"Resolving Green Turtle Foraging Grounds and Migration Routes Across the Pacific Ocean Using Satellite Telemetry and Stable Isotope Analysis"
INTRODUCTION AND MOTIVATING RATIONALE
Green sea turtles (Chelonia mydas) play an integral ecological role in marine and coastal ecosystems. In 2016, the U.S. Endangered Species Act (ESA) listed the Central South Pacific (CSP) and the Central West Pacific (CWP) green turtle distinct population segments (DPS) as Endangered, the highest protection level afforded under the ESA framework, while the Central North Pacific (CNP) green turtle was listed as Threatened. The navigational feats of green turtles in this region involve migrating hundreds to thousands of kilometers across the vast open ocean between their feeding grounds to small island targets, where they mate and nest. Green turtles are susceptible to a variety of natural and anthropogenic threats found among foraging and reproductive habitats, making it important to protect not only green turtle nesting grounds, but also their foraging areas and migratory routes. Resolving nesting green turtle foraging hotspots and migration paths will be the first step towards identifying and addressing threats in these essential habitats. Foraging grounds are typically shared by both adults and advanced juveniles, and since survivorship is lower for juveniles than for adults, protection of foraging grounds also ensures the conservation of future nesting populations. Identification of green turtle foraging grounds is still in its infancy and remains incomplete for those that nest in the U.S. Pacific Islands Region (PIR), which includes American Sāmoa (AS; CSP DPS), the Hawaiian Archipelago (HA; CNP DPS), and the Mariana Archipelago (MA; CWP DPS). Approximately 100 green turtles nest in the Rose Atoll in AS5 and preliminary research suggests they migrate to foraging grounds throughout the CSP, especially in Fiji6 (PIFSC unpublished). About 200 green turtles reproduce each year at Lalo/French Frigate Shoals (FFS) Atoll in the HA and migrate to foraging grounds within the Main Hawaiian Islands (MHI). About 80 green turtles nest in the Commonwealth of the Northern Mariana Islands (CNMI; Saipan, Tinian, Rota) and Guam in the MA. These post-nesting turtles were recently documented foraging off islands in the CWP and the coasts of East Asia.
PROPOSED PROJECT AND OBJECTIVES
The aim of this project is to contemporaneously use satellite telemetry and stable isotope analysis (SIA) to identify and characterize key foraging areas and migration routes used by green turtles that nest in the PIR. Two objectives that will guide this research include: (1) determining green turtle foraging hotspots and migration paths and (2) validating the SIA method for locating the associated feeding areas. Isotopic values of food are registered in an animal’s body tissue as nutrients accumulate from the diet over a long period of time and thus stable isotope values of animal tissue are indicative of its diet and the ecosystem in which it forages. For instance, the stable carbon (δ 13C) and nitrogen (δ 15N) isotopes can elucidate the origins of dietary nutrients and the trophic level of the consumer, respectively. Therefore, stable isotope values can reveal an animal’s previous location as it travels through spatially and isotopically distinct food webs. SIA has become an accurate, cost-effective method for studying elusive animals (e.g. sea turtles). Although satellite telemetry offers more precision in locating the feeding habitat of post-nesting turtles, it is expensive at $3,000 to $5,000 per satellite tag. This high cost generally produces information for only a small subset of the population. The affordability of SIA ($11 per sample for bulk SIA) allows for a larger sample size that is more representative at the population level, which can resolve foraging hotspots used by most of these post-nesting green turtles. SIA paired with telemetry data has emerged as an integrative approach to determine foraging hotspots of sea turtles, and the latter can be used to resolve migration routes. Together, these tools can help determine priority locations for conservation efforts for threatened and endangered green turtles.
RESEARCH QUESTIONS AND HYPOTHESES
Q1: What are the migratory paths and foraging areas used by PIR nesting green turtles?
H1: AS nesting green turtles will travel to and feed in several islands in the CWP, especially Fiji, and will use a consistent migration pathway for all destinations. HA nesters will migrate directly back to the MHI to feed. As revealed via recent telemetry data, MA nesting green turtles exhibit a unique pattern of foraging in the coasts of Japan, Taiwan, and the Philippines (PIFSC unpublished).
Q2: Can stable isotope analysis provide sufficient resolution to assign nesting green turtles to foraging areas throughout the Pacific?
H2: Isotopic signatures for AS, HA, and MA nesting green turtles will distinguish those that forage in the CSP, CNP, and CWP, respectively. Satellite telemetry data will validate isotopic differences for green turtles foraging throughout the South Pacific Ocean, the MHI, and eastern Asia, respectively. In this way, SIA can be validated as an accurate and efficient tool to confirm green turtle feeding areas.
I will work with the National Oceanic and Atmospheric Administration (NOAA) Fisheries’ Marine Turtle Biology and Assessment Program (MTBAP) to execute this study. Skin samples of non-tracked nesting turtles have been consistently collected for green turtles that nest in AS (since 2012), the HA (since the 1970s), and the MA (since 2004). These specimens include ≥30 AS, ≥800 HA, and ≥60 MA non-tracked females. Currently, 39 AS, 32 HA, and 26 MA post nesting migration routes and corresponding skin specimens have been collected from recently satellite-tagged females for preliminary tracking studies. Overall, ≥1,000 skin samples were obtained and archived at the Pacific Islands Fisheries Science Center (PIFSC) with nearly 100 belonging to tracked females and ~900 to non-tracked females.
Satellite Tag Data Processing. I will apply an analytical R toolkit currently being developed at PIFSC to create tracking visuals of archived telemetry data and determine the home range estimates of tracked nesting females. The precision of GPS coordinates from satellite telemetry will help infer green turtle migration paths and foraging habitats and validate SIA accuracy.
Sample Processing. Bulk SIA of δ13C and δ 15N of skin tissue collected for each tracked and non-tracked nesting green turtle will be conducted to compare potential differences in isotopic signatures between foraging areas. To confirm the accuracy of SIA for these populations, satellite telemetry data will be used to ground truth isotopic values for tracked turtles. Isotope values from tracked nesters will then be compared to the larger cohort of non-tracked turtles (with skin specimens) and used to quantify the number of nesters that utilize specific feeding areas and infer which are foraging hotspots. An amino acid compound-specific isotope analysis (AA-CSIA) will be employed if isotope values do not show differences between turtle habitats. If needed, an AA-CSIA of either δ 13C or δ 15N will provide a finer scale resolution that can substantially enhance our isotopic measurements.
Documenting Resights of Non-Tracked Turtles at Foraging Grounds. I will use location data provided by Citizen Scientists that reported resightings of post-nesting HA green turtles. The public is asked to take a photo of the individual turtle with its white, alphanumeric identifier on its shell and report the location details to NOAA Fisheries. These citizen science data will complement and confirm foraging hotspots that are identified with tracking data and provide additional validation on the resolution of the SIA method where telemetry data is not paired.
Sample Collection, Tag Application, and Permitting. I will increase the current sample size of both non-tracked and tracked nesters from the MA population by collecting skin tissue and applying satellite tags to nesting females encountered during night surveys around the peak of Guam’s 2022 and 2023 nesting seasons (May through August16). After a nester has deposited her eggs, superficial tissue (~6 mm diameter) will be obtained and preserved in a saturated salt solution. I will utilize safe capture, handling, and tagging methods outlined in the "Standard Research Protocols for Nesting and Basking Marine Turtles in the Pacific Islands Region" to complete this work. Methodologies by Mitchell and Jones et al. will be used to apply satellite tags to a minimum of three nesters per season. All permits have been acquired from the U.S. Fish and Wildlife Service and the Institutional Animal Care and Use Committees (IACUC) of the National Marine Fisheries Service (NMFS) and the UH to execute this research.
This study will be the first to identify foraging hotspots using stable isotopes for PIR nesting green turtles. Together, the spatial and SIA data will establish connectivity between PIR green turtle nesting habitats and feeding grounds that will help to describe and monitor this population and their movement throughout the Pacific Ocean for the effective management of this resource. Specifically, this study’s resolution of foraging habitats used by PIR protected nesting turtles will provide target areas that may need to be afforded more protection and/or research (e.g. quantifying diet quality and availability) for robust conservation. These data are critical for environmental managers and can support improved decisions on threat mitigation strategies, commercial fisheries practices, designation of priority habitats, and status reviews for PIR green turtle populations. Overall, this project will fill knowledge gaps by improving our understanding of the protected PIR green turtle species.
DISSEMINATION OF RESULTS
I intend to share this research among scientific communities by publishing a peer-reviewed paper and presenting the findings to local, national, and international scientific audiences. To reach the public, MTBAP will issue a press release about this research for media coverage with Hawai‘i’s local news. Also, I will be presenting this research via in-person and virtual community and classroom talks in the U.S. PIR. Utilizing already established working relationships with Office of National Marine Sanctuaries coordinators, I will use this study’s findings to update sea turtle exhibits in science centers in American Sāmoa and Hawai‘i. Altogether, this project will provide local communities with valuable information on their sea turtle’s movements in the PIR and the interconnectivity with international communities.