this colonisation event.

While increasing number of female sea turtles return each year to nest along the western Italian coasts, their foraging and wintering grounds remain largely unknown. What habitats do they utilise and how can their post-nesting movement strategies be characterised in relation to oceanographic features?

To address these questions, eight females were satellite-tagged after they had nested in the Cilento National Park between 2020 and 2024. At the time of abstract writing, our analysis included data collected from 2020 to 2023 (three turtles), as tracking for later periods was still ongoing. Once complete, the full dataset from all eight turtles will be analysed to improve the study’s robustness.

Location data were filtered and interpolated at 3-hour intervals using a continuous-time correlated random walk state space model. Foraging and travelling behaviours were classified using a Hidden Markov Model (HMM) via the “momentuHMM” R package. A move persistence index was calculated with the “aniMotum” R package and correlated with three environmental variables (Chlorophyll a, sea surface height, and current intensity). Turtles’ movement directions were also analysed relative to current flow. Finally, a General Additive Model was applied to assess the correlation between turtle movement efforts and current intensity.

Our results show that six turtles foraged in the pelagic waters of the Western Mediterranean, while one foraged on the Tunisian shelf, and another in the neritic waters along Sardinia’s northeastern coast. Preliminary findings suggest a positive relationship between current intensity and movement persistence (Z = 5.03, P < 0.001), with turtles preferring low-intensity currents for foraging as supported by previous studies. Low-current intensity likely enhances manoeuvrability and prey capture. Conversely, during travel, turtles selected high-intensity currents and optimized energy efficiency by moving perpendicular or aligned with the current. Current intensity was significantly negatively correlated with turtle movement effort (P < 0.001), indicating that active swimming effort decreased in stronger currents, likely as turtles took advantage of currents to aid their movement. These results suggest behavioural adaptations to exploit oceanographic features, minimising energy costs while maximising movement efficiency.

This study reveals an unexpectedly high proportion of female turtles foraging in pelagic habitats in the Western Mediterranean, rather than joining established neritic foraging sites commonly used by turtles from major Mediterranean rookeries. It also presents preliminary evidence of adaptive behaviours in response to current dynamics among loggerheads nesting in southern Italy, shedding light on their habitat use and movement strategies during post-nesting migrations. These findings highlight the need for further research on habitat connectivity to help identify priority areas for effective conservation efforts.

This study investigates habitat utilization, site fidelity, and life stage transitions of Green and Loggerhead Turtles along Florida’s Atlantic coast. Leveraging a 47-year capture-recapture dataset, we apply Hidden Markov Models to classify green and loggerhead turtles into three latent behavioral states—Nomadic, Resident, and Transitory—based on size and temporal patterns in habitat use. These classifications offer insights into turtles' adaptive behaviors across life stages, illustrating how habitat use evolves in response to individual growth and environmental factors. Our findings reveal distinct behavioral trends for both species, with clear shifts between states that align with known ecological strategies, such as post-dispersal and migratory behaviors. Temporal analysis shows that while the Nomadic state in green turtles remains stable over time, the Resident and Transitory behavioral states of green turtles, as well as all three states for loggerheads, have shown increases in average size, particularly since the 1990s. These size increases likely reflect reduced mortality and the progression of individuals through life stages, balanced by shifts in habitat use. These changes align with two major conservation measures: the introduction of Turtle Excluder Devices in United States shrimp trawl fisheries and the state of Florida's gill net ban. These findings underscore the efficacy of conservation actions in promoting population stability and influencing habitat utilization patterns. However, this sustained increase in sizes, especially within Resident and Transitory stages, may reflect an imbalance between high survival and low recruitment rates of smaller individuals, suggesting that while bycatch reduction has positively impacted survival, juvenile recruitment remains constrained.