Isotopic Signatures for Hawksbills (Eretmochelys imbricata) in the Eastern Pacific Ocean
For migrating animals, it is important to identify links between habitat use and spatial ecology. In September of 2018, we deployed SIRTRACK’s KiwiSat 202 PTT satellite transmitters on 4 nesting female hawksbills on the beaches of the Gandoca Manzanillo National Wildlife Refuge, located on the southeastern coast of Costa Rica. Satellite locations were analyzed to evaluate their internesting habitat and migration routes to distant foraging grounds. Gravid turtles selected internesting habitats close to beaches and may be exposed to anthropogenic threats such as entanglement in fishing gear, harvesting or boat strikes. Satellite tracks have revealed 2 distinctly different internesting behaviors, including remaining directly in front of the nesting beach, and moving 50 km south into the Bocas del Toro islands of Panama. This indicates that hawksbill’s internesting behavior is varied and may encompass areas significantly north or south of the nesting beach. Tracked hawksbills avoided strong coastal currents by moving in a circular pattern in the Caribbean Sea before migrating. Preliminary indications of postnesting movements are northward towards Honduras, remaining relatively close to the coast. This result shows that local ocean currents influence hawksbill behavior. Understanding routes from nesting sites to foraging areas is important in quantifying population-level impacts of anthropogenic threats and designing effective conservation responses to these threats. Elucidating to the need for an international approach for hawksbill conservation that spans many Central American countries.
Please click here to view Quintin's poster presentation at the 39th International Sea Turtle Symposium in Charleston, SC
Initial Project Proposal:
For migrating animals, it is important to identify links between habitat use and spatial ecology. Understanding specific associations between foraging and nesting habitats has been highlighted as a crucial step towards sea turtle conservation (Hamann et al. 2010; Rees et al. 2016). Using biogeochemical markers such as stable isotopes, biologists have been able to further understand the spatial and trophic ecology of marine animals (Ramos and González-Solís 2012). Stable Isotope Analysis (SIA) compares the ratios of isotopes of natural elements found in the tissues of predators and food items, which reflect their trophic ecology (Peterson and Fry 1987). The use of isotopes from C and N from skin tissue are well-studied and has given much insight to geographic distributions, trophic interactions, and habitat use of sea turtles (Pearson et al. 2017).
Supplementing the movements obtained from satellite telemetry, isotopic values can be collected for all encountered turtles to determine their foraging habitat and location (Hobson 2007). Due to cost most satellite telemetry studies tend to have small sample sizes but results in high-resolution movement for those turtles selected (Godley et al. 2008). By comparing the variation in N stable isotopes, the foraging habitat can be determined for turtles that are not selected for tracking (Zbinden et al. 2011). SIA has been used to support many satellite telemetry studies by establishing links between nesting and foraging habitats, as well as discovering foraging hotspots (Ceriani et al. 2017; Pajuelo et al. 2012; Robinson et al. 2016; Seminoff et al. 2012).
The hawksbill turtle (Eretmochelys imbricata) is listed as critically endangered due to the human exploitation of their carapace as tortoise shell products (Mortimer and Donnelly 2008). Hawksbills from the eastern Pacific Ocean (EP) have been largely neglected by research (Gaos et al. 2010; Gaos et al. 2017a; Godley et al. 2008; Pearson et al. 2017; Seminoff and Shanker 2008; Wallace et al. 2010). Hawksbills in the EP represent unique conservation needs based on their spatial ecology (Gaos et al. 2012) and nesting habitats (Gaos et al. 2017; Liles et al. 2015). Hawksbills have been shown to occupy different niches at various life-stages, and in the EP have been shown to exhibit natal foraging philopatry (Gaos, et al. 2017b). Identifying foraging habitats and habitat use will provide enhanced information for the management of this species (Carrión-Cortez et al. 2013; Gaos et al. 2010; Gaos et al. 2017a; Liles et al. 2011; Seminoff and Shanker 2008).
Anecdotal reports from local fishermen and a personal communication with a local Park Ranger (Luis Fonseca) have revealed an unknown nesting population of hawksbills on two islands off the coast of Costa Rica. The first, Isla Pelada is a part of the Murcielago islands, which is located inside of Santa Rosa National Park, a known forage area for hawksbills (Heidemeyer et al. 2014). The second, Isla Bolanos located in Salinas Bay, which is just north of El Jobo, Costa Rica. My objectives are to: 1) use isotopic values (δ13C and δ15N) to evaluate the foraging ecology and distribution of nesting hawksbills in the EP; 2) compare isotopic signatures of EP hawksbills to current isoscapes; 3) integrate SIA with satellite tracked hawksbills to locate foraging habitats.
Sampling and SIA
Nightly patrols of the beaches on Isla Pelada and Isla Bolanos will begin in May and conclude in September. Nesting hawksbills will not be approached until oviposition is complete and the nest is covered. Standard morphological data (CCL and CCW) will be collected and each hawksbill will be tagged (flipper and PIT). Using a sterilized scalpel, epidermis tissue samples will be collected from the trailing edge of a rear flipper and stored in a NaCl solution until prepped for analysis. Tissue samples will be dried for 24 h at 60oC, minced and homogenized. Samples will be prepared in tin capsules and sent to the Stable Isotope Facility (SIF) at the University of Wyoming for analysis of δ13C and δ15N. Samples will be analyzed using a Thermo Finnigan Delta Plus XP mass spectrometer attached to a Costech 4010 and Carlo Erba 1110 Elemental Analyzer. Stable isotope ratios are given relative to the isotope standard and are expressed in delta (δ) notation in parts per thousand (‰)
𝛿 = ( [𝑅 𝑆𝑎𝑚𝑝𝑙𝑒 𝑅 𝑆𝑡𝑎𝑛𝑑𝑎𝑟𝑑 ⁄ ] −1) (1000)
where Rsample and Rstandard are the corresponding ratios of heavy to light isotopes (13C/12C and 15N/14N) in the sample. To assign turtles to foraging areas using stable isotopes we will use the same approach as Wunder (2012). To predict foraging areas we will following the described methods from Ceriani et al. (2017) and Pajuelo et al. (2012).
Only healthy looking females will be selected for satellite transmitters. Upon the turtles return to the ocean, the turtle will be restrained with a wooden box for the attachment process. The carapace will be gently cleared of commensal epibionts, the top two dorsal scutes will be scrubbed with a bristle pad, then sanded and rinsed with acetone and water. Transmitters will be attached with a low-temperature curing 2-part epoxy, and molded into a hydrodynamic shape. Once epoxy is cured, the turtle will then be released to the ocean.
Expected Outcomes & Dissemination
SIA will reveal isotopic signatures for this nesting population of hawksbills, and combined with telemetry allows us to assign untracked turtles to foraging habitats. Comparing our signatures to current isoscapes we can determine the foraging niche for this nesting assemblage. This data will add to the growing body of literature for trophic ecology and movements of sea turtles. It is possible these hawksbills are foraging in nearby shallow habitats or mangroves (Gaos et al. 2012). Providing managers of Area Conservacion Guanacaste and Santa Rosa National Park with the location of hawksbill foraging habitats will allow them to continue protection and recovery for this endangered species.
All dissemination efforts will give appropriate recognition to the Boyd Lyon Scholarship. Results from this study will be distributed with a presentation to the local park rangers and managers of the National Park. In combined efforts with Equipo Tora Carey, local fishermen and families participate with the project by aiding in night patrols and transportation. Other outlets for publicity include: 1) conferences (ISTS19, IsoEcol19), 2) websites/social media (Paladino Lab, Equipo Tora Carey, The Leatherback Trust), 3) peer-reviewed journals, and 4) presentations to local entities (communities, resorts, dive shops, restaurants).